On Tue, Feb 01, 2011 at 07:48:40PM -0500, Ben Okopnik wrote:
>
> set your meter
> on a medium resistance setting and touch the leads to the terminals
> (take the bulb away, of course.) Next, reverse the leads and watch the
> meter as you touch them to the terminals: it should spike and then
> slowly come back to a relatively high resistance value. If it doesn't
> spike, or doesn't discharge to a high resistance value, it's bad.
On re-reading, I realized I should give some more details for anyone
who's never done this before. On the first pass, hold the leads on the
terminals for a good 5 seconds or more; you're actually charging the
capacitor from the meter (that's why you need it to be on the resistance
setting, which puts out a little voltage.) If the meter shows a steady
short, or some steady low value during that phase, you've already
diagnosed it: it's shorted. Next, when you reverse the meter, you want
to watch the scale very closely: depending on the value and your meter
setting (higher is better for small cap values), that spike might just
be a very short twitch of the needle. E.g., a 1μF capacitor when tested
with a meter with 100kΩ internal resistance will discharge in ~ .63 * R
* C seconds, or
.63 * 100e3 * 1e-6 = .063s
...less than a tenth of a second. Gotta watch it pretty carefully.
Fortunately, most AC caps tend to run from 5 to 50μF or so, which is
slow enough that you should even be able to roughly estimate if the cap
value is somewhere near what you want - as long as you know your meter's
internal resistance, that is.
Ben
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