Re: OLTC project
Dr. Shark, Sun Jun 18 2006, 06:23PM

That's a big brick, you might want to gather some experience from "cheaper" silicon before you give that one a shot.
If you intend to drive the brick within spec, you will be fine with the 20V on the gate, it is only if you want to push higher currents that a higher gate voltage helps.

The 0.5us is basically a funcion of your resonance frequency. IF you build a tiny high frequency coil, you want 100ns, whereas a huge beast might get away with 1us.

Best Coil-size fol OLTCs traditionally is: The lower the resonant frequency, the better, so you want 2000 turns of fine wire, and make it as big as you like. The IGBT you got should be in for several kW of power.

Re: OLTC project
Extreme Electronics, Sun Jun 18 2006, 08:41PM

Hi,
Thats a really nice brick !! but its be a shame to blow it...

I really suggest you start with a smaller OLTC first. to get the "feel" of OLTC's

You can build a nice OLTC with a single small $10 IGBT that will give 1' sparks and wont cost you the earth when you kill it.. take a look at 4pf50's tha are well proven in OLTC service.

take a look at

the "on" time is dependant on your quenching time, which is dependant on your coils frequency and coupling. Basically the IGBT needs to be on for as long as it takes to transfer all of your primary power to your secondary, and no longer to prevent over heating.

You Will need to overdrive even a small IGBT in OLTC service, a small OLTC can easilly have 400A primary current. Fortunatly IGBT's can be overdriven to around 2x their pulse rating with 25V gate drive. so a single 4pf50 can take a pulse of 400A.

Bigger OLTCs get much more complecated, as Steves site will show you, but most of that electronics is to protect the brick from over voltage & over current.

for a guide to coil sizes etc take a look at my OLTC calculatior I cant claim its accuracy, but its will give good ball-park figures.

Derek

Re: OLTC project
Jrz126, Tue Jun 20 2006, 12:01PM

I was thinking about building a smaller oltc, but I was thinking that it would be alot harder to blow this brick compared to a smaller TO-220 type igbt.
I've been reading up on how to use the IGBT bricks here Looks like there is quite a bit of good information there

Now for the secondary, do I want to keep the 4/1 H/W ratio? and whats a good resonant frequency to shoot for? I'm also planning on using 26 gauge (.404mm) wire, and probably an 8" cardboard concrete form.

Re: OLTC project
Dr. Shark, Tue Jun 20 2006, 12:24PM

Blowing big IGBTs can be very easy. E.g. if the gate is not protected properly, and switching with too high dI/dt causes a voltage spike, it is dead. Get some TO247 devices to play with, they are a good compromise.

Short and fat would be a good idea for an OLTC, even something like 2:1 ratio. .4mm wire would give you almost 2000T, so it sounds good.

Re: OLTC project
Steve Conner, Tue Jun 20 2006, 01:37PM

Meh. Calling a certain kind of Tesla coil obsolete doesn't really achieve anything. They're all variations on a 100 year old theme as far as I'm concerned, and hardly going to net you a Nobel Prize. Having said that, I admit the OLTC doesn't perform as well as the DRSSTC.

Re: OLTC project
Jrz126, Tue Jun 20 2006, 05:56PM

Well, I've only got one of these bricks, so it kinda limits my options.

If I use the 26 gauge wire, I'd need a height of 34" to get ~2000T. Should I make the coil diameter pretty wide? (they also had 12" concrete forms).
Is cardboard a good material to use?
In general, do I want to shoot for a lower or higher resonant freq?

Re: OLTC project
Dr. Shark, Tue Jun 20 2006, 06:46PM

Don't let Dan discourage you, I think he is joking. With the recent interest in Terrys Solid State Spark Gap, I think OLTCs are better than ever. Of course you can buy longer sparks with a DRSSTC, but it is a hell of a lot more complicated!

Making the coil diameter wider is going to give you more inductance per foot of wire, so you want to make it pretty wide. The lower the frequency, the better, as it allows you to use a bigger primary capacitor for more bang energy.

Re: OLTC project
Terry Fritz, Tue Jun 20 2006, 08:14PM

Hi,

So I have one of these IGBT's here and I want to make an OLTC.

Sweet!!! 20V should be fine on the gate unless you want to drive like 10,000 amps thorugh it, then go to 25 volts

0.5uS is fine. Mostly determined buy the resonant frequency of your coil and how fast the initial current will rise. You don't want to turn on "too fast" or the Ccg capacitance could over voltage the gate. But put a big 30V TVS across the gate to emitter anyway.

You want a big primary cap and small primary inductance. It is hard to get less than 1uH for the primary inductor so the biggest cap you can get. That usually drives down the secondary frequency making a secondary coil with lots of turns for high inductance. Be careful not to make the secondary coil too lossy though!! My 'first' OLTC suffered from high secondary losses since I got too carried away trying to make it 120BPS at 300V. You could go to like 900 to 1000V pretty much solving all that if you have the power supply to get that voltage. Or was your IGBT 2400V... if so you can really go to a higher voltage!

Everything begining with OLTC here:



Is about my 'first' one. But Steve's and latter ones or all modern with more of the details and problems figured out. But you can quickly click through the about just to check.

I don't think you "can" blow that brick. But start out running at lower voltage just to get everything tuned in before running at high power. Low break rates (like once every few seconds) also help setting up.

Now for the secondary, do I want to keep the 4/1 H/W ratio? and whats a good resonant frequency to shoot for? I'm also planning on using 26 gauge (.404mm) wire, and probably an 8" cardboard concrete form.

Sounds fine! Probably in the 50 - 100kHz range. See this for formulas and the JAVA on-line tool:





You wants lots of secondary peak voltage and lots of capacitor stored energy (1/2CV^2). Higher voltage really helps stored energy so you can use a smaller cap for higher resonant frequency and a more normal secondary.

I never thought about it till now, but the SISG drive circuit could run the single big high current brick OLTC too!!! In that case, you don't need nothin for a drive circuit )

You may want to check this out:





I hate to recommend something "new and never tried" to a new coiler though. But I think you are an engineer so you know the usual electronic details. The SISG drive is probably far more reliable and easy than any conventional OLTC drive. We won't even mention the complexity of DRSSTC's Newer DF-DRSSTC and SISG circuits will probably make them obsolete And the OLTC will always be the "daddy" But the DRSSTC is definitly the "thinking man's" machine!!! Once one has mastered the DRSSTC, everything else is "simple"...

If the SISG drive does not work, you are only out like $5 and 15 minutes of work But it should work fine. I guess it would be a SISGOLTC It would be the simplest "big" coil out there in that case with only "one" SISG section.

Be sure to use a minimum of copper too!! A "ten foot" length of 1/4 inch tubing is running $16 here!!!!

Cheers,

Terry

Re: OLTC project
Marko, Tue Jun 20 2006, 08:17PM

Don't let Dan discourage you

OK you mean steve.. ? :)

Actually it seems that oltc's are indeed quite outperformed by DRSSTC's.

Looking at steve conner's OLTC 1 and 2 small one barely outsparks it's secondary length in the end.
If you are building an OLTC I guess you should make it huge. You want lowest possible Fres for biggest possible bang energy with single primary turn.

Conner's big one made 6 foot sparks(if I remember) with whole mess of toroids, from the pic it seems somewhat more than 2 secondary lenghts.

Ward's DRSSTC1 makes equal sparks that look much more impressive from many times smaller coil.


That brick will make a quite good OLTC since you already have it, but then you will figure out that people make longer sparks with DRSSTC's based on some 40N60's, even if you draw out 'maximum' of that IGBT and the coil itself.

I guess that's what Steve wanted to say...


2500V IGBT also adds room for bigger sparks, if you jump from 1000 to 2000V cap voltage you get four times more stored bang energy.

Good luck with your coil...

Re: OLTC project
Steve Conner, Tue Jun 20 2006, 09:32PM

In the end the best I got was 81" from a 30" tall resonator, and it needed over 4kW of power input to do that. The OLTC is quite limited in its bang energy, and delivers it in one short wallop. The DRSSTC can achieve a much bigger energy, and delivers it more slowly, so it can grow long sparks from a short coil without flashing it over. My OLTC 2was flashing over the whole length of the resonator towards the end, although the sparks weren't that long.

To explain: An OLTC or conventional coil dumps all the energy into the secondary as fast as possible to minimize primary circuit losses. The streamers then form and suck it out. A properly tuned DRSSTC seems to deliver the power just as fast as the streamers can eat it and no more, so there's no harsh spike of voltage that would aggravate flashovers or racing sparks. In practice, we can slow the energy transfer in a conventional coil by loosening the coupling, but an OLTC can't tolerate the low coupling because of its high primary losses, so it ends up with this wham-bam energy transfer.

I think that giant 2500V IGBT has got to do something spectacular, though You should get twice the spark length of my OLTC 2 (which had a 1200A 1200V switch)

When I think about it, I get kind of depressed that I lost the "Battle Of The Giant SSTCs". I guess if it weren't for me, we wouldn't know that OLTCs sucked, so it wasn't all wasted effort :-/

Re: OLTC project
Marko, Tue Jun 20 2006, 10:09PM

When I think about it, I get kind of depressed that I lost the "Battle Of The Giant SSTCs". I guess if it weren't for me, we wouldn't know that OLTCs sucked, so it wasn't all wasted effort :-/

Yeah, but I don't think you should 'quit' TC's and everything because of 'frustrations' , funds etc.
You don't need to beat any of world records but for me such claims and putting 'wtf d00d' page over there seems boring.

For jrz126, I think he should try to make secondary as small as posible with tiny wire (winding 110mm or bigger tube with 0.1mm etc. wire could be an interesting battle) and then find ways to move breakout avay from it to avoid flashovers.

Maybe to leave some empty space between toroid and top secondary winding, or, who says that toroid needs to be at top of secondary anyway?
(small corona ring is now needed to remove danger of top-turn breakout)

Maybe with few more ideas you could make a pretty good coil in the end. Conner and Terry Fritz will probably help you the most in this area.

Re: OLTC project
Jrz126, Tue Jun 20 2006, 10:12PM

Thanks a bunch Terry, finally someone with some info :)

I’m going to read up on those SISG’s…I think I might already have all of the parts I need to build one. How would I set it up to fire off of 120Vac and 220Vac? Using a voltage doubler...I could probably figure it out on my own, but I figure I might as well ask.

That IGBT is rated for 2500V, and as of right now I am limited to a 220V outlet. I’m also going to use some snubber caps (1000VDC 0.56uf KPF type). I’ve got a total of 24 to use.

