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Beginnings of my first DRSSTC

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Shaun

Tue May 15 2007, 02:27AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

In the thread immediately before this one "Powering up an SGTC" I posted at the end that instead of investing in new parts to make my SGTC moke powerful, I decided to take the step up to a solid-state DRSSTC. Well, just now I was browsing eBay to see approximately what this undertaking would cost me, and I saw for about $22 5200' (a mile!) of 32ga magnet wire, no bidders. End time: 7 minutes. If anyone has ever been in this situation before, you know the mentality. Of course I had to get it, so now I pretty much have to build it; I can't bear to waste money.

My major goal will be durability over all other factors (within reason). I would like to be able to run it continuously for many minutes (15+) without worrying about overhating of anything.

Even though TeslaMap is for SGTCs, I still can use it to calculate specs for various parts.
Here's what I came up with:
Secondary- 2500 turns 32awg, 22" winding length on 4" thin wall PVC
Primary- Not sure yet; is copper tube advantageous over heavy gauge stranded copper wire (which I have excess of)?
Switching- H-bridge, I'll get some nice Bricks from digikey, I want to stress everything the least amount possible.
Driver- Something well built and widely know to work well- Probably the Steve Ward driver.
Toroid- A big one, window screen method.

I know the policy here is to not have other members do your homework. I have spent many hours searching through DRSSTC design guides, a favorite is this one. But I still have a few unanswered questions, which I feel are important.

What sort of power supply do I need, and how many watts should be use for a design this size?
IS A HELICAL COIL ADVANTAGEOUS OVER A CONICAL COIL??

And finally, I have heard that you can use the mains ground as an RF ground if properly connected thru a rugged low-pass filter. Is this true, or BS? Just because the added protability would be great.

If any of these questions sound obvious or noob-ish, forgive me, this is my first Solid State coil, second overall. However, I have extensive experience with low voltage semiconductor circuits, and I've seen the driver schematics and feel like they are well within my reach.

Any help would be greatlyappreciated.

-Shaun

Steve Ward

Tue May 15 2007, 02:40AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

What sort of power supply do I need, and how many watts should be use for a design this size?

A pretty typical power supply is just a bridge rectifier and some large electrolytic capacitors. If running on 120V mains and using 600V silicon (most common scenario), you can do a voltage doubler (im sure there is a schematic for one somewhere on my site, if not let me know). Otherwise, if 240V is available, just a full wave rectifier into the caps. For something in the 4-6 foot spark range, id suggest about 400J of storage caps.

IS A HELICAL COIL ADVANTAGEOUS OVER A CONICAL COIL??

No, not really. In fact, you can probably get by fine with a flat primary if you can get the coupling up to about .18 or so.

And finally, I have heard that you can use the mains ground as an RF ground if properly connected thru a rugged low-pass filter. Is this true, or BS? Just because the added protability would be great.

Well, i used mains ground on my coils up to 4 foot sparks, but after that it seemed pretty dangerous. In fact, on my DRSSTC 1, when producing upward of 5 foot sparks, i saw the RF ground ARC to the mains wiring on my terminal strip! This was when i was using an RF ground point in my backyard (which was likely better than my house ground setup). So just be cautious.

Shaun

Tue May 15 2007, 03:01AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

I already have an 8 foot copper ground rod in my lawn from the SGTC; it's good to know this will be suitable if needed. But I fully intend to try the mains ground as well (after unplugging everything else on the circuit)!

Also, when you reference a coupling of .18, is there an easy way to measure this? or is it usually calculated?
I remember reading an article on the coupling of 30 degree inverted conical versus flat spiral, and that a conical does couple better, but more is not always better with DRSSTCs, from waht I recall.

Steve Ward

Tue May 15 2007, 03:56AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

For DRSSTCs (and i think this actually goes for ALL TCs) is that the higher coupling produces better results, that is until insulation breakdown between the two coils.

I use JAVATC to do my calculations, and the .18 figure is based upon that program's predictions. Conical and helical primaries become more attractive in that they may be easier to shield from the sparks than a flat spiral winding.

