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Truant19

Fri Jun 29 2007, 08:19PM

Registered Member #855 Joined: Sat Jun 23 2007, 05:25PM
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Posts: 8

Hi guys, while im waiting to get my stuff for my large tesla coil I was thinking about building a small tesla just for some fun, and to learn a thing or two, but a got some questions..

I found a transformer i used for my amp that i built, and its center tapped but only on the secondary output, can I reverse this transformer and use it to step up the volts? should i ground the center tap?

also, how would i determin the size of the capicitor to use? ( as in farads), If i get say 9k volts, and i have put 3, 3k caps in series to mitigate the voltage across them, how would i come to determain the amount to put in parrel? or, how many farads whould i be shooting for here?

also, could i use a switch, rather then a spark gap? just so i can control the sparks, and hopefully not burn out a small transformer.

Thanks Guys, and if my questions suck, blame my proffs

Sulaiman

Fri Jun 29 2007, 08:35PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Assuming you're going for a spark gap Tesla Coil
the source of high voltage is one of the main obstacles
the simplest are NST and OBIT
then it gets complicated.

There is loads of (searchable) information here, tesla coil ring, pupman etc.

One one of the days that you aren't a truant, ask your proffs. why you can't do what you plan with your transformer.

Truant19

Fri Jun 29 2007, 08:48PM

Registered Member #855 Joined: Sat Jun 23 2007, 05:25PM
Location:
Posts: 8

haha, truant is my last name,

blah, so, why cant I use a small transformer to step up my voltage? is it because theres no current limiting device? is current limiting just for saftey? couldnt i just put on a fuse or breaker? I dont got school for another 2 months, and no other resources besides this forum and other googling methods which always seem way above what i know... blah!

J. Aaron Holmes

Fri Jun 29 2007, 10:20PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Truant19 wrote ...

blah, so, why cant I use a small transformer to step up my voltage? is it because theres no current limiting device? is current limiting just for saftey?

Huh? What kind of transformer is it? Like, a 120V-to-12VCT? Are you talking about putting 120V into the 12V leads or something? Don't forget about that little thing called "insulation"

A transformer winding rated for 12V probably isn't designed to stand up to ten times that. Likewise for the 120V winding. Forget about current limiting; nothing is current-limited once the insulation fails and boils away.

Cheers,
Aaron, N7OE

...

Fri Jun 29 2007, 11:28PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

meh, the 12v winding would take 120v no problem, and if it was a decent transformer the 120v winding would take 1.2kv without too much trouble, but you run into something called 'saturation', which prevents the transformer from working correctly (or at all) with more than its rated voltage. Read more about it in the 4hv wiki, or google it.

J. Aaron Holmes

Sat Jun 30 2007, 03:54PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

... wrote ...

meh, the 12v winding would take 120v no problem, and if it was a decent transformer the 120v winding would take 1.2kv without too much trouble, but you run into something called 'saturation', which prevents the transformer from working correctly (or at all) with more than its rated voltage. Read more about it in the 4hv wiki, or google it.

Indeed, yes! Forgot to mention that. But like insulation failure, saturation is often overexpected and underobserved; a friend of mine ran is 14kV pig with the 120V windings in parallel but still applied 240V. ~28kV was produced without any unexpected current draw, however during a particularly brutal coil run with an async gap and large cap, the transformer HV winding flashed over internally and was ruined. Of course, this was only 2x the rated voltage of the windings, not 10x! At 10x, I don't doubt it would saturate immediately. Pigs, of course, are build with some margins. The 12VCT trannies you get from Rad Shack probably aren't built for much over the stated rating, and so pushing them in various ways is probably more apt to result in blown breakers or flames or both!

Regards,
Aaron, N7OE

Sulaiman

Sat Jun 30 2007, 08:13PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The only way to use a higher than rated voltage for a transformer winding is to use a proportionately higher frequency
e.g. a 120:12 Vac @60 Hz transformer, used in reverse at 120Hz could be 24:240 Vac etc. etc.
Once you have a core of a particular cross-sectional area and material the volt-seconds per turn is set.
With a coil of a fixed number of turns the Volt.Seconds for the coil are set.

Shaun

Sun Jul 01 2007, 06:16AM

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

Well, by that math, you could use this transformer backwards, if you could somehow give it 600Hz insteasd of just 60. Now, of course this can be done, but not without some simple brainwork and circuit design, lol.

It might be easier (maybe not cheaper) to use multiple transfomers and wire the primaries and secondaries in series with the primaries and secondaries of the other transformers. Ideally you would have ten transformers in series for equivalent volts per turn, and for these EXTREMELY common little 12/120s I bet you can get 10 cheap on ebay or something. Plus wouldn't that also solve the secondary insulation problem?

More importantly, wouldn't 1.2 kv be horribly inefficent for an SGTC? I don't know for sure, maybe someone else has experience in this area.

You also talk about replacing the spark gap with a switch to control the sparks. Although this is the base of SSTCs, doing that with a mechanical switch would TOTALLY ELIMINATE the entire underlying principle in a tesla coil: resonance.
The purpose of the gap is so the circuit only fires when the capacitor reaches a full charge. Also a mechanical switch would probably arc over and be damaged, plus you'd be exposed to a high-voltage capacitive discharge. In a small coil this wouldn't be lethal, but you'd get one hell of a scare.

Or maybe I'm completely wrong, who knows?

Marko

Sun Jul 01 2007, 01:41PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It might be easier (maybe not cheaper) to use multiple transfomers and wire the primaries and secondaries in series with the primaries and secondaries of the other transformers. Ideally you would have ten transformers in series for equivalent volts per turn, and for these EXTREMELY common little 12/120s I bet you can get 10 cheap on ebay or something. Plus wouldn't that also solve the secondary insulation problem?

This was already discussed and is not possible because primary/secondary insulation.

Seriesed secondaries will, at point, flash over to core and to primaries (since all primaries are connected together).

You also talk about replacing the spark gap with a switch to control the sparks. Although this is the base of SSTCs, doing that with a mechanical switch would TOTALLY ELIMINATE the entire underlying principle in a tesla coil: resonance.
The purpose of the gap is so the circuit only fires when the capacitor reaches a full charge. Also a mechanical switch would probably arc over and be damaged, plus you'd be exposed to a high-voltage capacitive discharge. In a small coil this wouldn't be lethal, but you'd get one hell of a scare.

My school actually owns a small TC switched by a vibrating relay, and it still pretty much uses resonance. The relay switch can easily be used to clamp shut when cap is charged, simply by connecting the relay winding in series with it.

Much bigger problem is really nasty erosion of contacts wich for that reason need to be quite bulky. Such coils usually operate on low voltages and don't have some spectacular spark output, but they work.

To power really small SGTC's you can pass with flyback transformers. (100-200W at most). For anything larger, be ready to buy a NST, and period.

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