Re: Derating HV Caps
From: Thomas McGahee[SMTP:tom_mcgahee-at-sigmais-dot-com]
Sent: Sunday, July 20, 1997 11:42 AM
To: Tesla List
Subject: Derating HV Caps
> From: Larry Robertson[SMTP:LWRobertson-at-msn-dot-com]
> Sent: Saturday, July 19, 1997 10:34 PM
> To: bertpool-at-flash-dot-net
> Cc: Tesla Builders
> Subject: Big Cap
> Hi Bert ...
> Thanks for volunteering information on those Sangamo
> caps. The price sounds good, but I guess when contemplating
> a 250 lb. purchase one would want to know where it is more or
> I live about 25 miles south of San Jose, CA in what used
> to be a small town called Morgan Hill. The widening of Hwy 101
> fixed the small .
> A local company has Aerovox caps 1.84 mfd. at 60 kV
> p/n PX300D28 (which also have a substantial heft) for $200 plus
> $10 (!) shipping.
> If your friend lives nearby or we could arrange shipping
> for a reasonable sum I might rather buy from him. Let me know.
I have seen a number of posts about buying bargain-basement HV
capacitors, and I would like to remind the unwary of a few facts.
When buying any commercial capacitor for use in Tesla service you
will almost always have to de-rate it substantially. Most HV
capacitors have a DC rating. As soon as you apply AC to them, you
have to de-rate their *VOLTAGE* rating by dividing the voltage rating
by 1.414 just to account for the difference between PEAK and RMS
values. Then you have to FURTHER derate the NEW value to account for
the additional stresses caused by AC RIPPLE and POLARITY REVERSAL
taking place once per cycle! In normal DC service the AC ripple is
small compared to the DC level. In AC service the ripple is 100%. If
you are JUST connecting the capacitor up to good old 60Hz AC, (not
using it for Tesla service...) then the de-rating factor here isn't
TOO bad. Say 30%. But when subjected to PULSES of large current being
drawn from it (which is the essence of Tesla service...) then you may
have to de-rate anywhere from 30% to 80%. Remember, that in a DC
power supply the capacitor only has to handle the RIPPLE currents.
But in AC operation it has MUCH larger "ripple" currents. The real
killer, though, is PULSE mode operation, because here the DISCHARGE
takes place in an extremely short time period, and so the currents
And keep in mind that MANY capacitors simply CANNOT support pulsed
mode of operation for more than a few seconds before severe
overheating and physical damage occur. This is FURTHER aggravated
when using a rotary spark gap, because now the number of discharges
per second can exceed two times the mains frequency. High BPS rates
are MURDER on capacitors. They literally get BEATEN to death!!
When a HV capacitor is rapidly discharged, there are SEVERE
mechanical stresses as well as electrical stresses. These mechanical
stresses can cause rapid failure. If your capacitor get really warm
when running a Tesla coil, it is not only wasting energy, but it is
also a potential source of great danger.
When using any capacitor not specifically rated for Tesla service, be
VERY careful. ASSUME the worst. Protect yourself against accidental
explosions. Give it several test runs at REDUCED input levels. Let
them run for at least a minute and then remove power, discharge the
capacitor, and then check for heat rise, oil leaks, hissing, or any
other sign of trouble.
There are some really good bargains out there, but there are also a
great many capacitors that are totally unfit for Tesla service. Let
the buyer beware! Especially *good* buys are the MICA capacitors
designed for radio transmitter and other RF work. These are sealed,
and even 40 year old units work great because the mica does not
decompose or dry out like the paper capacitors. You can often find
these at hamfests ...if you get there before Richard Hull or any of
the OTHER savvy members on this list :) I have also found a number of
radio surplus outlets that usually have some of these hanging around
at reasonable prices. The bigger units are especially good finds.
What is a good price? Well, figure out how much it would cost you to
make, say a .01 mfd 10KV Pulse duty Tesla capacitor. If you can BUY
enough mica units to achieve the same rating at the same or less
cost, then definitely buy them up!!!!
Fr. Tom McGahee