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Re: A new cap failure mode?
"At 20:18 4/02/99 -0700, Terry Fritz wrote:
> When one places a nice safety gap across a capacitor, and it is
firing for
>any reason, what is the current in the discharge pulse? Since Tesla caps
>are designed to be very low inductance and low resistance, there is very
>little limiting the current when the safety gap fires. If a cap is charged
>to say 20kV and the resistance of the mess is say 0.5 ohm - we get 40000
>amps!! That is enough to do some real internal damage to any capacitor.
>The suggestion comes up that perhaps a safety gap placed directly across a
>primary cap needs a little resistance in the circuit to keep this current
>to a "safe" level. "
Among the sources of capacitor failure are dielectric breakdown caused
by either puncture by too high voltage, or deterioration due to corona,
leading to breakdown at lower voltage, and internal heating due to the
current flowing through the internal resistance. If a capacitor does
indeed have low internal resistance, the heating effect should be
relatively small. To figure heating you need to use the RMS current,
not the peak current.
Example:
The reactance of an 0.1 mfd capacitor at 100 kHz is about about 16
ohms; if the Q is 100, the ESR will be about 0.16 ohms. If the RMS
current in the circuit is 100 amps (a pretty high value), the total
power loss due to ESR would be 160 watts. This shouldn't heat a massive
capacitor very fast. Real high-power experts - comments please.
Ed