Re: OLTC project
Terry Fritz, Tue Jun 20 2006, 11:43PM

Hi Jeff,

I just realized it was "you"

I "think" voltage doublers will charge in 1/60th of a second so the BPS would "always" be 60 BPS max? So we end up with either 300V at 120 BPS or 600V at 60BPS.... I am thinking the 600V would be better. Maybe there is a simple way to switch between them? I really think you want to use the 220VAC but you could use the 120VAC too for testing like me. Make the input power stuff real "flexible". At 300V we might be able to run at 240 BPS too which seems to have odd advantages... Wysock and Parisse love 240 BPS and their coils spark super good!! My new test coil will look into that, but it is a few weeks out.... We are now learning MUCH about streamer and how to make them long

I used resonate charging but that was sort of a complex thing requiering a pretty finicy inductor. I would avoid that even though it "could" do wonderful things.

I "think" a voltage doubled 220VAC thing would give the best bang for the buck. Maybe others would know better? If you have a 220V variac that could go to 256.7 VAC that would be perfect? Do you have a variac and if so what kind?

I think OLTCs are hurt a little by secondary losses so we need to be careful of that. I think Bart's program does Rac of the secondary prediction but I have always used MandK since that matches measurements for me. Paul has a ton of great data about Rac and how to keep it low. Coupling is also an area where I really had no choice on my OLTC. But maybe ScanTesla can show us a better approach. It can itterate though all the variables looking for the "best" configuration. It works fine for OLTCs too

We should study Steve's coil too that he says sucks to see if there are any areas that could be improved.

I have not played with the OLTC other than my original one. The goal then was just get it to work "at all".... But we know so much more now...

SISG firing voltage is controlled by the number and value of the SIDACs used. You can set it for anything...

SISG don't quench on like the first notch "yet"... So tht is a bit of an issue but my OLTC did not perform that much better anyway with first notch quenching and I think Steve found that too... I really see "no" reason not to make it an SISG... The firing system I had needed current taps off the line and all for sync... That all seems usless now with the SISG...

The OLTC may not be the best performing coil on earth, but it is the "simplest" in many ways. The SISG combined with the OLTC might make it the simplest "period"...

Cheers,

Terry

Re: OLTC project
Jrz126, Wed Jun 21 2006, 01:14AM


Yeah, it's "me"...kinda sounds like you recognize me from somewhere?

I "think" voltage doublers will charge in 1/60th of a second so the BPS would "always" be 60 BPS max? So we end up with either 300V at 120 BPS or 600V at 60BPS.... I am thinking the 600V would be better. Maybe there is a simple way to switch between them? I really think you want to use the 220VAC but you could use the 120VAC too for testing like me.

Could I set the SISG up so that I can control the breakdown voltage by using a jumper? Lemme see if I can get a schematic...the girlfriend only has dial up, so its quite slow...well I made a quick sketch...

I used resonate charging but that was sort of a complex thing requiering a pretty finicy inductor. I would avoid that even though it "could" do wonderful things.

well that can always be an upgrade I suppose...

I "think" a voltage doubled 220VAC thing would give the best bang for the buck. Maybe others would know better? If you have a 220V variac that could go to 256.7 VAC that would be perfect? Do you have a variac and if so what kind?

I have only have a 10 amp 120V variac :(

Well I better get going, the girlfriend is giving me the evil eye right now, and saying something about a "joining a 'my boyfriend is an electrical engineer' support group".

Oh yeah, I noticed steve used some flexable cable to connect his primary to the bricks. What should I look for in this cable?

Re: OLTC project
Terry Fritz, Wed Jun 21 2006, 03:00AM

Yeah, it's "me"...kinda sounds like you recognize me from somewhere?

The only one I know to get "another" DF-DRSSTC working )

Could I set the SISG up so that I can control the breakdown voltage by using a jumper? Lemme see if I can get a schematic...

Yep!! You got keep all the "nodes" "open" for "modification"... Nobody knows what really to do, so you have to be able to do it "all"...

Oh yeah, I noticed steve used some flexable cable to connect his primary to the bricks. What should I look for in this cable?

"If it ain't RG-8, it can't be toooo great" )) Radio Shack# 278-980.

Cheers,

Terry

Re: OLTC project
Steve Conner, Wed Jun 21 2006, 10:11AM

The cable I used is coaxial speaker cable for PA systems. It has about 12AWG worth of wire crammed into it.

To drive that brick off 220v you probably want a voltage tripler or quadrupler to get you up to 1kV DC bus voltage

Firkragg, I just changed the wtf d00d page to something a bit less "dum". I was ill and real depressed when I wrote it

In response to the original question about gate charge, I found that my new gate driver design with discrete MOSFETs rated 12A@60V can drive a 600A brick fine. What affects the turn-on and turn-off times the most seems to be the stray inductance of the wiring between driver board and brick: not the choice of MOSFETs or whatever.

Most big bricks seem to have gate resistors built into them that limit how fast you can turn the thing on. I believe the popular CM600HA-24H has 0.8 ohm worth of resistance in there, so if a gate driver for these bricks goes to 20V, there's no point in designing it to deliver more than 25A.

Re: OLTC project
Jrz126, Wed Jun 21 2006, 12:11PM

Well I just put in an order to Digikey...so it will hopefully be here Monday or Tuesday. That will give me some time to get the brick ready to go.

A voltage Tripler?...I didnt know they existed.
I should probably figure out what voltage I am going to run at...My Cap bank is going to be somewhat difficult to connect up. They have the tabs, not the axial leads.
here's the datasheet. (1000VDC working volts and 0.56uF.)
So I'd prefer to not have to reconfigure it too much to run at a higher voltage.

Re: OLTC project
Steve Conner, Wed Jun 21 2006, 12:53PM

Yup, you get voltage triplers, check out I believe they draw a DC component from the line however, which can be a real pain in some circumstances.

I would connect the cap bank for 2000V right from the word go. The extra voltage will give your coil a real kick in the back side, I wouldn't be surprised if it turned out the best performing OLTC ever.

Re: OLTC project
Jrz126, Wed Jun 21 2006, 03:23PM

Whats the benefit of using the coax cable? Is the coax cable just being used for the shielding conductor? Old Monitor cables had a steel braided sheilding over the cable, would that also work? I've got plenty of both lying around, but the braiding from the monitor cable would be much more flexible.

Also, how should I go about heatsinking this monster?

Re: OLTC project
Steve Ward, Wed Jun 21 2006, 05:35PM

The coax is to provide a low inductance line between the IGBTs and primary coil. This is very crucial because you want all of the inductance to be in the primary. You dont want stray inductance in the circuit for a few reasons: it lowers the overall coupling between the primary and secondary, and it increases your surge impedance unnecessarily so. Ideally the IGBTs would probably become part of the primary coil like Terry did in his first OLTC design. But, since you are dealing with a giant IGBT with a big heatsink, its not an easy way go to. The coax is the next best option, but you need to parallel enough of them to handle the currents.

Re: OLTC project
Jrz126, Wed Jun 21 2006, 08:17PM

So is the coax being used for the inner or outer conductor? I'm assuming outter due to the high current involved...so would the braided shielding work then?

Also, Are there any eaisly obtainable alternatives to using the cardboard form for the secondary? I can see that getting broken down the road.

Re: OLTC project
Steve Ward, Wed Jun 21 2006, 08:53PM

No, the coax is used as coax (both conductors are used, and seperately). It is used for the send and return of the primary connections. You are basically reducing the loop area of the wiring, thus reducing the inductance.

Re: OLTC project
Terry Fritz, Thu Jun 22 2006, 12:30AM

Hi,

Yeah, it's "me"...kinda sounds like you recognize me from somewhere?

I had you confused with James Zimmerschied who is also "JRZ". I get confused easy #:-)


The Steve's

Thanks for explaining the Coax and how it is used to reduce wiring inductance!! I never realized what all that was about *:-) I might do that on my new coil where primary inductance needs to be rather low.

Must study the voltage doubler and trippler link! The DC SISG and MOT stuff is all new to me. Mark and some use resonant charging on them too...

Somedays, it is like I don't know anything 0:-)

For a durrable secondary form. I like cut to length polycarbonite from Mcmaster-Carr.

Cut-to-Length Length

Id OD P# Cost/foot
1" 1-1/4" 8585K15 $4.58
1-1/4" 1-1/2" 8585K16 5.32
1-3/8" 1-1/2" 8585K43 3.24
1-1/2" 1-3/4" 8585K17 6.51
1-3/4" 2" 8585K18 7.00
2" 2-1/4" 8585K46 8.19
2-1/4" 2-1/2" 8585K19 8.73
2-1/2" 2-3/4" 8585K34 10.01
2-3/4" 3" 8585K21 10.70
3-1/4" 3-1/2" 8585K33 13.25
3-3/4" 4" 8585K22 17.06
4-3/4" 5" 8585K45 24.38
5-3/4" 6" 8585K23 29.92
7-3/4" 8" 8585K48 33.36

But it costs a whole lot more that concrete tube!! Very nice for little coils though. You can also get sewer and drain pipe from a plumbing supply contractor type place but you usually have to get full lengths that are like 20 feet long... Or just hang out there and follow the guy that just bought 50 of them and see if he has any left over )) You can often call plumbing places and ask if they have an extra bit they would sell you too. Most contractors just throw the extra away. The trick is to be there at the right time.

Cheers,

Terry

Re: OLTC project
Jrz126, Thu Jun 22 2006, 01:08AM

ouch. thats quite expensive.
I'm going to make some calls tomorrow and see if I can find some for cheap...If not, I'll probably just buy a concrete tube, plug the ends and fill it with expandable foam.

My dad has a 18.5" diameter piece of pvc, 48" long, left over from when we built our house...I think its alittle too big?

Re: OLTC project
..., Thu Jun 22 2006, 01:33AM

What size are you looking for? Public Missiles Ltd has 48" pieces of phenolic tube for $40 for a 6" piece or $80 for a 12" diameter... If you only need 4" I would get the 'quantum tube', (it is that grey plastic tube that is shown at the top of the airframes section of the webstore) it isn't as strong (but still stronger than you need) but considerably easier to work with.

Re: OLTC project
Steve Conner, Thu Jun 22 2006, 11:10AM

When I was making my OLTC 2, I went to one of those plumbing supply places and spoke nicely to them, and they cut off a 3 foot piece of 10" HDPE pipe with 3/8" wall thickness for me. It cost about $35.

The secondary for my DRSSTC was another piece of smaller HDPE piping that I found lying in the road one day.

Re: OLTC project
HV Enthusiast, Thu Jun 22 2006, 12:38PM


Check EBAY. There is a supplier there that sells acrylic tubes for dirt cheap. Although, i'm not sure what the largest diameter is.

Re: OLTC project
J. Aaron Holmes, Thu Jun 22 2006, 06:35PM

Now that we have the SISG in our toolboxes, what about using SISGs to create an "off line" Marx generator to power your OLTC? Take three 300V SISGs (one SIDAC each) and make a three-stage Marx that'll spit out 900V. Then attach TC primary. I'm told SIDACs are slow, so perhaps the rule would be that the output of the stack ought not to exceed the voltage of any single IGBT. Else the rise time might outpace the SIDACs after the first stage fired and you'd get deep-fried IGBT. But if you stay below the IGBT voltage, as in my example above, wouldn't that work?

