RE: Series-linked Microwave caps? any good?

From: 	Jim Lux[SMTP:jimlux-at-earthlink-dot-net]
Sent: 	Tuesday, December 02, 1997 10:29 AM
To: 	Tesla List
Subject: 	Re: Series-linked Microwave caps? any good?

> Subject: 	Re: Series-linked Microwave caps? any good?
> > From:   Matthew Mills[SMTP:megavolt-at-usa-dot-net]
> > I have a number of Capacitors from microwave transformers laying around
> > and I was wondering whether or not I can series link these for Tesla
> > use....
> > 
> > My theory is that if each one is rated at 2100vAC eff and have a value
> > of around 0.95uF then I should be able to run 10 of them in series for
> > capacitor rated at 21000vAC -at- 0.095uF which sounds quite reasonable....
> > any comments?
> > 
> These caps are typically in a metal can, and are constructed using Mylar
> (PET) dielectric. While these will work just fine for HV DC
> applications, DON'T use them in a Tesla Coil tank circuit. The mylar
> dielectric heats up very fast, since it's quite lossy under RF
> conditions. The relatively low thermal conductivity makes the center of
> the cap heat up faster than the rest of the cap. The heat makes the
> Mylar even MORE lossy, leading to a thermal runaway condition. The outer
> cans of these caps has been known to explode without warning, showering
> the area (and the coiler) with hot oil and pieces of capacitor! Please
> don't use them in your coil - we want you coiling safely next year!

Comments on both of the above:
1) Aside from lossy nature of the PET dielectric, if you hook up a bunch in
series, you are going to have a pretty significant series inductance there,
which will cause problems. These caps aren't made for low inductance pulse
discharge, and aren't even all that great as filter caps for DC. In an
oven, you don't really care about the ripple or power supply induced
frequency shifts, etc. As long as the RF power gets out, and is within the
band, who really cares what the precise waveform or envelope is.

2) That said, the resistor in parallel won't hurt, and it certainly makes
it safer. The microwave oven cap I have in front of me here is 3
microfarads and 1 Megohm, which works out to a 3 second time constant.
Figure that it will take about 5  timeconstants to get the 3000 Volts down
to around 50 volts, so the cap is "safe" to touch after 15 seconds. DON'T
TRUST THE BLEEDER THOUGH! Always discharge with an insulated wire to be
safe. The bleeder will get the energy down to where your screwdriver or
cliplead won't get chunks melted out of it.

3) Capacitors such as this have pressure relief ports built into them. I
would find it unlikely that they would ever explode. Rupture and leak oil,
possibly. Bulge the sides (I've seen this one), also possible. Recall that
this is a component used in UL listed devices, and UL is a fiend on
preventing explosions, fires, boiling oil, and the like. Granted, if I hook
up my 75 kV, 40 kJoule cap bank to the poor thing, it probably will
explode, but that is an anomalous situation.