BIG Capacitors (don't use electrolytics)

To: "'Tesla List'" <tesla-at-pupman-dot-com>
Subject: BIG Capacitors (don't use electrolytics)
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 20 Feb 1998 23:49:46 -0600
Approved: tesla-at-stic-dot-net

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From:  Jim Lux [SMTP:jimlux-at-earthlink-dot-net]
Sent:  Friday, February 20, 1998 10:39 AM
To:  Tesla List
Subject:  Re: BIG Capacitors (don't use electrolytics)

> ----------
> From:  Edward V. Phillips [SMTP:ed-at-alumni.caltech.edu]
> Sent:  Tuesday, February 17, 1998 11:54 AM
> To:  tesla-at-pupman-dot-com
> Subject:  Re:  BIG Capacitors
> 
> Justin:
> 
>         Any idea how big a 100 uF 18 kV capacitor would be?
> You can get 4800 ufd 450 volt electrolytic capacitors pretty
> cheaply at swap meets (less than a buck around here).  40 of
> those in series would be 120 ufd at 18 kV, if you could 
> equalize the voltages with shunt resistors across each 
> capacitor. 

A can crusher application (or a similar underdamped discharge) will put a
significant reverse voltage on the storage capacitor. If the caps are
electrolytic, they'll be destroyed in short order.  Several years ago, I
built a massive electrolytic cap bank as a brute force filter for a 300
Volt DC power supply running the caps in parallel (30 caps at 15-20
thousand uF each, as I recall). Indeed, the discharge is impressive, but
after a few dozen shots demonstrating this phenomenon to my friends , I
noticed that the caps were failing: the leakage current started rising, and
one of the caps developed a suspcious bulge and got pretty hot.  This was
before I learned about the practicalities of voltage reversal, etc.