Then you'd have the Sidac Igbt Spark Gap Marx Generator Tesla Coil (SISGMGTC)

...would be "interesting".

Regards,
Aaron, N7OE

PS: Yay! I'm finally signed up!

Re: OLTC project
J. Aaron Holmes, Thu Jun 22 2006, 07:31PM

(edit: Few more thoughts)

Basically, the schematic I had in mind was:



...where each SISG is set for 300V (one SIDAC).

Hmmm... Maybe the voltage doubler ought to be separated out with bigger caps for smoothing to allow for higher break rates, I don't know. Haven't thought enough about it. Add in DC resonant charging and higher-V IGBTs and you might really get something out

Re: OLTC project
Jrz126, Thu Jun 22 2006, 08:39PM

So I think I'm going to go with the cardboard tube...(although, I think I might have enough 26# wire to do a 34" coil on that 18.5" diameter pipe that I have ). Will I have to coat the cardboard before I wind it?

My SISG parts wont arrive until Tuesday

Re: OLTC project
Jrz126, Fri Jun 23 2006, 07:16PM

Sweet! the fed ex package tracker dealy is way off. The parts are scheduled to be delivered today.

I have a 4.5" x 18" coil already wound, I think I'll set up a little test rig for now. I dont know if Ill be able to use the SiDac circuit, since it will only fire at 240-280V. I should have ordered some lower voltage ones.

I was reading up on how to drive these some more (see this ). on page 16,

An IGBT will be off when its gate voltage is zero. However, in order to ensure that the IGBT stays in its off state when dv/dt noise is present in the collector-emitter voltage an off bias must be used. Use of reverse bias also decreases turn off losses. The relationship between reverse bias voltage and switching losses is shown in Figure 4.45. For H-Series IGBTs an off bias of -5 to -15V is recommended.

So does this mean my Gate Drive should be capable of going negative?

Also, I was reading Steve's OLTC stuff and he mentioned something about secondary flashover...what is this exactly?

Re: OLTC project
Marko, Fri Jun 23 2006, 08:43PM

So does this mean my Gate Drive should be capable of going negative?

Yes, It probably isn't that important here but it's main advantages are that it speeds up turn-off, and holds the gate strongly under zero.

Noise needs to be greater than this voltage (plus few volts) to accidently turn the IGBT on.

In high frequency bridge applications this is very important because miller (gate-drain capacitance) couples part of high supply voltage to the gate.
When IGBT closes shut voltage on it's drain quickly rises to supply voltage, and miller capacitance couples this dV/dt change to gate.
If gatedrive failed to hold off that 'spike', gate will turn on, IGBT will open when it was supposed to be closed, undergo shoot-trough with another IGBT in halfbridge, shorting the supply, and both die a horrid death.

In OLTC things are different, you have only one switch and switching is slow, so holding gate isn't necessary. As I figured out Steve Conner didn't use negative drive in his final design.

Also, I was reading Steve's OLTC stuff and he mentioned something about secondary flashover...what is this exactly?

As steve explained, single turn primary and MMC, switched by IGBT, aren't actually a very good resonant circuit.
Since we are operating on low voltage we try to get the tank capacitance as big as possible, and we must cut down the primary to single turn if we want a reasonably high Fres.

Due to losses on Primary circuit it must be closely coupled to secondary, so the energy is transferred as fast as possible before it's lost.

Fast energy transfer causes secondary voltage to rise rapidly, wich leaves little time for streamers to 'grow' and secondary tends to flash over and make racing sparks. It's hard to get multi-secondary-length streamer 'growing' off the breakout point and miraculously not flashing ovver, like with DRSSTC and less-coupled SGTC's (where energy transfer is much slower, especially with DRSSTC).

Re: OLTC project
Jrz126, Sat Jun 24 2006, 03:32AM

I made a quick test setup... secondary + rig
the brick

closeup of the brick

Now all I need is a gate drive

Re: OLTC project
Marko, Sat Jun 24 2006, 10:41AM

Your MMC and secondary look pretty small for that brick. (Hihg Fres).

But for quick lash-up we can say it's OK.

What wire thickness did you use for this secondary?

Re: OLTC project
Jrz126, Sat Jun 24 2006, 10:53AM

I've got 24 of those caps to use.

I used 1/4" fridge tube.

I'll be working on it later tonight :)

Re: OLTC project
Jrz126, Sat Jun 24 2006, 10:40PM

I'm designing my Gate drive right now, I'm going to use a IR21844 IC, a half bridge mosfet driver which has a programmable 'dead time' to prevent shoot through.
I'm going to use a 555 timer to trigger it. What duty cycle/on time do I want to use to drive my IGBT?

Re: OLTC project
Marko, Sat Jun 24 2006, 11:13PM

Maybe we misunderstood?

I guess you meant to drive gatedrive-mosfets with this driver, and shoot-trough is not a danger there.
This is a high voltage driver for SMPS use, seems to be generally slow but you could use it if you really wish, for mosfet driving.

Look at Steve Conner's OLTC pages, you have lots of OLTC knowledge there.


You can use it but there are much simpler ways to do the same thing, and much faster.
I'd stick to Steve Conner's driver for the beggining, for a big OLTC improved version (with zero cross detection and crowbar) is very advisable.



NE555 and simple mosfet driver may be good for the beggining, at low power. Just take care not to blow up that nice IGBT.

Re: OLTC project
Steve Conner, Sat Jun 24 2006, 11:21PM

Jrz and Aaron, 48 hours between double posts please If you want to add something in a hurry, edit your original post.

Using the SISG as a power supply is pretty pointless. If you don't like voltage multipliers running off the 60Hz line, just go to an electrical salvage yard, get a 120 to 480v transformer, run it backwards, and rectify the 480V AC output. If you want to do something really 1337, make a boost converter power supply like Steve Ward's 7kW unit.

Re: OLTC project
Terry Fritz, Sat Jun 24 2006, 11:30PM

Hi,

I looked into the voltage doubler and trippler stuff. Pretty cool!! But the caps have to be pretty darn big and all!! In general, I would think one would be better off just using a MOT. Of course, maybe some folks like the voltage doublers and tripplers so more "power" to them ))) But a very simple MOT run at low voltage off a variac can kick some pretty big arcs too )

Cheers,

Terry

Re: OLTC project
Marko, Sat Jun 24 2006, 11:45PM

With 2500V IGBT one needs to lower the input voltage of a mot just a little.

To get 2 kilovolts it would take a pretty big voltage multiplier to get enough power ($$$ caps)

Re: OLTC project
Steve Conner, Sun Jun 25 2006, 01:04AM

You only need 1kV since DC resonant charging automatically doubles it for you. My OLTC 2 used a 600V DC bus made by voltage doubling the 240V AC supply.

Re: OLTC project
Jrz126, Sun Jun 25 2006, 11:37PM

I calcuated the resonant freq of my testing secondary to be 189.95 khz. Now how do I determine what frequency to drive my IGBT at?

Re: OLTC project
Steve Conner, Sun Jun 25 2006, 11:43PM

The OLTC isn't like other SSTCs where you drive the switches around the resonant frequency. If you want 120 sparks per second, you feed 120 pulses per second to the IGBT, so you could say you're driving it at 120Hz.

However, the length of the pulse for best performance is related to the resonant frequency of the coils and the coupling between them. You want the pulse to end when the primary current reaches its first notch, usually after 3-4 cycles of the resonant frequency.

Since OLTCs lacked bang energy, we ended up operating them at insane breakrates to try and make the sparks bigger. I never ran my OLTC 2 at breakrates higher than 600Hz because the power supply couldn't take it, but I've seen other people use well over 1000Hz.

Re: OLTC project
GimpyJoe, Mon Jun 26 2006, 12:40AM


You can eliminate the need for a variac by simply making a new tap point on your MOT secondary. Just do a cut off turns and re-solder when you have the output voltage you want.

Re: OLTC project
Marko, Mon Jun 26 2006, 11:57AM

Maybe, if MOT likes unwinding.
Problem is that this involves sawing the core in half, and resonant charging with doubler or etc. is a less pain than that.

Re: OLTC project
Steve Ward, Mon Jun 26 2006, 04:28PM

Re: Cardboard forms: I use a cardboard form on my largest coil. The cardboard performs just fine at standing off the voltages, but you must properly prep the form. Make sure you have a dry tube, not something thats been sitting outside. First thing you have to do is take a knife and "score" (shallow cut) along the seam of where the paper wrapping overlaps (its a spiral around the tube). This should allow you to remove the outer paper easily with careful peeling. Once you get rid of that outer layer of paper (you want it gone cause it has black, conductive, ink), you are ready to begin the process of coating and sanding. Coat the tube with your typical polyurethane varnish, then give it a light sanding. Repeat this process about 3 times or more, until you have a suitably smooth surface to wind your coil on. After winding the coil, seal it again with 3 or more coats of the varnish. You may also wish to coat the ends (the edges) as well as the inside of the tube with varnish to keep it from absorbing any more water. So far my coil has withstood 144" sparks from an 8" diameter 45" long winding. I used the concrete form tube from menards, but the stuff from home-depot has worked fine too. I think its all pretty much the same, as ive heard other successes with cardboard too.

Re: OLTC project
Dr. Drone, Mon Jun 26 2006, 07:21PM

Re: OLTC project
Jrz126, Mon Jun 26 2006, 11:38PM

Thank you all for the info...

I found a 555 circuit that allows me to vary the duty cycle with a pot. I had the on time set to about 50usec. with 144 bps. It ran pretty good off of a car battery charger, pulling about 4-8 amps. When I use my variac, it hums like mad...Am I drawing too much current through it? how do I make sure I dont pull too much current from the wall outlet?

Re: OLTC project
HV Enthusiast, Tue Jun 27 2006, 01:08AM

i use cardboard sonotubes for a number of coils also. i don't bother removing the outer layer though. i think the entire tube, being considerably hygroscopic, will have a higher conductivity than a little black ink. if it makes you feel better, then by all means remove it, but i don't think its necessary.

Re: OLTC project
Jrz126, Wed Jun 28 2006, 08:55PM

I've started work on my winding jig. I'll try to have some pics up later tonight.
I've got a few questions, first off, how much polyurethane did you guys use? I'd prefer to not buy the 1 gallon can if I dont have to.
I also ordered parts to build a gate drive similar to Steve Conner's. How is the Rogowski coil implemented? I'm mostly looking for the position with respect to the primary, and I assume I'll need to use shielded cable?

Well thats all for now, hopefully I'll be able to get some work done tonight

Re: OLTC project
Steve Ward, Thu Jun 29 2006, 01:15AM

1 quart of varnish was plenty for my 8"x48" coil. I wouldnt get the really small can though.

