Re: Old Capacitors

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Reply To: 	bert.hickman-at-aquila-dot-com
Sent: 	Tuesday, December 02, 1997 7:50 AM
To: 	Tesla List
Subject: 	Re: Old Capacitors

Tesla List wrote:
> From:   Alan Sharp[SMTP:100624.504-at-compuserve-dot-com]
> Sent:   Monday, December 01, 1997 4:46 AM
> To:     INTERNET:tesla-at-pupman-dot-com
> Subject:        Old Capacitors
> Greetings all,
> I've just aquired six 0.1uf 10kv DC capacitors.
> They are 7" long, 1" diameter brown cylinders with stud terminals
> marked Hivoltage Capacitors Limited
> Londonderry N.Ireland
> +8% ripple 50Hz
> TPM 100-114
> I was going to put 3 of these in series,
> then 2 series in //  for my 7.5KV AC 550mA supply - to give 0.066uf 30kV DC
> but I'm wondering if they will take the current at 200kHz -
> would I be better to use 4 or 5 in series and get the other 0.04uF from
> my other capacitors? Or reduce the voltage to 5kV AC.
> It also occurs to me that moving to a lower frequency - bigger coil
> will ease the load on the capacitors.
> Any thoughts.
> have fun,
> Alan Sharp (UK)


These sound like they're Mylar, made for DC filtering applications.
Assuming that you're running at a gap firing voltage of, say, 9 KV, and
matching it with a primary inductance of about 9.5 uH to get 200 kHz
operation, your maximum tank current will be about 750 Amps, shared
between the two paralleled groups, ot about 375 Amps/cap. While this is
nothing for a properly constructed pulse cap, these caps don't sound
like they're pulse caps! 

Aside from the lossy dielectric, the other "weak link" in these
capacitors is the end terminations. These are usually flame-sprayed
metal over the metallization on each end, soldered to the end studs, and
may not be robust enough for high current pulse discharge application.
The end connection to the capacitor roll can separate under the
repetitive shock and thermal stresses of TC use.

Alan, it's really hard to say how long these will last - keep your
run-times down so as not to overheat them, and put them in a container
to catch the piece if one decides to "go ballistic".

Good luck, and safe coilin' to you, Alan!

-- Bert --