Author Topic: Keep blowing capacitors  (Read 316 times)

Offline pmlapl

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Keep blowing capacitors
« on: September 27, 2021, 11:21:43 PM »
Hi,
Working on my first SG coil. Started with a series of ceramic caps, which  died. Switched to film and they blew also. Finally bought a 30kv 10nf cap which worked about 5 minutes and then died. So I plan to get a larger cap but would like some help. My coil is 1300 plus turns at 4.68 OD x 10 inches with a 4.5 x 1.375 inch toroid. Primary is 3 turns at 4.5 inches OD. Capacitor needed is 10 NF. Coils are driven with an oil ignition transformer, 10kv x 23ma. Resonant frequency is 670kHz that yields about a 4 inch spark. Just wondering if I could use the 30kv cap but with a rotary spark gap to keep the load down. Perhaps a 40kv would work without the rotary SG or just go for two 40kv 20nf in series. Suggestions would be greatly appreciated.
 

Offline Mads Barnkob

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Re: Keep blowing capacitors
« Reply #1 on: September 28, 2021, 11:18:34 AM »
You need to calculate all the vital parameters and design a MMC that can withstand voltage and current peaks, at the resonant frequency.

You should read this guide on MMC: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/mmc-tank-capacitor/ and good MMC capacitor: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/good-mmc-capacitors/

You can use the MMC calculator to help you design a MMC that will last as long as you can afford to: https://kaizerpowerelectronics.dk/calculators/mmc-calculator/
https://kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics
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Offline pmlapl

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Re: Keep blowing capacitors
« Reply #2 on: September 28, 2021, 04:28:43 PM »
Thanks for the info but the examples are for much larger coils than mine. Also many of the variables in the calculations are unknown to me or how to find them, such as peak primary current. The calculations appear to be for a solid state coil as well and for an electrical engineer which I am not. I still appreciate your reply. Thanks again.

Offline MRMILSTAR

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Re: Keep blowing capacitors
« Reply #3 on: September 28, 2021, 04:46:54 PM »
Use JAVATC to get an initial Tesla coil design. This will tell you the parameters that you will need so you can then use Mads' tool to design a MMC array. JAVATC is actually designed for spark gap coils but much of it can be used for any type of Tesla coil. You will still have to gain an understanding of what all of the parameters mean. You don't have to be an electrical engineer to design and build a Tesla coil but you do have to invest the time to learn how they work and how to use the design tools.
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline pmlapl

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Re: Keep blowing capacitors
« Reply #4 on: September 28, 2021, 07:21:43 PM »
Thanks much Steve,
I used JavaTC to design my coil but it gave me a 12.5 turn primary which didn't yield much spark. I checked the resonance of the secondary and primary with my oscilloscope and ended up needing only 3 turns. It also showed I needed 10nf capacitance. It worked great until the caps died. I now am using a variac to lower the transformer voltage for now until I get higher voltage caps. I found out that I need some that can take 2 to 4x more voltage than the peak to peak supply. Turns out to be 40 to 50kv. There's a place in China (hvstuff.com) that sells some at a reasonable price.
     I do understand much of the electronics of a TC, it's just that sites that I was referred to have way more detail in design than I needed. I just have to grit my teeth and spend the money on the right kind or enough capacitors if I want my coil to work longer than a few minutes. My next project will hopefully be a solid state version.
Thanks again

Offline johnf

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Re: Keep blowing capacitors
« Reply #5 on: September 29, 2021, 07:35:03 AM »
Your last answer has all the clues
you had to reduce primary turns and increase the capacitance to get good sparks

ie "Q" was not enough
 "Q" = high circulating currents and if these cannot be got the voltage rises

you need pulse rated capacitors that have enough voltage stand off and current capacity

Offline johnf

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Re: Keep blowing capacitors
« Reply #6 on: September 29, 2021, 07:39:27 AM »
I tried to "Modify" my post but the board refused to act!!!

I meant to say that tuned circuit "Q" is made better by increasing capacitance with reducing inductance ie "Q" is proportional to "C" in a tuned circuit

Offline Twospoons

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Re: Keep blowing capacitors
« Reply #7 on: September 30, 2021, 01:46:07 AM »
[quote author=johnf link=topic=1768.msg13419#msg13419 date=1632893967
I meant to say that tuned circuit "Q" is made better by increasing capacitance with reducing inductance ie "Q" is proportional to "C" in a tuned circuit
[/quote]

That may be somewhat true in practice, buts its not because Q is proportional to C (its not). Its because reducing L to keep the same Fres also tends to reduce the R in the tuned circuit (unless you have really shitty capacitors).  Reducing R is the real reason the Q rises.

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Re: Keep blowing capacitors
« Reply #7 on: September 30, 2021, 01:46:07 AM »

 


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