Re: OLTC project
Steve Conner, Thu Jun 29 2006, 08:58AM

Hi jrz

The Rogowski coil is a toroidal air cored current transformer. I made mine by wrapping some left-over secondary wire round a 6" length of plastic hose (about 3/8" diameter) and then bending the hose round into a donut shape and joining the ends with tape.

I put it round a conductor of the primary circuit so that the full primary current flows through the middle of it. The whole point of the Rogowski coil is that it only detects currents that pass through the donut hole. To avoid the single turn effect and help improve the cancellation, I passed the wire from the far end back down the middle of the hose.

Re: OLTC project
Jrz126, Tue Jul 04 2006, 08:37PM

I've got my winding jig ready to go and I put 4 coats of poly. on the bare form. I'm just about ready to start winding it. It was suggested previously in this thread to use about 2000 turns and a 2:1 ratio. Now do I want to stick to the 2:1 ratio or 2000 turns?
If I stick with the 2000 turns, I'll end up with a 2.7:1 ratio. and if I stick to the 2:1 ratio, I'll end up with 1400 turns.

Re: OLTC project
Steve Ward, Tue Jul 04 2006, 10:32PM

I see no reason at all for a 2:1 ratio. By all means, use a longer winding and get the lowest operating frequency you can. Even a 3:1 ratio is a little "fat" in my experience. I typically go with 4:1. But really, winding ratio isnt as crucial as some might say it is. Getting a lower Fr so you can use more bang energy *is* crucial.

Re: OLTC project
Steve Conner, Wed Jul 05 2006, 02:59PM

I'd go with the 2.7:1 and 2000 turns. The reason for a "Stubby" secondary is that it allows higher coupling, but the OLTC isn't so demanding of coupling as the DRSSTC. I got plenty coupling on the OLTC 2 with its 3:1 (30" x 10") secondary.

Re: OLTC project
Jrz126, Thu Jul 06 2006, 01:50PM

There is a seam in the cardboard that is exposed now from when I removed the yellow paper. How should I fill it in? I put a piece of masking tape over it, but its still alittle flimsy. Should I put some Poly. over the masking tape?

So should I put more than 2000 turns on it? Is it possible to have a resonant frequency that is too low?

I ordered a bunch of parts last wednesday, hopefully they will be here shortly.

Re: OLTC project
Steve Ward, Thu Jul 06 2006, 04:18PM

There is a seam in the cardboard that is exposed now from when I removed the yellow paper. How should I fill it in? I put a piece of masking tape over it, but its still alittle flimsy. Should I put some Poly. over the masking tape?

Is there a deep grove left? The seam in my tube just left a bit if a bump, so i just wound over it.

So should I put more than 2000 turns on it? Is it possible to have a resonant frequency that is too low?

Well, its only too low if you cant tune your primary low enough to match, so keep in mind how many caps you will have, and what your primary coil will be. There probably isnt much to be gained from more than 2000 turns. Be sure to top it off with a good sized toroid, generally the outer diameter of the toroid should be equal to the length of the coil (thats my general rule anyway).

Re: OLTC project
Jrz126, Thu Jul 06 2006, 06:18PM

the groove is pretty big, I'd say about 1/16th wide and 1/16th deep. the masking tape covers it, but its pretty flimsy. I had about 10-15 turns on it and I noticed it was mis-shaped there. I'll try putting some poly over a small section and see how that does.

I've got 24 0.56 caps to use, so I'll be able to have 13.44uf at 1000V or 6.72 uf at 2000v. Which brings up another question...Would I be able to use a double-sided PCB for a bus bar to mount these caps? I have some 1-oz double-sided boards lying around. The 1-oz is pretty thin, but I might be able to beef it up with some addiional wire.

Do I need to have the large topload on there all the time? or is it for when I start getting into higher powerlevels? I have 4" dryer duct toroid already made up (15" overall), that I'd like to use for testing at lower powerlevels.

Re: OLTC project
Marko, Thu Jul 06 2006, 06:33PM

Yep, you need huge topload and secondary.

I hope i'm 'breaking no rules' with this, pic, , but just to get the image of size of toroid and secondary Steve used. 10x30inch is pretty big. Maybe you could get away from this by using thinnger wire, but you should anyway keep frequency very low, 50-60kHz in overall.

I was suprised too by it's giantness...

Re: OLTC project
Jrz126, Sat Jul 08 2006, 08:59PM

Well I think I'm ready to wind my secondary.

I think the Post office lost my package of electronics, its been about 10 days now :( and the online tracker says it was delivered 8 days ago.

This project seems to be taking forever...

Re: OLTC project
Jrz126, Tue Jul 11 2006, 10:03PM

I guess the post office thought I moved or something, so my package was forwarded to some other address. Guess thats the last time I ever ship with the US postal service.

How perfect does the secondary have to be? I'm having a tough time keeping the wire tight, so there are a couple spots where it sticks up alittle. Also, that groove is also causing it to be alittle funny shaped. I definitly dont want to fry it because I didnt spend alittle more time prepping it.

Re: OLTC project
Steve Ward, Tue Jul 11 2006, 11:24PM

My cardboard coil has a pretty obvious deformation due to the overlap in the form. I wouldnt worry about it being perfect. So long as the turns arent overlapped anywhere, you should be OK. Be sure to coat it right after winding so that the coil cant loosen up and come undone!

Re: OLTC project
Marko, Tue Jul 11 2006, 11:36PM

Be sure to coat it right after winding so that the coil cant loosen up and come undone!

Don't tell it happened to you too?

I'd say it's really one of most frustrating things that can happen with coils..

Re: OLTC project
Nik, Wed Jul 12 2006, 12:12AM

Depending on how slow your winding jig is you can coat the windings as you wind them. My jig is a mindnumbing 4rpm so I have no problems winding the coil and putting on soem varnish at the same time.

Re: OLTC project
Desmogod, Wed Jul 12 2006, 02:20AM


So it takes you c4 Hours to wind a 1000 turn secondary??!!
I did my last secondary (c1600t) quicker by hand!

Re: OLTC project
Jrz126, Wed Jul 12 2006, 02:27AM

ok, I just wanted to make sure that it didnt have to be super perfect. As for the unwinding, I've kept a hot glue gun handy since I can only wind for so long before I need a break, so I just put a dab of glue on there to hold the wire. It peels right off too.

Now for some pictures...
4" down, only 30 some to go
the jig I've got a nice DC motor hooked up, ~4rpm at 6v and about 8 rpm at 12V.

Close up of the coil

I've been wondering how everyone else holds the initally when they start to wind it...This is the solution I came up with, a bunch of cable ties strapped around the form and held in place with some hot glue. Seems pretty simple.

Re: OLTC project
Steve Ward, Wed Jul 12 2006, 02:39AM

I've been wondering how everyone else holds the initally when they start to wind it...This is the solution I came up with, a bunch of cable ties strapped around the form and held in place with some hot glue. Seems pretty simple.

Either glue the wire down to the form, or tie it to one of the screws holding the end cap on the form.

The winding looks fine so far. Sounds like you could use a faster motor. I did my 45" winding in about 1 hour i think.

Re: OLTC project
Dr. Drone, Wed Jul 12 2006, 03:39AM

Re: OLTC project
Jrz126, Wed Jul 12 2006, 01:39PM


You can wind it at that speed? how do you make sure you get a nice tight coil?
I'd rather go slower to make sure I dont screw up.

Oh yeah, would I be able to use somthing like this for my Primary to brick connection?

Re: OLTC project
EDY19, Wed Jul 12 2006, 02:35PM

When winding coils, I put the former in the drill, and once the wire is started, if you make sure there is tension on it and it is only a couple of degrees off perpendicular with the form towards the starting end, the winding is really easy, as the wire just "rolls" or slides down the previous winding right into place. I've never spent more than a half hour on any of my coils- even a 4" by 12 inch wound with 35AWG wire. The biggest thing is making sure that whatever you are spinning the former with is perfectly centered, this helps a LOT! I usually wind my coils at about 180RPM+ once they are started

Re: OLTC project
Tom540, Wed Jul 12 2006, 04:44PM

You guys do guide the wire with your fingers though right? Theres no way I can let it wind and hope it falls into place.

Re: OLTC project
EDY19, Wed Jul 12 2006, 05:12PM

Yes, I hold on to the wire (with higher speed, I use leather gloves) and keep it at a couple degree angle so it "falls" into place. Most secondarys are only a matter of a half hour or less

Re: OLTC project
Jrz126, Thu Jul 13 2006, 04:14AM

I dont think I can wind mine much faster, that groove is causing problems.

I've been thinking about my overall design and I came up with a pretty good idea, but I just want to make sure its ok.
I have 24 0.56 1000Vdc caps and I'm planning on running 2 paralleled strings in series. It would be so much easier to wire it up like this sketch. The IGBT connects across the 2 circles and that scribble is supposed to be a primary.

Re: OLTC project
Steve Ward, Thu Jul 13 2006, 05:25AM

That wiring scheme is fine.

Re: OLTC project
Jrz126, Tue Jul 18 2006, 11:57PM

Finished winding the secondary on sunday and I'm working on coating it right now. There are a few spots where one of the turns sticks up alittle higher than the others, so I'm probably not going to sand it.

I'm attempting to build a Rogowski coil now and I could use some advice...what size wire should I use? what length should I use for the loop part? etc.

Re: OLTC project
Jrz126, Thu Jul 27 2006, 12:49PM

Time for an update and some questions...

I picked up these diodes here:
There was still 4 days left on the auction, but I just emailed the seller and asked if I could purchase them. I paid alittle extra, but atleast I didnt have to wait.
Would they work for my voltage doubler/tripler?

Next, My primary is just about finished, it's not too pretty looking, but I hope it will get the job done. I went with 5 parallel single turns of 1/4" copper. Does this seem reasonable?

The secondary is almost finished, I just have to trim it down alittle bit. Also, how should I seal the ends? I have some 12" MDF circles that fit somewhat snugly inside the cardboard. Should I just put some polyurethane or fiberglass resin over the ends?

I'm going to be putting in some time tonight, and I'll try to get some pictures.

Re: OLTC project
Steve Ward, Thu Jul 27 2006, 06:48PM

First, those diodes are overkill for a multiplier, but they will work very well indeed. They should be heatsinked if running more than 5A or so.

The primary sounds fine, though i have never built an OLTC. Some pictures of the proposed layout would give more information.

I have sealed many coils with either plywood or particle board ends, sealed with polyurethane varnish. It seems to work just fine.

Re: OLTC project
Jrz126, Fri Jul 28 2006, 01:38AM

Here's the pics...
The cap bank and IGBT They;re attached to a doublesided PCB, 12 on each side. (13.44uF) I used some pieces from an old board game as insulators.

A closeup of the insulators.


The rogowski coil...



The primary bolts. I'm going to be attaching ring terminals to these, and then to the top of the double sided pcb.

The spacing of the primary conductors.

the secondary in its final position.

and finally the oscopes... Purchased from the junk room at work for only $12.00 ($0.25 per pound) The digital scope has a few problems, but its good enough for now.

