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Flyback based SGTC, help greatly appreciated.

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brtaman

Sun Jun 07 2009, 09:08PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hello 4hv members,

I have been a long (very long) time lurker on these forums and decided it is about time I join these forums. Well my main motive for joining is that I was hoping you guys could assist me in building my first time tesla coil, which has been in the making (theoretical :)) for quite a while now.

I am quite well experienced with high voltage completely understand the immense danger associated with them, an example is the fact that I treat large capacitors with the same respect I would give to a hand grenade. (principle of operation isn't too far off if used incorrectly.) I am no Electrical Engineer, but I do pick up things quite quickly, so you will only have to explain once. :)

As already mentioned, this is my first tesla coil and for this reason it will be small in size and power. Here are some preliminary specs of the build.

-Flyback based (relatively large flyback, scavenged from a Phllips 17 inch PC Monitor)
-MMC (This is where I need most of the help)
-secondary coil: 1inch x 4.5 inch polypropylene (apparently one of the best materials for secondary coil)
-Primary coil: 12 turn 10AWG? (Probably what do you guys think?) spacing around 1cm ?
-static spark gap (multiple) will be using a couple of PC fans to move as much air as physically possible.

What do you guys think of the specs, remember this is my first of probably many TCs and I want to keep it nice and simple. I have been doing a lot of reading on Richie's Tesla coil site which is a great place for info!

Ok so now onto my questions:

Since Flybacks generally operate at very high voltages compared to NSTs MOTs etc., my MMC calculations are off the charts!

I am thinking of using a MMC built out of these caps, I can get other capacities (of same model) based on your recommendations:

FKP 1 pulse capacitor

EPS

Dimensions: (L x W x H) 31.5 x 13 x 24 mm
Flank Steepness: 11000 V/Âµs
Nominal voltage: 2000 V/DC/700 V/AC
Tolerance: Â±10 %
Reliability: Î»02fit
tan: 3 x 10-4/1 kHz
Test class to IEC: 55/100/56
Electrical Capacity: 0.015 ÂµF
Dielectric: Metallfolie und doppelseitig metallisierte Polypropylenfolie
PITCH: 27.5 mm
Temperature range: -55 - +100 Â°C
RoHS compliant: yes

How many of these do you think I would need in series and parallel for my tesla coil...(I know it is a best guess atm, as I have to calculate resonance etc.), but my main concern is blowing them due to the flybacks incredibly high voltage. ( ex. 50kv(?)...at least 35 in series, ouch my walled hurts already, say it aint so :I).

Or if you have any other recommendation on good but cheap to use on my coil, I would be most grateful.

What sort of choke would you guys recommend, should I hand wind it, should I install one at all?

Since I am in Europe, I am having quite a hard time finding a nice NST, which I think is a much better choice for the TC? Anyone know a guy who knows a guy, that might have a 230v high powered NST on his hands who lives in Europe?

Thanks a lot! This is it for now, I am sure I will have many more questions throughout the build. :) (hopefully not, I try and read as much as possible before asking questions.)

Thanks
brtaman

Plasma Lover

Sun Jun 07 2009, 09:32PM

Registered Member #1911 Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165

Welcome to the forum, brtaman,

My advice is to wait and try to get an NST. It takes a while to get a flyback driver running if you don't have one already. If you already have a flyback driver, I can give you advice on making your TC with either skype IM or MSN messenger. My IDs for both are on my profile page, if you're interested.

Would you mind letting us know what materials you have and what your budget is, as well as if you have any special sources for things like those capacitors?

Once again, welcome,

Christopher Karr

brtaman

Sun Jun 07 2009, 09:41PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hey,

Thanks for the quick reply! Just added you on MSN.

brtaman

Mads Barnkob

Mon Jun 08 2009, 06:34PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

brtaman wrote ...

-secondary coil: 1inch x 4.5 inch polypropylene (apparently one of the best materials for secondary coil)

I recommend you make a larger secondary, you will end up with a very high frequency even with thin wire, go for some 40mm diameter and 20cm height. Should be pvc pipe at that size available in hardware stores in europe.

brtaman wrote ...

-Primary coil: 12 turn 10AWG? (Probably what do you guys think?) spacing around 1cm ?

Even AWG16 (1mm^2) would be fine, I get the feeling you mean a flat coil when talking about spacing that way.

brtaman wrote ...

Or if you have any other recommendation on good but cheap to use on my coil, I would be most grateful.

You can use magnet wire from a microwaveovens fan motor to wind the coil, its 0,13mm so its thin and brittle, but you will be able to make a small secondary at a reasonable resonant frequency. I used this for the tesla coil I showed you.

Find some cheap MICA capacitors on ebay to use as you primary tank capacitance, and tune with primary turns, you just have to do some calculations with fex. JAVATC to get somewhere near the value you need for about 6-10 turns on the primary so you have something to work with

brtaman wrote ...

What sort of choke would you guys recommend, should I hand wind it, should I install one at all?