Oh, and id suggest a tank capacitor around .3 to .4 uF at 6kV. That should allow you to figure out your primary.

Shaun

Tue May 15 2007, 05:18PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Funny coincidence, I actually have a pair of 10KVDC .5uf caps (Plastic Capacitors LK 100 504A). One should be good for this application, good thing I have two cause I don't know if they're pulse rated, and it could easily blow. The capacitance is out of the range you gave me, but I figure If anything I'll get hotter arcs. Of course, I could always series the two for .25 uf, but I'd rather be over the range than under it (and use only one cap!).

So here's what JavaTC turned out, nothing here is really set in stone yet except the secondary wire gauge. These are just specs I made up that would be easy for me to put together, and that get the k in the right range.

Units are inches.

----------------------------------------- -----------
Secondary Coil Inputs:
------------------------------------------ ----------
2.125 = Radius 1
2.125 = Radius 2
18 = Height 1
40 = Height 2
2500 = Turns
32 = Wire Awg

--------------------------------------------- -------
Primary Coil Inputs:
------------------------------------------ ----------
6 = Radius 1
6 = Radius 2
18 = Height 1
24 = Height 2
10 = Turns
0.375 = Wire Diameter
0.5 = Primary Cap (uF)
3 = Total Lead Length
0.162 = Lead Diameter

---------------------------------------- ------------
Top Load Inputs:
------------------------------------------ ----------
Toroid #1: minor=6, major=22, height=48, topload

----------------------------------------- -----------
Secondary Outputs:
----------------------------------------- -----------
83.13 kHz = Secondary Resonant Frequency
90 degÂ° = Angle of Secondary
22 inch = Length of Winding
113.6 inch = Turns Per Unit
0.00085 inch = Space Between Turns (edge to edge)
2781.6 ft = Length of Wire
5.18:1 = H/D Aspect Ratio
456.36 ohms = DC Resistance
60137 ohms = Forward Transfer Impedance
59766 ohms = Reactance at Resonance
0.53 lbs = Weight of Wire
114.423 mH = Les-Effective Series Inductance
108.457 mH = Lee-Equivalent Energy Inductance
120.54 mH = Ldc-Low Frequency Inductance
32.034 pF = Ces-Effective Shunt Capacitance
29.99 pF = Cee-Equivalent Energy Capacitance
44.791 pF = Cdc-Low Frequency Capacitance
10.08 mils = Skin Depth
25.262 pF = Topload Effective Capacitance
547.3 ohms = Effective AC Resistance
109 = Q

----------------------------------------------- -----
Primary Outputs:
----------------------------------------- -----------
40.15 kHz = Primary Resonant Frequency
51.7 % high = Percent Detuned
90 degÂ° = Angle of Primary
31.42 ft = Length of Wire
0.225 inch = Average spacing between turns (edge to edge)
3.875 inch = Primary to Secondary Clearance
31.366 ÂµH = Ldc-Low Frequency Inductance
0.11666 ÂµF = Cap size needed with Primary L (reference)
0.055 ÂµH = Lead Length Inductance
356.405 ÂµH = Lm-Mutual Inductance
0.183 k = Coupling Coefficient
5.46 = Number of half cycles for energy transfer at K
66.62 Âµs = Time for total energy transfer (ideal quench time)
-------------------------------------------- -------------------------------

THe only thing here that worries me is the 5.18:1 H/D ratio on the secondary. Is this acceptable, or should I move up to 6" pipe? Does it even matter that much?

btw I took my AP chemistry exam about 25 minutes ago. There's not a shred of doubt in my mind that I got a five :)

Brett Miller

Tue May 15 2007, 05:30PM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

Steve,

Since, like Shaun, I am working on my first DRSSTC, I had a question regarding the bridge electrolytics. I have two 2500uf 350v Cornell Dubilier electrolytics, which would probably work great for my half bridge doubler. However, they are huge and would cramp up my lower level guts or force me to go with a larger base for my coil. I also have 4 1000uf 200v electrolytics which I got off of ebay a few months ago when I thought I was going to design something smaller. Thinking about what would happen if I screwed up the OCD or failed to achieve ZCS (at severl hundred Khz) worried me into going with the 12 x 4 31 awg secondary.

