[TCML] ANOTHER ONE DOWN, ANOTHER 37667 BITES THE DUST
k.schraner at datacomm.ch
Thu May 17 05:00:15 MDT 2012
Hi Tuomas, Michael, All,
Tuomas, I concur with you. Would never use one of these caps, without heavy
voltage derating. Jim Lux has made available a method for cap live
estimation before, on this list. Please see:
The method allows to estimate cap live, in respect of the 2 parameters
"discharge voltage" and "voltage reversal". I've made an excel out of it, in
order to evaluate my own Maxwell caps. Jim has given me permission to give
this excel to interested coilers. So, I can send it, if someone likes to try
it. Please mail me offlist on k.schraner at datacomm.ch, if you want one. The
excel is easy for Maxwells, for which you have a datasheet with all needed
Just purchased 6 of the 37667 on Ebay, owning 8 now. Those can be combined
for a sort of "Maxwell-MMC" with i.e. 60nF/70kVDC. And I'm not so much
afraid of short live, if it's not for a bad history of those caps.
From: Tuomas Koivurova
Sent: Thursday, May 17, 2012 10:14 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] ANOTHER ONE DOWN, ANOTHER 37667 BITES THE DUST
I would never ever push used cap with unknown history close to their
maximum voltage ratings. As i said, in AC driven coils the charge
voltage can be *higher* than transformer peak voltage. Headroom is
always very good idea, even with brand new caps.
I got this information regarding reversal ratings from general atomics
application engineer :)
2012/5/17 <mddeming at aol.com>:
> Hi Tuomas, Michael, All,
> Everything in Electronics has a "tolerance band", usually ~5% for the
> pretty good stuff.
> Using this as a rough measure, a 1.15 x 35kV-rated cap would hit its limit
> somewhere between 38,200V and 42,300V. This could be part of the reason
> why some folks have no problem with one cap and some have nothing but
> trouble with one of the same type and size. As always in science, a
> conclusion based on a single observation or experience is not very
> It would be more accurate to say that some of these caps may be seeing an
> overvoltage of about 2%-4%, depending upon manufacturing tolerances
> Matt D
> -----Original Message-----
> From: Tuomas Koivurova <tkoivuro at gmail.com>
> To: Tesla Coil Mailing List <tesla at pupman.com>
> Sent: Wed, May 16, 2012 8:50 pm
> Subject: Re: [TCML] ANOTHER ONE DOWN, ANOTHER 37667 BITES THE DUST
> 37667 Maxwells are not useless!
> You are simply overvolting them.
> 37667 is a 500pps polypropylene capacitor with 30nF 35kVDC ratings.
> hat is most important is their reversal rating which is quite low,
> nly 15%.
> So what does it mean? It means that when the capacitor gets charged to
> ransformer peak voltage and then discharged through spark gap to a
> oil creating LC circuit with relatively high Q there will be voltage
> eversal which is very close to 100%. That reversal creates potential
> ifference over dielectric which is equal to 2x Vpeak of your
> With 14.4kVAC pig we have 20.45kV peak-to-peak and after first primary
> ycle 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
> ffects that can be seen with async AC driven coils you are exceeding
> he voltage ratings of your capacitor.
> tay within the ratings and you will be fine. I have trusty pig driven
> C spark gapper where i have 80kVpp reversal handling capability (45kV
> C and 80% reversal rating) and my worst case reversal is 60kVpp.
> ever had any troubles with that and i have pushed it so hard that my
> rimary was hot to touch! All caps are still fine and will be for long
> 012/5/13 MICHAEL HARDY <mhardy4 at wi.rr.com>:
> When I first decided to build a pig powered coil (2004 ), I searched for
> aps on the internet, and of course the infamous 37667's were everywhere.
> at $100 each. Then found out they weren't very good for TC's. I've since
> unning 3 TC rated Maxwell 0.2uF, 20 kV caps in series, for 0.066uF @
> his on a 14.4 kV Pt powered 6" coil system running around 4800 watts with
> roblem. I have recently completed a 15" coil running at about 15 kW. I
> used 2
> anks (including the one above) in parallel for 0.14 uF at 60 kV. I still
> unning the smaller coil routinely. Changing out one bank is a pain, so
> o try a pair of 37667's in parallel (0.06 uF @ 35kV). Ran good with only
> armth to cases of caps. Short 30 sec runs with 3 minute pauses. After a
> our I saw steam from the cap bank. Checked, and one of them had split
> ON'T BUY 37667 MAXWELL'S FOR TC USE!
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