Monday, April 20, 2009

used pulse capacitors on eBay

Before you decide to spend any of your hard-earned money on used pulse duty capacitors from eBay or any other source, be aware that pulse-rated capacitors used in pulse-discharge service have a finite shot life.

That shot life can be calculated if you know all the circuit parameters and a fair amount about the capacitor.
Of course, buying random used caps, you cannot know how they were used and/or abused.

However.  Most of the big ones came from government projects.  Those people know everything it is possible to know about the capacitors and the circuits they are used in.  They calculate the shot life, and then, when a capacitor is within 10% of the end of its predicted life, they pull it from service and surplus it.

THAT'S what you're buying.

Now, this may not be such a bad thing.  The biggest enemy of pulse capacitors is voltage reversal.  That's the "ringing" which occurs when a capacitor discharges into any inductive (ie; not purely resistive) load.  Pulse capacitors usually have a rating for maximum voltage reversal, expressed as some percentage of the original charge voltage.  Generally, they "like" to discharge into a critically damped load (having little or no voltage reversal) but in the real world, that isn't often possible.

But if you run a pulse cap at something like half of its rated voltage, the worst voltage reversal you could cause should be well below its maximum rating.

I'm not saying don't buy them at all, I'm just saying it would be foolish to expect them to last for more than a few hundred shots (maybe less, depending on the original design spec) at their maximum (label) ratings.

Caveat Emptor.

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