I guess my question has to do with the lower limit of energy storage for an H or half bridge bus caps. Is there a method for calculating the minimum energy you need there? I would like to be able to use caps small enough to mount them right on the half-bridge pcb, but naturally the 2500uf ones would have to be mounted off the board and wired in, like on your DRSSTC-3 and Chris Hooper's DRSSTC-1. I've looked all over the popular coiling resources and can't seem to find any insight here...I suppose I could have looked at SMPS design web sites, but I have done this before and don't recall any formulas for determining this.

Sorry for barging in on your DRSSTC thread Shaun, but perhaps this will benefit you too.

-Brett

Steve Ward

Tue May 15 2007, 05:30PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Can you get a picture of your capacitor? I will say that a DRSSTC tank cap is a pretty hard life. The HV is not the bad part really, but the high currents. And, not so much the peak current (should be less than 1kA here for sure) but the RMS currents. The RMS currents tend to be rather high, id guess around 40-50 A for a coil with 4 foot sparks running 120BPS. Its the RMS current that causes the heating, and ultimate failure of the capacitor. So figure out if its worth the risk, or if you should just buy a good MMC bank.

The H/D ratio is a little extreme in my book, but not too crazy really. If you could get 6" pipe, then it might be worth it, otherwise id suggest just making your secondary a little shorter. Of course, it will likely work just fine as is. It depends on how long you want the sparks to be i suppose.

Congrats on the AP chem exam, i suppose that means no chem for you in college?

Brett Miller

Tue May 15 2007, 06:08PM

Registered Member #593 Joined: Tue Mar 20 2007, 12:32AM
Location:
Posts: 50

Steve and Shaun,

Ok, it is discussed in the tips for constructing the bridge inverter in Steve's design guide. I guess I have been focussing so much on reading and re-reading the driver theory section, that I had not perused the bridge tips in a while.

Size the filter caps so that they store 20-50x the intended bang energy. Thanks again Steve.

-Brett

Shaun

Tue May 15 2007, 08:18PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Wow, another weird coincidence. I have some lytics very similar to Brett's that I also got off Ebay, from when I was attempting a coilgun. I was hoping to use them in the doubler for the DRSSTC. THe attached pic shows the two types of lytics, I have 4 each of the ones on the left and right. They are Mallorys, which I'm pretty sure is a division of Cornell Dublier. The center one is the one I mentioned in an earlier post as a possible primary tank cap.

Any comments?

(BTW Brett, feel free to barge in as much as you need to, I'm in no rush)

And Steve, I think I will go to a 6 inch secondary based on your advice. The only reason I started at 4" was because I have an 8 foot section of it in my garage, left over from my first coil.

Edit: Haha...it looks like there's a tiny coil of wire sitting on top of the left cap, but its a 2 pound spool of 24 ga.
Reminds me of how my friends and I used to take pictures of people standing in someone else's hand.

Steve Ward

Tue May 15 2007, 08:40PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Brett: the 20-50X bang energy is a very broad range, that seemed to work well for all of my coils. If you make the lytics too small, they likely arent going to be rated for enough RMS current, and will overheat from use. I also like a good bit of energy storage so that the supply doesnt sag too much during the bang. Of course, lots of storage leads to more damage in the event of a failure, but i dont think it really matters much since its just a matter of how loud the IGBT will pop :P.

Shaun, that cap you show is definately not going to work. Its a DC filtering cap (im almost positive of this), and would probably be ok at 60hz AC, but surely not RF work and not at this current level. From what ive seen, for a DRSSTC, the MMC can be made from metalized film caps (rather than film foil) to save some money. The reason for this is, that you will generally have a few parallel strings of caps, and its the RMS currents that stress the cap bank. Metalized caps RMS rating is better per $ than foil caps. In either case, your MMC will usually well exceed the peak current requirement.

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