Subject: Capacitor testing
From: "John"
[email protected]
Date: 4/1/2004 8:03 AM Central Standard Time
Message-id: <
[email protected]>
Hello.
I have a question on capacitor testing (plenty on the web about caps but not
basic testing).
You have no access to a capacity meter, you only have a DVM, plenty of
bulbs and some batteries.
I know that using a moving coil meter you just select resistance, as the
capacitor charges the meter movement moves.
How can I test a cap with a DVM (no cap range) to indicate that it is
functional.
Thanks.
Hi, John. DVMs measure resistance the same way the older Simpsons did -- put a
current across the resistor, then measure the voltage across it.
You can do a quick cap check for larger value capacitors the same way -- set it
to measure the highest resistance range, or use the range hold button to hold
it in a meg-ohm measurement mode, and just attach the leads to the cap (red to
cap +, black to cap -). You should see the reading on the DVM gradually go up
from zero to full scale. Especially for the cheapie DVMs, you might want to
make sure the cap is discharged before you test.
If your cap isn't tantalum, you also might want to try reversing the leads, and
seeing the reading go from negative overload up through zero and then to
positive overload.
If you don't have a range hold button but do have autorange, scrounge a
high-value resistor at the lowest end of your highest resistance range (say, if
your highest range is 20 Meg, get a 2.2 Meg resistor), and charge the cap up
with the resistor in series. That will keep the autorange from shifting down
(and also increasing the current across the DUT). This will also prevent the
meter from starting at zero, but you will still see the reading gradually rise.
You can't do the lead reversing thing here, though.
Note that this is only a very basic test to see if the cap is dissipating
current slower than the DVM is sourcing it -- I'd expect a little more from a
cap test. Also, for smaller value caps, it won't tell you if the cap is open
instead of shorted, which frequently happens. The voltage across the cap rises
too fast to distinguish between the two.
Good luck
Chris