Discussion in 'General Electronics Chat' started by Rino, Jan 19, 2009.
How can you know if a capacitors is still good or not?How can you use a multi-meter to check it?
Of course the first test is to determine if the cap is shorted with the ohmmeter. If it is, toss it.
After shorted caps are eliminated, a VOM or DVM can perform crude condition evaluations of capacitors above a value that depends on the characteristics of the ohmmeter function. The basic test consists of observing the response of the meter at the instant of connecting the probes across the cap. If the capacitance is large enough and the ohmmeter series resistance is high enough you will see the pointer or display plunge toward zero ohms, then slowly (for a large cap) or very quickly (for a small cap) rise again toward infinite resistance. This behavior results from the charging of the cap by the internal circuit of the ohmmeter.
A low sensitivity analog ohmmeter (for example, my inexpensive 1000 ohms/volt analog meter I built from a Heathkit more than 50 years ago) can only show a response to a cap of perhaps 1.0 MFD or more, and that value causes a barely visible tic of the pointer. My 20,000 ohms/volt Simpson 260 analog VOM shows a barely visible pointer response to a 0.01 MFD cap. My Fluke 77 autoranging DVM shows a fairly strong response to a 0.03 MFD cap.
What you are seeing is the meter responding to what is essentially a short circuit across its leads at the instant of contact with the leads of a discharged cap. The capacitor charges up to the ohmmeter probe voltage at a rate dependent upon the internal voltage applied by the ohmmeter circuit and the internal series resistance in the ohmmeter circuit. If the cap is good and has negligible leakage, the ohmmeter will eventually reach an infinite resistance reading. A leaky cap will cause the meter to stall part way to the infinite reading. A shorted cap will cause the meter to show a very low resistance without any recovery. If the cap is large and the response is too slow for your patience, switch to a lower resistance range. Typically for small caps the highest resistance range gives the best response.
For a non-polar cap, the meter response will be about doubled if you reverse the leads on the component because the cap has already received a charge during the initial contact prior to lead reversal. Be sure not to touch both leads when reversing the leads because your body resistance will quickly drain off the charge left on the cap.
This is a very handy go/no-go test of a cap in the field. It typically won't work reliably with both leads of the cap still connected to the circuit due to parallel resistance.
If the cap is large enough that you can assess the speed of response, you can make a crude estimate of capacitance by comparing the meter response to that from a known good cap of known value close to that which you are trying to evaluate.
The meter tic can be useful but also misleading if you then assume the cap is ok
Cheap DMMs are available with a cap tester.
I got one for 5 Euros.
It's a start.
It may be useful to know that the approx time to charge is RC for a resistive source with constant voltage or ( C/i ) V for a constant current with a max voltage of V