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Re: [TCML] Help with custom capacitor design

Hello Greg,

I can't claim to be an expert on this but I will give you my opinion. Since you are discussing a custom-built capacitor I am assuming that you are thinking of an oil-filled energy discharge capacitor such as built by General Atomics (formerly Maxwell), Aerovox and others. Be prepared to pay a lot of money for a new custom unit. Most of us that use oil-filled pulse discharge capacitors, such as myself, just use what the second-hand market offers us in order to keep the cost reasonable. Then we just design the rest of the coil around that. In my case, I used 6 of the Maxwell 37667 types which were available on E-Bay at the time. Of coarse, the problem is that you don't know what will be available or when. Since they are used, you also don't know how much life is left in them although I have never had a problem with that.

As far as voltage rating is concerned, I would want a 3x specification. So, for an 18 KV charging voltage, I would want at least a 54 KV maximum DC voltage rating. For an MMC array you could get by on less since they are self-healing. The peak current rating sounds OK. For BPS, I would want a spec of 2x the expected break rate. That would put you at 1000 BPS maximum pulse rating spec.

For the other spec points that you mention, I would just look at the manufacturer's data sheets for some of their current capacitors that are closest to the capacitance, break rate and lifetime that you want to give you an idea of what is achievable. I would then use the specs (RMS current, ESL, etc.) for one of those capacitors in your request-for-quote. Use reasonable extrapolation if needed.

One spec that you didn't mention that is very important is the maximum number of shots that the capacitor can deliver before failure. You can estimate this requirement by taking your break rate (500 BPS) and how long you want your capacitor(s) to last and make the calculation. For example, the Maxwell 37667s that I use have a maximum life of 3x10^8 shots. If these capacitors were new, they would have a life time of 347 hours at maximum rated voltage for my coil (240 BPS). Since you will be running your coil at less than the maximum rated voltage, they will last even longer than the rated lifetime. The General Atomics web site has several very helpful papers concerning the design and application of oil-filled pulse capacitors.

I have never built an MMC array but from what I have read over the years on the TCML this may be your best approach to obtaining the custom specs that you want, although it is not cheap either. If you decide to use this method, be prepared for soldering hundreds of capacitors and resistors into some kind of reasonable assembly that will not flash over and be reasonably easy to service. Given that daunting challenge you can see why some of us prefer the Maxwell-type oil-filled capacitors if we can get them for a reasonable cost.

Steve White
Cedar Rapids, Iowa

----- Original Message -----
From: "Greg Peters" <greg.j.peters@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Wednesday, October 10, 2018 4:54:12 AM
Subject: [TCML] Help with custom capacitor design

Hi All,

I’m working on a large 3-phase DC resonant charged magnifier coil and would like to attempt to order a custom capacitor to enable a small, neat and compact design.

The coil is going to run at about 15kVA with the primary cap charged to near 18kV at 500BPS. I expect Fres to be between 75 and 100kHz, so peak primary currents of perhaps 2000 to 3000 amps. 

I was thinking I would specify a 40kV DC and 5000 amp peak rating – how does that sound? 

Now, is anyone able to offer some advice regarding the specs I should request for things like:

RMS current,
Ripple Voltage (assume 100%???)

Any advice would be greatly appreciated!



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