Re: [TCML] ANOTHER ONE DOWN, ANOTHER 37667 BITES THE DUST

[Tuomas Koivurova <tkoivuro@xxxxxxxxx>]

37667 Maxwells are not useless!

You are simply overvolting them.

37667 is a 500pps polypropylene capacitor with 30nF 35kVDC ratings.
What is most important is their reversal rating which is quite low,
only 15%.

So what does it mean? It means that when the capacitor gets charged to
transformer peak voltage and then discharged through spark gap to a
coil creating LC circuit with relatively high Q there will be voltage
reversal which is very close to 100%. That reversal creates potential
difference over dielectric which is equal to 2x Vpeak of your
transformer!

With 14.4kVAC pig we have 20.45kV peak-to-peak and after first primary
cycle the capacitors dielectric "sees" voltage stress equal to 40.9kV.

37667 is rated for 1.15 x 35kV = 40.25kV peak-to-peak!

So even with perfectly tuned sync gap with no resonant rise or other
effects that can be seen with async AC driven coils you are exceeding
the voltage ratings of your capacitor.


Stay within the ratings and you will be fine. I have trusty pig driven
DC spark gapper where i have 80kVpp reversal handling capability (45kV
DC and 80% reversal rating) and my worst case reversal is 60kVpp.
Never had any troubles with that and i have pushed it so hard that my
primary was hot to touch! All caps are still fine and will be for long
time.




2012/5/13 MICHAEL HARDY <mhardy4@xxxxxxxxx>:
> When I first decided to build a pig powered coil (2004 ), I searched for pulse caps on the internet, and of course the infamous 37667's were everywhere. Bought 5 at $100 each. Then found out they weren't very good for TC's. I've since been running 3  TC rated Maxwell 0.2uF, 20 kV caps in series, for 0.066uF @ 60kV. This on a 14.4 kV Pt powered 6" coil system running around 4800 watts with no problem. I have recently completed a 15" coil running at about 15 kW. I used 2 banks (including the one above)  in parallel for 0.14 uF at 60 kV. I still am running the smaller coil routinely. Changing out one bank is a pain, so decided to try a pair of 37667's in parallel (0.06 uF @ 35kV). Ran good with only slight warmth to cases of caps. Short 30 sec runs with 3 minute pauses. After a half hour I saw steam from the cap bank. Checked, and one of them had split open. DON'T BUY 37667 MAXWELL'S FOR TC USE!
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