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

[mddeming@xxxxxxx]

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 meaningful. 
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@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
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
ransformer!
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
ime.


012/5/13 MICHAEL HARDY <mhardy4@xxxxxxxxx>:
 When I first decided to build a pig powered coil (2004 ), I searched for pulse 
aps on the internet, and of course the infamous 37667's were everywhere. Bought 
 at $100 each. Then found out they weren't very good for TC's. I've since been 
unning 3  TC rated Maxwell 0.2uF, 20 kV caps in series, for 0.066uF @ 60kV. 
his on a 14.4 kV Pt powered 6" coil system running around 4800 watts with no 
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 am 
unning the smaller coil routinely. Changing out one bank is a pain, so decided 
o try a pair of 37667's in parallel (0.06 uF @ 35kV). Ran good with only slight 
armth to cases of caps. Short 30 sec runs with 3 minute pauses. After a half 
our I saw steam from the cap bank. Checked, and one of them had split open. 
ON'T BUY 37667 MAXWELL'S FOR TC USE!
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