In a flyback powered tesla coil, dont bother. This will be your first coil and a learning experience, you will move on with your first acquired knowledge, so keep it simple to start with :)

brtaman wrote ...

Since I am in Europe, I am having quite a hard time finding a nice NST, which I think is a much better choice for the TC? Anyone know a guy who knows a guy, that might have a 230v high powered NST on his hands who lives in Europe?

I called and emailed all the neonsign companies in my town and finally found one that still had transformers, so I went to their workshop and bought a used NST out the back, this is your best choice, we never had a neonage in europe so they are not on every streetcorner waiting to be picked up

brtaman

Mon Jun 08 2009, 07:06PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Hey Kaizer,

Thanks for the tips! Only problem is that I just wound the secondary (3/4) of the way there. It took me 4 hours -.- of meticulous winding, I am using wire from a wall adapter, which is extremely thin, I dare say 38AWG perhaps even less. By my calculations the resonant frequency in theory is 714.6 kHz. Is this way to high, I am really in love with the secondary I built took great care. There are few things as satisfying as seeing a secondary slowly come to live. I really want to use it. :D

By my preliminary calculations I will need around 0.36 nF. Other than price are those capacitors good, they would not cost me that much, they seem to have pretty nice specs:

This version seems to be the one for me:

29000 V/s
2000V/DC 750V AC
6800pf

Based on calculations, I will need a ~50cm wide primary with 20-21 turns...Is this what you meant with the high frequency?

I am having quite a large problem finding a NST as well, as you said there was no neon craze here in Europe, I guess I will have to keep on looking for my next SGTC.

Thanks again for the help,
brtaman

Mads Barnkob

Mon Jun 08 2009, 07:24PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

My little flyback SGTC was around 600kHz, so you will be fine :)

You need to find a better ratio between capacitor size and primary turns, try to find a capacitance that would get you at resonant frequency with 10 turn on the primary.

brtaman

Mon Jun 08 2009, 07:36PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

GREAT NEWS! :D After seeing your response I went into my workshop and started looking with great sadness at my secondary. :)

Now I will be able to use it. :D

What do you think I should do about capacitance?

It seems that my LTR for a static cap is 0.3 nF for rotary static gap would be 0.4 nF and resonant 0.2 nF.

So (complete guess out of the blue), I am assuming (big assumption, it is just for now until I test flyback):

Output: 50kV ??? (Probably not...just dont want my caps exploding :P)
Current: 2ma

So I was thinking of going with 25 caps in series (2000v, 6800pF)...Giving me 50kv and 0.27nF??

Thanks
brtaman

plazmatron

Mon Jun 08 2009, 09:39PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Flyback driven tesla coils can be great if driven correctly.
However I have found that it is necessary to use a rotary gap to get excellent performance.

DC flybacks produce a series of HV pulses, rather than continuous DC, and this contributes significantly to heating up the tips of the spark gaps (even tungsten), and thus increasing switching losses, resulting in poor performance.

A static gap `wants` to fire, eveytime it `sees` a HV pulse. With a neon sign transformer this is 120 times a second (twice the mains frequency). This is fine, since there is significant delay between each time the spark gap fires, for the power to couple to the secondary, and the streamers.

When using DC flyback, we have upwards of 20,000 pulses per second arriving at the gap (or even smooth DC if the tank cap is large enough). Now what happens is the gap `wants` to fire, and stay `lit`, since the voltage never dips low enough for the arc to be properly extinguished. All that happens then, is we get little or no transfer of energy into the resonant circuit, and a glowing spark gap.

I made a small rotary gap for mine with a small DC motor and various odds and ends, and the performance with such a small coil is impressive!

Les

brtaman

Mon Jun 08 2009, 09:50PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Thanks for the tip. I will definitely try it, but perhaps on my next TC? This is my first one I am trying to keep it stupid simple. But based on your post, I think that there is a good chance I will get around to making one.

How critical is the balance of the wheel on a rotary spark gap? Seems like something I would mess up. :)

Thanks
brtaman

plazmatron

Mon Jun 08 2009, 10:11PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Ok then begin with a static gap, and then once you have it working, replace it with a rotary. You will find that whilst a static gap will work, the performance with flybacks can be poor to abysmal, but once you have gotten sparks out of the top of your coil, then it is time to improve it

For my gap, I just used a 1 inch diameter plastic pulley from a tape recorder, as it just happened to fit on the motor shaft.(larger would be better) 4 holes were drilled around the circumference, and small bolts were glued into each. (I connected the bolts together electrically, the gap electrodes face opposite bolts, so the gap fires twice per revolution ) The balance isnt too critical, since it aint an angle grinder motor, and its only a small nylon disk, so it would hardly dismember anyone if it failed catastrophically. However reasonable balance is desirable, to reduce vibration.

Hobby shops sell plastic pulleys, sometimes with convenient holes drilled around the edge.

I really must tidy up my flyback coil, and stick some photos up, maybe I will get on with that this week, whilst other projects are awaiting parts........

Les

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