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Goober
05-07-2011, 10:47 AM
Can one completley test a capacitor with only a bog standard multimeter?

I don't ever remember being taught how to test a capacitor. Unless its obviously blown apart, short citcuited (using ohm tester) or burned out I've never for certain known if its faulty by using a multimeter. IF in doubt I always used my Hermetic Compressor tester or just replaced the suspect Cap. I know some multimeters today have a Cap test function, but I'm talking about a bog standard meter I would have used.

I do remember somewhere someone telling me that if the ohms resistance rises steadily when metered acros the terminals that the Cap was ok. Another more doubtfull test was to put a live and neutral onto termianls and flick the power to the cap, then do the old short out the terminals to check if Cap discharges......not even sure if that last test is real?

Can anyone shed some light on this long to murky bit in my knowledge.

Just talking about hermetic compressor and fan motor Caps here...is there any other?

Stevo

Brian_UK
05-07-2011, 10:53 PM
A small test, not a complete test, as stolen from another forum...

You can use your multimeter as an ohmmeter to test the capacitor.

1) Discharge the capacitor by shortening its leads. That is - use a wire and connect the leads of the capacitor together. This will discharge it.

2) put your multimeter in the high ranges 10K-1M

3)CONNECT MULTIMETER TO CAPACITOR LEADS(OBSERVE THE POLARITY IF ELECTROLYTIC). AT SOON AS THE LEADS MAKE CONTACT, THE METER WILL SWING NEAR ZERO. IT WILL THEN MOVE SLOWLY TOWARD INFINITY. FINALLY THE METER WOULD COME TO BE INFINITE OHMS BECAUSE THE CAPACITOR IS BEING CHARGED BY THE BATTERY OF THE MULTIMETER.

4) IF THE CAPACITOR IS BAD, IT WILL GO TO ZERO OHMS AND REMAIN THERE. THIS IS CALLED A SHORTENED CAPACITOR

5) IN THE CASE OF AN OPEN CAPACITOR THERE WILL BE NO OHMMETER INDICATION.

6)SOME CAPACITORS HAVE A LOW DIELECTRIC LEAKAGE. YOU WILL KNOW THIS IF THE OHMMETER COMES TO REST AT A POINT LOWER THAN INFINITE. TEST A KNOWN GOOD CAPACITOR OF THE SAME TYPE TO BE SURE

install monkey
05-07-2011, 10:59 PM
the more doubtful test does work but make sure u have spare undies in ur toolbox for when u discharge it! the best way is get a capacitor tester for about 70. some capacitors on vrv systems are 10,000 microfarad

mikeref
05-07-2011, 11:30 PM
Goober. Firstly full marks to Brian on explanation, and install monkey for the reality factor:D, but for me, i use Fluke with test feature on board. Start caps have a bleed resistor that alter readings. use a megga if you want to test charge caps then the handy screw driver to discharge.( Guess most did that in trade school :)

Goober
06-07-2011, 03:09 AM
Brian UK, yep, found that same site explanation myself....

Install Monkey, I knew it was real...

Mikeref, Meggar another way to test...not on the tools anymore, become a shiny trousers and am slowly forgetting all my hands on knowledge, so don't really need to by one of them new fangled multimeters. But do still drool over them and those digtial gauges whenever looking at the tools at the suppliers!

Thanks for the replies, but it does seem that we can only do so much with a multimeter and not categorically test a Cap.

Magoo
13-07-2011, 03:45 AM
Hi Goober.
splash out and puchase a Fluke multimeter. Removes all the doubt and speculation.