Re: Series-linked Microwave caps? any good?

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Sunday, November 30, 1997 6:37 PM
To: 	Tesla List
Subject: 	Re: Series-linked Microwave caps? any good?

Tesla List wrote:
> From:   Matthew Mills[SMTP:megavolt-at-usa-dot-net]
> Reply To:       megavolt-at-usa-dot-net
> Sent:   Sunday, November 30, 1997 1:56 PM
> To:     Tesla List
> Subject:        Series-linked Microwave caps? any good?
> Hi, thanks to all the people on this list who have helped me so far, as
> you can see I am still asking questions....:^)
> 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 a
> capacitor rated at 21000vAC -at- 0.095uF which sounds quite reasonable....
> any comments?
> PS I hear that these caps have a resistor built in (to drain off the
> charge maybe?) would this make them unsuitable? can i remove the
> resitor?
> Matt.


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!

Safe coilin' to you, Matt!

-- Bert --