Re: OLTC project
Jrz126, Sat Aug 05 2006, 09:40PM

I was finally able to get some testing done on the OLTC, I've spent the past couple days repairing my Oscope.
Using a 12V battery charger and chicken stick, I was able to pull a .5" arc off the topload.

How do I calculate the inductance of my primary? I have it wired up as 5 parallel turns.

Oh and is there an easy way to measure the Fres of my secondary?

Re: OLTC project
Steve Ward, Sat Aug 05 2006, 10:09PM

If you really want to measure the resonant frequency of the secondary, you can hook the base of it up to a signal generator (ground the other output of the gen). Hang a scope probe in the air near the coil, and sweep the frequency until you find the lowest freqency that really peaks on the scope. It should be close to whatever you calculated. If you get a number thats way off, be suspicious. If you dont have a signal generator, you could use a simple 555 circuit too, it doesnt have to be fancy.

To measure the primary inductance, the best way would be to dry fire the primary (that is, remove the secondary). You shouldnt need but a few volts on the tank cap before firing. You can watch the voltage across the tank cap, or across the primary on the scope. Try to capture the ringdown, and work out the oscillatory frequency from that. Once you know this frequency, and you are fairly certain of the tank cap value, then you will know the *total* inductance of the tank circuit (which is really what matters for tuning). There might be ways of calculating the inductance, but my method would be the most direct way of determining the total loop inductance of the setup.

I think most guys tune their OLTC's by changing the number of capacitors in their bank, rather than trying to tap the primary.

Re: OLTC project
..., Sat Aug 05 2006, 10:26PM

Inudctance of primary: Calculate inductance of a single turn of whatever diameter the primay is (uH~[diameter in inches]/18 if I remember correcly, is a very rough estimate) then count divide by the number of turns.

Frequiency of secondary: Hook it to a function generator and see when it resonates. I just take the the 600ohm output of a function generator anf hook one side up to the gnd of the secondary, the other up to a ground plane under it, then set a scope probe a few inces away from the top of the secondary, set the time base of the scope to about what I expect it to resoante at, then sweep the frequiency until I get the biggest signal picked up... works great It also works great to show how object getting near the coil accect the resonay frequiency

Re: OLTC project
Steve Ward, Sun Aug 06 2006, 02:04AM

then count divide by the number of turns.

Not so fast... Coupled inductors do follow the rule 1/Ltot = 1/L1 + 1/L2... that *uncoupled* inductors do. The inductance is reduced somewhat from an ideal single turn (maybe by a few %), but its definately not 1/5th, in this case. Just trying to keep the facts straight.

Re: OLTC project
Jrz126, Tue Aug 08 2006, 06:44PM

Well I was able to measure the primary frequency, 53 khz, which gives me an L of 0.66uH, my C is 13.44 uF.

So now I think I'm in a bit of trouble. My secondary calculates out to 399mH and 17.73 pF. With a 6x24" topload (30.02pF), I'll have a resonant frequency of 36.45 kHz.

In order to match this with my current primary, I'll need to increase my cap bank to 28.4 uF. Which means I will have to buy some new caps. Or I could try to make a 2 turn primary...Is it possible to parallel several 2-turn primarys?

Do my numbers make sense? Are they too extreme?
I'm using TeslaMap to estimate my secondary frequency.

Re: OLTC project
Steve Ward, Tue Aug 08 2006, 07:50PM

Your numbers sound OK to me for the primary (though i calculate .67uH for the primary, good enough).

Given your L and C values for the secondary, i calculate nearly 50khz for resonance, but this doesnt take into account all the distributed effects in the secondary, which may change that rough number.

In any case, you might want to measure the secondary frequency since there is some question here...

I suggest getting more capacitors so you get more bang energy. If you charge your current tank cap to 1kV, thats a bang energy of 6.72J. Thats something like 806W input power at 120bps. Anyway, i think you might only need a few more caps to bring the system in tune, so maybe you will end up at around 8-9J bangs, which should make for 5' sparks if you really wanted .

Id really suggest eventually making a new MMC for 2kV operation if you really want to get some long sparks. In that case, it may be worth doing 2 primary turns so that your tank cap wouldnt be a 30J monster (i dont know that your coil is tall enough for 30J bangs, which would be about 8-9' sparks if its efficient).

Re: OLTC project
Dr. Drone, Tue Aug 08 2006, 07:55PM

Re: OLTC project
Steve Conner, Wed Aug 09 2006, 11:16AM

I second what Ward said, pleeeeese use 2kV, it'll make the thing so much better. My OLTC2 ran at a charging voltage of 1kV (DC resonant charged from a 600V bus) and it was never really more than "OK" performance wise.

Re: OLTC project
Jrz126, Wed Aug 09 2006, 05:47PM

Chris, I dont quite follow your diagram there...I'm not sure what that weird box thing is by the 10K pot.

There's only one problem with using 2kV to power it, and that is that I dont have a 2kV source. How much current should I expect to draw from the line at that voltage? Would I be able to use a MOT to power it?

Also, where would I be able to get some nice caps from? I'd like to be able to use the ones I have if possible since they were free.

Can I run several 2-turn primarys in parallel? Maybe I'll attempt it with some 10 gauge wire to see how well it will work.

Re: OLTC project
..., Wed Aug 09 2006, 07:27PM

I think that Chris is trying to rectify the output to give an average voltage (which would be highest when it is in tune) but as long as you have a scope you don't need any of that.

The current will be proportional to the break rate... If you are using 10J bangs at 2kv 100bps is going to draw 500ma, just about right for a mot... If you want to crank things up you would need to parallel a few more on

As to the caps, a standard CDE 942 .15ufs/2kv cap is about 1/3J, so for for well under $100 you could make a mmc... You should be able to get away with just paralleling them up, but you would need to make sure you don't have very many spikes...

I would assume that multiple 2 turn primaries should work fine...

Re: OLTC project
Dr. Drone, Wed Aug 09 2006, 07:47PM

Re: OLTC project
Jrz126, Mon Aug 14 2006, 12:23PM

Well I ordered 200 caps (0.68 uF and 1200V) based on MR Ward's recommendation. They are being delivered today. I think there must be a Murphy's Law that no matter when you order a package, it will always be delivered on a monday. So you sit around all weekend waiting for it.

What are some things to keep in mind while trying to do a low inductance layout?

Re: OLTC project
Steve Ward, Mon Aug 14 2006, 03:25PM

Muahaha! Now this coil can become the largest OLTC when its ready .

For the MMC, since its "only" 2400V, you can rely on the PC board material to insulate just fine. So, what im envisioning is a large 2 side PCB, set up very similar to your current MMC, where each side of the board is one of the termination points. Now you just have to series 2 caps before connecting them to the board. I would suggest putting 2 caps sitting parallel to each other, so they form a upside down U shape (the leads connect the 2 legs of the U). That should be the most space efficient, and keep the inductance down. You could probably find some way to put caps on both sides of the PCB like you did with your current MMC. Anyway, careful thought is required to minimize the inductance on a 100 cap MMC bank!

If you want, i can draw a picture of what i was trying to describe.

Re: OLTC project
Jrz126, Mon Aug 14 2006, 11:54PM

I think I understand what you are talking about. Now how would attach the caps to the front side of the pcb? I could drill a hole for the one lead to go through, but there isnt an easy way to secure the second one.

Also, I may need to use more than 1 double sided PCB. I can only fit 82 caps on a 6x9" board. =14.7uF

This is definitly going to be a challange...

Re: OLTC project
Jrz126, Wed Aug 16 2006, 06:28PM

I think I might try to use some of these to mount my capacitors. The plug will be mounted on the PCB so that it is only conducting on one side of the PCB. Then I'll stick the leads of the capacitors (2 caps, one on each side of the board) into it and solder them in.

It'll be pretty complicated and a PITA since I will be trying to cram as many caps as possible onto this board. But its the best idea I can come up with at the moment.

Will the stray inductance have a significant affect on the output of my TC? It'll be alot of work to build this low inductance cap bank, and if I'll only lose 3-4" of spark from building a simpler design, then I'm ok with that.

Also, Is it possible to operate a microcontroller near a tesla coil as long as it is well sheilded? I'm thinking about building a microcontroller based SCR powersupply, and I'd hate to go through all of that work and have it screw up once I start making streamers.

Re: OLTC project
Marko, Wed Aug 16 2006, 06:44PM

Will the stray inductance have a significant affect on the output of my TC? It'll be alot of work to build this low inductance cap bank, and if I'll only lose 3-4" of spark from building a simpler design, then I'm ok with that.

It's not about the output, but rather your ability to put most of inductance you have into the primary.

Using2 primary turns would cut your surge current in half, reducing losses havily and allowing you to use lower coupling.

Also, Is it possible to operate a microcontroller near a tesla coil as long as it is well sheilded? I'm thinking about building a microcontroller based SCR powersupply, and I'd hate to go through all of that work and have it screw up once I start making streamers.

You can build a 'solid state variac' easily from 2 SCR's adn few other components:

Re: OLTC project
Steve Ward, Wed Aug 16 2006, 11:17PM

Provided the uC is electrically isolated and in a shielded box, it might just work . Id really suggest just finding some other controller IC thats made for what you want to do. uCs and tesla coils together just dont give me a good feeling.

As to the inductance in the cap bank. Basically, it will add to your primary inductance. This normally doesnt matter, but if you only have 1 primary turn to start with, your leakage inductance could be a good % of the primary inductance. What this does is lowers the overall coupling coefficient to the secondary coil. You might be able to make up for that with raising the primary, but at some point, the coils will arc over. If arc over is your limitation, then to get the maximum coupling, you need to have the least leakage inductance in circuit as possible.

I think if you stick with the idea of a double sided PC board to construct the MMC, you will do just fine.

Re: OLTC project
Jrz126, Thu Aug 17 2006, 07:19PM

Well the parts are ordered for the cap bank. Hopefully it wont be too time consuming. I think I might go with a 2-turn primary so I can get away with less caps.

Where can I get/make some high current inductors for currentlimiting? Whats a good inductance to shoot for?

I'm testing my SCR rectifier using a 19Vrms AC input from a transformer as my AC input. I'm getting ~34 volts out. When I try to connect up my oscope probe, the output drops to ~10-15 volts. I checked the voltage between my probe ground and the SCR power supply with my DMM and its reading ~30 volts.
anyone know whats going on here?

Re: OLTC project
Jrz126, Thu Aug 24 2006, 12:19PM

I recieved my banana plugs from the ebay auction a couple days ago. I was kinda surprised by the size of the box.
It turns out that they are individually packaged.
Yep, there were a total 479 in there (should have been 500).

I think I might be able to get 120 caps (there's 200 in that box) onto a 6x9" board .

I'll try to post some more tonight.

Re: OLTC project
Marko, Thu Aug 24 2006, 12:27PM

Lol for bannana plugs

Why don't you just solder the caps to piece of copper clad boaqrd or prefboard?

