Author Topic: Need second opinion on tesla coil capacitors  (Read 492 times)

Offline ResonantEMF

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Need second opinion on tesla coil capacitors
« on: July 27, 2021, 09:14:01 AM »
After hours of searching, I found some capacitors that will work with my dual MOT Tesla coil (at a price I can afford).

https://www.ebay.com/itm/144069961451?hash=item218b3d26eb:g:zpsAAOSw5ZBWQs6C

I get 50 for around $50. I'm planning to use 6 in series and 4 strings in parallel to get a 12kv 66.6 nF MMC, so that's enough to make one MMC and have over an entire MMC worth of spares, should they be needed. Do these capacitors seem to be suited for the task? I don't know their dv/dt rating, but that doesn't seem like it'll be an issue.

Personally, I'm quite sure they'll work great. But some confirmation would make me feel better about purchasing!

Additionally... If anyone knows where I can get some good caps for cheaper, that would be great too. :D

Online Felix B.

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Re: Need second opinion on tesla coil capacitors
« Reply #1 on: July 27, 2021, 04:58:46 PM »
Hello

I built 4 MOT RSGTC about 2 years ago and knew pretty much nothing aobut tesla coils (maybe becaus I was just 14 at the time). First I just used a bunch of MOCs in series whis was a very bad idea. They exploded because of the low voltage and pulse rating. Then I used amazon pulse caps but the also exploded because I thought 15kV would be enough but it wasn't. Finally I upgraded to WIMA MKP10 capacitors for a final voltage rating of 30kV (DC) and a capacity of 64nF. In a DRSSTC you can calculate the needed voltage rating pretty easy but in a SGTC the voltage across the MMC will be pretty messy and I would go with at least 20kV voltage raping and well known pulse film capacitors. Here is a video of my SGTC:

Offline davekni

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Re: Need second opinion on tesla coil capacitors
« Reply #2 on: July 27, 2021, 06:48:04 PM »
No way to know without specifications.  If you try these, I'd use most of them initially, say 8S6P.  Reduced stress on individual parts is of more value than spares.  (Essentially same as Felix was saying.)
David Knierim

Offline ResonantEMF

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Re: Need second opinion on tesla coil capacitors
« Reply #3 on: July 27, 2021, 07:38:47 PM »
No way to know without specifications.  If you try these, I'd use most of them initially, say 8S6P.  Reduced stress on individual parts is of more value than spares.  (Essentially same as Felix was saying.)

Sorry, I should have specified more. I'm using 2 MOTs in series and my frequency is 103 kHz. I need about 55nF but 66nF can be tuned back to account for that. I have an adjustable stationary spark gap but I'll upgrade to a rotary one soon. Tell me if you need any more details. Sorry if I seem difficult to work with.

Offline MRMILSTAR

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Re: Need second opinion on tesla coil capacitors
« Reply #4 on: July 27, 2021, 08:45:26 PM »
I believe that David meant capacitor specs.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline ResonantEMF

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Re: Need second opinion on tesla coil capacitors
« Reply #5 on: July 27, 2021, 09:12:05 PM »
I believe that David meant capacitor specs.

Oh... Well, I contacted the seller about dv/dt ratings so I should be able to get some more data soon.

Offline davekni

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Re: Need second opinion on tesla coil capacitors
« Reply #6 on: July 27, 2021, 11:26:15 PM »
AC voltage and RMS current ratings are likely more important for capacitor choice.  dV/dt can be helpful, but that is a rating for pulses, not for sine-wave bursts.
David Knierim

Offline ResonantEMF

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Re: Need second opinion on tesla coil capacitors
« Reply #7 on: July 28, 2021, 01:02:30 AM »
AC voltage and RMS current ratings are likely more important for capacitor choice.  dV/dt can be helpful, but that is a rating for pulses, not for sine-wave bursts.

In that case, I'm completely lost at this point... I can't find anywhere where I can buy caps that will work for me. Not with my $50 budget, at least.

Offline davekni

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Re: Need second opinion on tesla coil capacitors
« Reply #8 on: July 30, 2021, 04:45:12 AM »
The 0.1uF 2kV caps you listed may work fine.  It is hard to tell.  Another option is the Chinese 0.33uF 1200Vdc 630Vac induction cooker capacitors that I use on my DRSSTC.  Here's a couple links to posts where I describe the abuse-testing I've done on these:
https://highvoltageforum.net/index.php?topic=937.msg6287#msg6287
https://highvoltageforum.net/index.php?topic=1051.msg7413#msg7413

I can't guarantee you will have success with these, or that the ones you order will behave like mine.  If it were my money, I'd gamble on caps designed for high-frequency AC and with abuse-testing up to 2100V.  Depending on budget, you could use 20 caps (10S2P) or 45 caps (15S3P) for better margin.  There are many EBay sellers of what I believe are the same capacitor, and other outlets of such goods.
David Knierim

High Voltage Forum

Re: Need second opinion on tesla coil capacitors
« Reply #8 on: July 30, 2021, 04:45:12 AM »

 


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