I don't get why you nees a ton of bananna plugs (exscept for increasing overall inductance)?

Re: OLTC project
Jrz126, Thu Aug 24 2006, 01:09PM

Thats the plan. I'm going to have 60 on each side of the board. The problem is trying to attach the lead that doesn't go through the board. This is where the banana plugs come in. With the banana plugs and some plastic insulators, I'll be able to have the lead stick through, but it will still be attached to the front of the board.
I dunno if that makes sense but I'll try have some pics up later tonight or tomorrow.

Oh, and I'll only need 60 of the plugs, but the auction was for 500. I only paid $26 for them.

Re: OLTC project
Jrz126, Fri Sept 01 2006, 07:01PM

I started soldering the cap bank yesterday, its going surprisingly well considering I'm attaching 120 caps onto a 6"x9" pcb. Hopefully I dont regret not hi-pot testing the bare pcb...

My girlfriend and I were heading out to get something to eat and we drove past this huge microwave sitting at someone's curb. I went back later and picked it up. I'm not sure of the wattage, but it has a huge transformer in it. Is there anything special that I have to do to use it for my OLTC? or even a link to how the transformer operates would be helpful. I dont quite understand how it works.

Re: OLTC project
Steve Conner, Sun Sept 10 2006, 11:32PM

Hi guys

I found some high resolution pictures of the cap bank on my old OLTC2 coil:

Re: OLTC project
Marko, Mon Sept 11 2006, 12:32PM

I always liked steve's pics..

And flat-cable for low L gate drive idea, hmmmmm

My girlfriend and I were heading out to get something to eat and we drove past this huge microwave sitting at someone's curb. I went back later and picked it up. I'm not sure of the wattage, but it has a huge transformer in it. Is there anything special that I have to do to use it for my OLTC? or even a link to how the transformer operates would be helpful. I dont quite understand how it works.

You have lots about mot's in aarchives adn around web.

Problem is that voltage is going to be somewhat too high for your use.

I would stay with 900V multiplier and resonant charger for now..

Re: OLTC project
Jrz126, Fri Oct 06 2006, 01:42PM

Havent really had much time to work on it lately. Senior year is really kicking my butt at the moment.

I've got Steve's circuit all soldered up and I'm currently testing it.

I'm going to redesign a 2-turn primary using some copper flashing. I was able to pick up a roll of 10" x 20 ft. How high should I make it? I was thinking of going with 5" or 2.5". I'm going to use some of that weatherstip foam stuff (.5" thick, adheasive on the one side) to insulate and support the foil. Anything else that I need to consider in my design?

Also, I figure I'm going to have a Fres of 22-27khz once I get my toploads on there. (A 4"x24" to proect the top and then a 4"x36" for the streamers). Is there a problem with a very low Fres?

Hopefully I can finally get this thing running before it starts to snow...

Re: OLTC project
Steve Ward, Fri Oct 06 2006, 04:25PM

For the primary i would probably go with 5" wide strip, just to keep the resistance down. The primary current is likely to bunch near the top of the ribbon due to higher coupling with the secondary, but all of the strip width will still contribute something beneficial. Also, beware that inductance calculations for using really wide flat conductors is somewhat tricky. Anyway, the inductance is smaller than that of a coil made with a dimensionless strip width. I picture the coil to really be composed of infinitely many coils, stacked upon each other. Since each coil has a coupling of slightly less than 1 with respect to the others, the inductance goes down (parallel inductors). Anyway, you are gonna have to fine tune the machine anyway, so just take a best guess at the primary size, and tune it by adding/subtracting tank caps.

There shouldnt be any problems/anomalies with the low operating frequency. Luckily, its not audible for humans.

Re: OLTC project
Jrz126, Wed Oct 18 2006, 05:28PM

Well now that the week from hell (mid-terms, career fairs, etc) is over, I might have some time to work on it this weekend.

How much distance should I put between my primary and secondary? I was thinking about using an inch or 2. (the secondary is 12 3/8" in diameter).

Do I need to have the ends of secondary air tight? I have a couple of MDF disks attached to the ends, but I didnt put any sort of sealer over them.

Is there an alternative to the black drain pipe stuff for the topload construction? I'm having some trouble getting a 4" diameter piece into a 24" circle.

some pretty basic questions, but I figure it would be better to ask and get it right the first time.

Re: OLTC project
Steve Ward, Wed Oct 18 2006, 06:53PM

I sealed my sonotube coil just to keep moisture out, but maybe make sure its dry before doing this so you dont seal too much moisture in. It probably doesnt matter too much though.

Im not sure on what spacing the primary should be, i would probably guess about 3" or so.

About the black drain pipe, it shouldnt be too hard to form. Make sure you have the right kind! They make rigid black drain pipe, and then the flexible stuff. I have regularly made 16" diameter toroids from the 4" stuff, so 24" should be relatively easy to form.

Re: OLTC project
Jrz126, Fri Nov 03 2006, 01:18AM

Finally got around to posting some pics of the cap bank...
here
closeup autofocus > my camera

The coil is starting to take shape as well...
coil

edit:
I was going to add more info last night, but a vhdl assignment was kicking my butt.
Anyhoo...
For my small toroid, how "tight" does it have to be to the top turns of the secondary to protect the top turns?
Is there a better method for attaching the aluminum foil to the toroid? My hot glue gun isnt quite up to the task, so I was wondering if there might be a spray-on adheasive that would be suitable?
Finally, what method do you use to hold the drain pipe in a circular shape? I'm having alittle trouble shaping the smaller toroid.

For The cap bank, I think I went alittle overboard on the spacings...hopefully they wont generate too much heat

I've come up with a possible design for the primary, so I think I might duck out of work early to go work on it.

Re: OLTC project
Jrz126, Mon Nov 06 2006, 05:58PM

I finished the primary...
wood form
Inner tap 1
inner tap 2
I'm going to attach another piece of copper to this and have it run to the cap bank. I'm planning on just using a block of wood and some screws to hold it together.

the finished primary

???? I dont know how I convinced the girlfriend to do that...

The smaller secondary
The string was just temporary. I ran some drywall screws in through the pipe to secure it.
taped up
I'm currently attaching the foil. Hopefully I will have the foil finished up pretty soon. I was showing this to my professor the other day and he mentioned that I can borrow a frequency generator from school for tuning.

Would these be suitable for pulse transformer use? I'm going to try to order some sample inductors from them for a high power LED driver. Maybe get a couple of these while im at it.

Hopefully I will be making sparks soon

Re: OLTC project
Steve Conner, Tue Nov 07 2006, 10:44AM

The Coilcraft SD250-1 ought to do. It's not really that critical: I also got it to work with a common mode suppressor choke from a SMPS. The insulation between windings just needs to be good. And you probably want to dremel away that bit of the groundplane to get some more safety clearance (if you haven't already)

Re: OLTC project
Jrz126, Fri Dec 29 2006, 02:31PM

Well last semester turned out to be the semester from hell, but atleast its over. I was too busy working on an inverted pendulum for a final project. here is a video of it finally working.
Most of the time was spent trying to get the H-bridge circuit to work. here is the breadboard. and here's the breadboard after some mosfet failure with no current limiting.

Right before i quit to work on the pendulum, I was able to get some 1" sparks from it using a battery charger for my supply. I set the break rate to very low and all of the florecent lights in the garage would flicker.

Here's a pic of what it looks like now.

I might try to get some sparks out of it before school starts again, but it might have to wait until spring time. I might try to incorporate it into a school project

Re: OLTC project
Steve Conner, Fri Dec 29 2006, 03:11PM

That coil is gigantic I'm looking forward to seeing what it does!

Re: OLTC project
Marko, Fri Dec 29 2006, 04:45PM

O__O Cant wait to see it fired up... and see if it outsparks Steve's OLTC and becomes largest in the world...

Re: OLTC project
Jrz126, Fri Jan 05 2007, 06:12PM

I've been thinking about the work remaining and I have a few questions.

First off, I need to make a strike rail. My plan is to use some 1/4" copper tube and mount it acouple inches up on the wooden form of the primary. Do I need a second strike rail on the outer side of the primary?

The powersupply will consist of a basic voltage doubler and Variac. I'm still trying to locate a suitable inductor to use for the choke. Any suggestions on where to look?

I'm going to solve my pulse transformer problem by switching to fiber optic. I'd have to buy some shielded cable and whatnot for the pulse transformers anyways, so might as well go for the fiber optic.

Another concern that I have is the coating of polyurethane on the secondary. I'm thinking that I might not have put enough on. Anyone have a problem from not coating the secondary enough?

Re: OLTC project
Marko, Fri Jan 05 2007, 06:30PM

I always used single coat of poly ant it always worked well. (although all were much smaller secondaries than this one )

Some use epoxy, also a single coat.

I think two strike rails are good idea. Steve did exactly same on his oltc

Re: OLTC project
Steve Ward, Fri Jan 05 2007, 06:45PM

First off, I need to make a strike rail. My plan is to use some 1/4" copper tube and mount it acouple inches up on the wooden form of the primary. Do I need a second strike rail on the outer side of the primary?

I would say that you will want more than 1 strike rail, in fact, you may want more than 2. The output sparks of the tesla coil tend to follow a general path, arcing outward from the toroid, then bending back in near the base of the coil (where the primary is). So, assuming that sparks are coming in horizontally at your primary, you need to be effective at catching them before they hit the primary. Id say put one strike ring about 1" higher than the primary, but add about 3" of radius. Then do another strike ring at say 2/3 the height of the primary, but make its radius maybe 5" greater than the primary. Then maybe finally, a strike ring mounted right to your table at an even greater radius. They dont have to be copper rings... any metal will be good enough (so i dont suggest spending all that money unless you want copper for some reason). Even really small wire will work fine here, minding structural integrity.

I'm still trying to locate a suitable inductor to use for the choke. Any suggestions on where to look?

Do you have any idea what inductance you need? Pspice would probably help you figure that out since it doesnt seem (at first thought) like a nice simple problem to optimize by hand. Im thinking you could likely modify a MOT in some manner, probably seperating the E and I sections with some sort of gap (to increase the current it can handle before saturation, and to reduce the inductance). I think that any inductor you would use will have an air gap for energy storage (someone correct me if im wrong) so most of what you might find will need modification of the core.

Anyone have a problem from not coating the secondary enough?

Normally this isnt a problem unless you have problems with sparking between primary and secondary, or the secondary arcing back onto itself. The thick layer of poly doesnt *prevent* this, but can sometimes save the secondary from much damage. My large DRSSTC has seen a fair share of really nasty arcs between the coils, with no visible damage to the winding. I think i used something like 6-8 coats of poly varnish (it was the whole 1 quart can). So i wouldnt say its a necessity, but it could prevent you needing to do some sanding and refinishing if something bad happens.

Re: OLTC project
Jrz126, Fri Jan 05 2007, 10:09PM

Do you have any idea what inductance you need? Pspice would probably help you figure that out since it doesnt seem (at first thought) like a nice simple problem to optimize by hand. Im thinking you could likely modify a MOT in some manner, probably seperating the E and I sections with some sort of gap (to increase the current it can handle before saturation, and to reduce the inductance). I think that any inductor you would use will have an air gap for energy storage (someone correct me if im wrong) so most of what you might find will need modification of the core.

hmm...I've kinda stayed away from Pspice. Not quite sure how to simulate it, and also not quite sure what I would be looking for in the output. I guess its about time to try to figure it out. So...anyone have some Pspice models that I can use?
I have a couple MOTs lying around that I can 'modify'. Now do I need to rewind it? I'm thinking I can just remove the HV winding and use the primary winding as is. Its 18 gauge wire BTW.

Re: OLTC project
Finn Hammer, Sat Jan 06 2007, 12:35AM


I`ve used (more or less modified) mot`s for the OLTC`s and OL-BRISG`s that I have made, and have microsim models that will predict a good size.
If you tell me the size of the primary cap, as well as the bus voltage, I`l be happy to run the numbers for you.

Cheers, Finn Hammer

Re: OLTC project
Jrz126, Sat Jan 06 2007, 01:56AM

Well I got the MOT apart. The angle grinder made quick work out of it. I also welded a bead across each side to prevent the laminations from coming apart. What frequency should I try to measure the L at?

My primary cap is 12.92uF, and it'll be running off ~600V. Maybe alittle less, my diodes are only rated for 1200V each.

Well I have to say that I am quite modivated to get this thing working now. I figured I'd be up to my waist in snow by now, so I really wouldnt be able to test it too much in the garage. But the weather is 50*F+ and sunny, so I might as well try it.

Re: OLTC project
Steve Conner, Sat Jan 06 2007, 12:03PM

The inductor value isn't too critical. I just took an MOT, sawed the core apart, got rid of the HV winding, clamped the core back together with a 1/8" piece of plexi to set the airgap, and used that. Of course our MOTs are 240V and yours are 120, so you'd end up with one quarter of the inductance. Maybe you could fit two MOT primaries onto the same core, or use a couple of units in series? Or maybe it would work with 1/4 of the inductance, I don't know.

Re: OLTC project
Finn Hammer, Sat Jan 06 2007, 12:38PM


The charging inductor size has to be big enough to keep the current rise during ringdown within a reasonable level. The de-Qing diode is a limiting factor here. And it should be small enough to allow the capacitor charging current to drop to zero, before the next Bang.
If it is too big for that, the voltage rise will increase beyond the 2x bus voltage that we ordinarily assume.
Anything over a 100mH choke with 600V buss and 12.92uF cap, the voltage of the cap will increase to over 1500 volts after the first 15 bangs.
To get a predictable voltage across the cap, the inductor should be maximum 25mH for maximum 500BPS.

Another factor that determines the final voltage across the cap, is the ringdown time, or quench time.

If the coil is set to quench at less than 100µS, then the final voltage stays pretty much at 2x the bus voltage, but longer quench times start to have an impact on the final voltage.

If we still assume your 12.92uF cap, 600 Vbuss, 25mH choke and 500BPS, then the final voltage on the cap, and the peak charging current, varies like this with quench time:

100µS.....1199V.....13.7A

200µS.....1220V.... .14.2A

300µS.....1257V.....14.9A

400µS.....130 6V.....16.1A

500µS.....1336V.....17.5A

So you see that things start to get funky, and again, with Tesla cols, there is that little extra "twist" to already established theory, which makes it fun to work with.

What this all boils down to, is, that you really owe it to your self to build a crowbar circuit, so that you don`t send all your igbt`s to the overvoltage failure bin.
It is your guess how I have come to this conclusion .

To measure your choke, apply 50(60) hz across it with a variac, measure voltage across it, and monitor current trough it.

Then u = wLi

I hope this helps.

Cheers, Finn Hammer

Re: OLTC project
Jrz126, Mon Jan 08 2007, 11:28PM

Thanks for the help Finn. I'm definitly gonna use a crowbar ckt. Which wont be hard at all since I'm using Steve Conner's OLTC2 circuit board.

I did some testing over the weekend and I was able to make some decent progress. Mostly because Steve Conner was in the chatroom and I was able to get some realtime advice.

The problem I am running into is that I'm not able to see the first notch in the primary.
http://www.personal.psu.edu/jrz126/images/oltc/no_notch_1.JPG
The top trace is just a probe hanging 5' away. the bottom trace is across the primary coil. The bottom probe is set to 10X.
Any ideas why I'm not seeing the first notch?
I tried decreasing the coupling by moving the secondary up about an inch. but I didnt see any improvent.
I also removed and added a bunch of caps, and I could see some increase in output as I remove the caps.

Heres a few pics:
The 'powersupply' that I am using:
http://www.personal.psu.edu/jrz126/images/oltc/powersupply.JPG
I'm driving the transformer with my Variac so that my function generator and Oscope dont ground out the supply.

The modified MOT:
http://www.personal.psu.edu/jrz126/images/oltc/modified_mot.JPG

The Cap bank and rogowski coil:
http://www.personal.psu.edu/jrz126/images/oltc/cap_bank_rogowski.JPG
It's not completely finished. Hopefully the stray inductance isnt killing the performance?

Finally, A video of the small sparks I'm getting now:
http://www.personal.psu.edu/jrz126/images/oltc/TC_and_pendulum%20059.mpg

Re: OLTC project
Steve Conner, Tue Jan 09 2007, 10:21AM

Hmm, from the look of the scope traces, your coupling is too tight and you're out of tune. The IGBT and capbank assembly will have considerable stray inductance due to the way you laid it out. The loop area between caps and that big thick return wire is pretty huge, and that is probably what makes the tuning be out. You could try moving the return wire round the other side of the cap bank.

Also, you ought to check that the zero current turn-off thingie is working well before you crank the voltage up. I can see quite a spike when it turns off. It should be obvious when it starts working: the Rogowski coil LED will change state, and the gate drive pulse will suddenly get shorter, and will "snap" to the nearest negative half-cycle. As you adjust the hold-on time pot, it will jump along in increments of one cycle. It should never turn off partway through a cycle.

Of course, if you put the Rogowski coil on backwards, it turns off at exactly the WRONG time for maximum spikage So better check that. I always just tried it both ways round while using a second Rogowski coil to look at the primary current and check that I got it right. The turnoff should always happen during a negative half-cycle, that is to say, one with the opposite polarity to the first half-cycle you see.

The capacitor C28 in the Rogowski coil circuit might need made bigger, since your coil is lower frequency than anything I ever tried, and that cap has to integrate the Rogowski coil signal (they naturally measure di/dt)

Glad to see someone is using my driver board btw! It was designed for a commercial installation, so it's a bit more complex than it needs to be for a hobbyist project. Good luck with it!

The schematic:
A page describing how I tested my primary circuit:
Some more pics I found while looking for the schematic

Re: OLTC project
Jrz126, Tue Jan 09 2007, 01:29PM


I was talking to Steve Ward last night on AIM. He also mentioned the tuning. So I went and removed 8 more caps and I started to see the notches. I couldnt fire it up because everyone was sleeping. I'm down to 9.4 uF and still not in tune And I cant run the wire around the back side because there are 60 more (unused now) caps back there.

I think I might try to go down to a single turn primary, but I'm gonna remove some more caps and see if I can get it to throw some bigger sparks.

The rogowski coil is going to need some debugging. I think I may have swapped a couple resistors. I will also try changing C28.

Well I better get to work, I'm planning on ducking out alittle early though.

Re: OLTC project
Finn Hammer, Tue Jan 09 2007, 07:01PM

Your coupling is up towards 0.28 to 0.3. You should raise the secondary up to the point where the bottom turn is level with the top edge of the primary.
Later you can lower it a bit.
The tuning is off, and may well be off by what amounts to 1/3 the primary caps.
This is a suggestion based on simulating and trying to get similar waveforms as yours.

With a 2 turn primary, I`d think you have just enough Q to resonate the circuit up with a function generator across the cap. The IGBT has to be shorted out with a soldered connection, or perhaps just turned on permanently.
However, in a situation like this, solid evidence supplied by a measurement is the way to go.
Measure your primary and secondary resonant frequency ASAP, and you will have your answer.
Building low inductance caps is not at all easy. But an OLTC can produce great sparks even quite far out of tune.

Cheers, Finn Hammer

Re: OLTC project
Jrz126, Tue Jan 09 2007, 10:16PM

I got the rogowski coil circuit to trigger! It was a bad solder joint. Its turning off when the voltage across the primary is near the negative peak. does that mean the current is 0 at that point?

Finn,
Thanks for the info, what program are you using for the simulation?
You were just about dead on with the 1/3rd extra capacitance. I'm down to 7.5 uF and I made that measurement when I was at about 10uF.
I'm seeing the first notch after 2.5 cycles.

Still no improvement in the arc length though

What should I try next?

Re: OLTC project
Steve Conner, Tue Jan 09 2007, 10:32PM

The peaks in primary voltage are the zeros of primary current (V=L*di/dt and all that). A negative peak means that the primary current is passing through zero, changing from positive to negative. IOW, the current just finished flowing in the IGBT and is going to spend a half cycle going through the inverse diode. This is just the right time to turn off.

That assumes that the voltage probe is wired up correctly such that the first quarter cycle of primary voltage was positive.

The next step is to adjust the timing so the IGBT turns off at or immediately after the notch. Then the setup is complete and you can crank the voltage higher. Probably best not to crank it too high until you've installed the crowbar circuit, though. Note it needs a high voltage resistor which isn't installed on the board for safety reasons. A 1M resistor gives a trigger voltage of about 1kV.

Re: OLTC project
Jrz126, Wed Jan 10 2007, 01:11PM

Well I tried going alittle higher on the charging voltage and I was able to gain some arc length (up to ~5" now). I also started causing some interference on the TV too.

Should I try to switch over to a 1-turn primary so I can get my tank cap back up?

EDIT:::

I decided to switch to a 1 turn primary and I was able to get it to notch much better than before.
http://www.personal.psu.edu/jrz126/images/oltc/single_turn_tuning_4.JPG
I am able to get ~1' arcs now with a DC bus voltage of about 180V (~400V after the choke) and 60 bps.

Steve Ward mentioned that many people have blown their garage door openers. Does this happen from a direct strike to the opener itself? or a strike to anything the opener is connected to? What should I do to make sure I dont kill my opener?

Woot Fed-ex just dropped off my fiber optic supplies. Time to get back to work.

Re: OLTC project
Jrz126, Fri Jan 12 2007, 02:28PM

Time for some pictures:
Me and the coil (I'm still trying to come up with a clever name for it)


The cap bank and rogowski coil. I'm gonna try to minimize the stray inductance sometime soon


The work area where I did the low voltage testing


My view of the coil. (yes, it is quite scary)


Sparks!


Still no breakout to air, I'm trying to debug the crowbar circuit. It trips every time I draw an arc. I think I just need some sheilding. (scared the crap out of me when it tripped the first time, not used to everything just going silent all of a sudden)
The strike rails are about 90% complete. Just need to add another half turn and connect them to ground.
Once I get those 2 things fixed, I should be able to give it some power. If I'm not too scared
More pics will be posted soon.

Re: OLTC project
Finn Hammer, Fri Jan 12 2007, 06:39PM


You might consider calling it "Cohiba" because the primary sits on it like the label on a cuban cigar.
Bear with me, I`m just trying to subtly suggest that you raise the secondary up. You will get a much smoother running coil that way.
Always start out with low coupling.

Cheers, Finn Hammer

Re: OLTC project
Dr. Drone, Sat Jan 13 2007, 02:21AM

Re: OLTC project
Jrz126, Sat Jan 13 2007, 01:43PM

Ok, I moved it up a few inches so the bottom turns are slightly above the primary.

Turns out my crowbar was tripping because I was probing that signal. I also switched over to some shielded cable.

I cranked it up to 480V on the bus and was able to get some 22" sparks to my chicken stick. Unfortunitly, I'm maxed out there, so I need to rework my powersupply

Re: OLTC project
Jrz126, Tue Jan 16 2007, 06:22PM

omg Updatez:
Here's a pic of the 22" sparks. I like the dimly lit flourescent light in the background
http://www.personal.psu.edu/jrz126/images/oltc/22inch.JPG
I built my power supply into a wooden box. I'd say it cleaned up rather nicely.
the old supply
http://www.personal.psu.edu/jrz126/images/oltc/powersupply.JPG
http://www.personal.psu.edu/jrz126/images/oltc/new_supply_insides.JPG
http://www.personal.psu.edu/jrz126/images/oltc/new_supply_installed.JPG
I also replaced that 2 gauge wire with some copper sheet. I added an additional 5.4uF back into the tank circuit to get it back into tune. (I'm running all 120 caps at about 20.4uF now )
I havent done much testing though. When I first gave it some power after the cap upgrade, something arced from my function generator to ground, so I decided it was time to switch over to a fiber optic trigger and get my expensive equipment out of 'the kill zone' (the FG isn't even mine, I borrowed it from school about 4 months ago). My fiber optic reciever has a minimum operating frequency of about 100kHz, so I need to work some stuff out before i can test it further.

I got the crowbar upgraded to trip out at about 2330 V (it was 23,300V until I realized that my probe was set to 10X). Should I set this lower? my cap bank is rated at 2400V and my brick is 2500V.

Also, I think I had a flashover while testing the fiber optic circuit. I think the choke voltage was charged up to roughtly 300V before the trigger started working. When it did trigger, there was a pretty good sized bang at the base of the coil. It was on the back side of the coil so I couldnt see it. When it does flash over, would it tend to go to my strike rail? or the primary coil? I raised the secondary up so that the bottom turns were slightly above the primary before this happened.

Unfortunitly I am back to school now, so my free time is going to drop drastically . I'll still try to squeeze some work in here and there. I may even see if I'll be able to incorporate it into a project for my embedded systems class. Though I dont think the prof would let me bring it in to school

Oh yeah, whats the best way to photograph the coil? I'd like to get some nice shots of the smallish purple arcs to air.

Re: OLTC project
Jrz126, Mon Mar 19 2007, 01:37PM

Updates...
First off, I have a new pic of me with the coil... here. I really like this pic...I also took a copy of it to my interview with Lincoln Electric. Turns out the VP of sales there is a TC nut as well

I figured out how to use the PCB machine at school. I hope they dont mind if I use it for a few personal projects. (I dont really care if they do )
here are the PCBs. The two on the left are the SCR controller and the other one is the breakrate generator. They both will use a fiber optic link for remote control. I'll post the schematics once I make a few corrections.

I had it running on the SCR supply on saturday. I noticed the current meter was behaving alittle weird...I was getting a nice steady current (about 3A) until it started arcing. Once I was drawing an arc, the meter would bounce around randomly. sometimes getting up to 12-15A. Is this normal? I'm kinda thinking I'm getting some interference on my SCR controller and misfiring the SCRs?
Also, the output was kinda poor. Running off the 120Vac, I was able to get a max arc of like 15". I tried increasing the breakrate, but there was no increase in output. I'm still in tune too. Any ideas?

Well time for class

edit: Oh yeah I also updated my website with alot of pics and stuff.

Re: OLTC project
Jrz126, Tue Mar 27 2007, 04:35PM

Funny updates....

I was playing with the coil last night and was running off the measly 120V line and everything was working just fine. I wanted to try the 220V line since my dad was out of town, there would be little opposition. So I get it all wired up and I flip the big switch that feeds the 220V to the welder. All the lights in the garage immediatly go out, but no asploding of the SCR powersupply, and no tripping of the circuit breakers.
So I figure I popped the fuses in the breaker panel since they react quicker than the CBs. I go to the basement and remove the cover to the panel and realize that there are no fuses anymore. (my dad just upgraded to a 200A service, so no more fuses?). I also noticed that it is just one of the 120V lines that is down.
After about 10 minutes the power came back on its own. So I think I tripped something that was further upstream?

This morning my mom calls me at work and says that half the power is out again. I told her to call the electric company and report it. I guess she was the only one to report a problem. The power came back about 20 minutes after she called. About an hour later, one of the linemen shows up at the house.
About an hour later one of the linemen shows up at our house to take a look. The garage happend to be open, and he noticed the tesla coil (kinda hard to miss). So my mom told him about it. He said that maybe I was loading the line too much or something, I really wish i was there so I could explain it better.
My mom talked with him for about 15-20 minutes, and apparently I went to kindergarden with his son (about 17 years ago only my mom would remember something like that). He said that he would really like to see the coil running, and he also left me a pair of those super heavy duty insulated gloves that they use for the HV stuff . I havent seen them yet, but its a pretty sweet freebe for nearly knocking the neighborhood off the grid.
Looks like I have a friend in the electric company

Re: OLTC project
Terry Fritz, Wed Mar 28 2007, 07:46PM

Hi,

You may have a bad connection to one of the phases to your home. It might be blowing open and reconnecting depending on the weather or something. That happens a lot. See if any of the others on the same circuit as you in the neighborhood notice it. Power company breakers will drop "both" phases in all cases and never just one (unless something is really screwed up!). They will also drop power to the whole area. This sure sounds like a problem with the wiring to your house...

my dad just upgraded to a 200A service...

Hmmmmm... Did this problem just happen to start at that exact same time I would start looking for the problem at that point

If they did not tighten down one of the incoming wires to the new box.... Defective breaker, etc... Call the electrician back and have it rechecked...

Cool! coil you have there too!!! I have not studied it much but it looks great!!

Cheers,

Terry

Re: OLTC project
Jrz126, Thu Mar 29 2007, 03:17PM

You called it Terry. It was the feed at the junction box by the curb. The electric company had to come out 2 more times on tuesday...One guy just diagnosed the problem, then he called another crew to do the repair.

I was able to fire up the coil (off 120V) for the last crew too. They were pretty impressed with it.

Re: OLTC project
Jrz126, Mon Apr 30 2007, 05:17PM

Finally finished my senior design project so I had some free time for the coil...

I talked my prof into letting me use it for a final project in my embedded systems course, I had to attempt to audio modulate it. (He wasn't gonna let me do it until I showed him one of Steve Conner's modulated DRSSTC vids). Mine will sound nothing like it though since its just an OLTC. I made sure he was aware of that as well.

I just finished up the controller box for it.

I had some issues with it resetting, so I converted it to run off batteries, but havent had a chance to try it yet.

I was thinking though, there is about 10" wiring between the controls and the PCB in the box, would this extra wire cause some noise issues even though it is in my enclosure?

Re: OLTC project
Steve Ward, Mon Apr 30 2007, 05:36PM

Can you snap a picture of the inside of the box?

Also, was anything else (like the RCA cable) hooked up to the controller when it reset?

And the output is in fact fiber optic cable now, right?

How is your reset pin hooked up?

Where did you get that LCD by the way? Ive been looking around for a decent one and found a few on all electronics, but the cool ones are all EL backlight based (which is slightly annoying needing a little converter in there). Oh, and speaking of LCDs, im wondering if its possibly an EMI leak into the box, and maybe that is the issue? Next time try taping some foil over the screen completely so as to get rid of the large hole the LCD creates. Im guessing there is something that exists to shield displays like this, but is optically pretty transparent (seems so useful that it *must* exist, right?).

Very nice work on that box!

Re: OLTC project
Tom540, Mon Apr 30 2007, 06:19PM

Mouser and Digikey sell those lcds.

Re: OLTC project
Jrz126, Mon Apr 30 2007, 07:05PM

Mouser and Digikey do sell them, but I think they might charge an arm and a leg for them? I got mine from for 10 bucks. and that was because it was blue characters with the LED backlight.
They also have some great prices on other stuff too. Like those 0.1" connectors that are used on PC mobo's for the power switch, speaker etc.
Their shipping is kinda gay though, they wont notify you that your order has been put on hold for 4 weeks because a $0.50 part is out of stock.


Here's a kinda bad pic of the inside of the box:

I was powering the entire system off the wall outlet and I think thats what was killing my uC. I switched over to a 9V, but I still need the AC input so I can detect zero-crossings for my powersupply. I have that optically isolated.

I have my MCLR pin hooked to a switch so that I can either tie it to VCC directly while running the coil or have it pulled up with a 10k resistor for programming.

I'm gonna go give it a try now...

Re: OLTC project
Tom540, Tue May 01 2007, 10:20PM

I once bought an LCD just like that but it was a 4X20. I'm pretty sure it was pretty cheap too but I can't remember the place I got it from. They only sold LCD's. I know that's not much help.

Re: OLTC project
..., Tue May 01 2007, 10:39PM

Really ebay is the best place to get them, orders of magnitude cheaper than most real companies.

Re: OLTC project
Jrz126, Mon May 07 2007, 07:07PM

Well it turns out that the AC Zero-crossing input was causing my reset problems. I temporarily switched over to the variac and the controller works just fine.
I talked my prof. into letting me bring the coil in for a live demo. It was kinda scary transporting it there, but it made it in one piece. I did my presentation and then did the live demo. I surprised everyone by getting the audio to work at the last minute (I was up until 2AM programming the theme song to super mario brothers into it).

I have a short video of it here

I also tried burning some CDs too
After awhile my chicken stick caught on fire

I'm working on another SCR controller that will get rid of the ZC input to my controller now. Once I get that working, I should be able to plug into the 220V line and really light it up :)

Re: OLTC project
Brett Miller, Mon May 07 2007, 07:51PM

Jrz126,

Excellent! Nice sparks and great modulation effects. I look forward to seeing you power that huge OLTC up on 220v!

-Brett