Testing Caps - Bad, leaky, etc

priglmeier

Well-known member

Donor 2016
Joined
Feb 26, 2008
Messages
1,696
Reaction score
25
Location
richfield, Minnesota
So I'm a bit rusty on some of my electronics repair... it's been a looong time.

Testing for bad caps on a PCB after discharging them with a screwdriver. I hook my digital multimeter up to the caps measuring Ohms:

If the cap is bad / stuck it reports more than 1 Ohm?

If the cap is good it reports OL on the meter.

If the cap is leaky?

Please clarify
 
I'm not sure if you can test a cap with a regular meter. I use a capacitance meter, which will show you the capacitance reading of the cap so you can compare it to the capacitance rating marked on the cap. I usually check them out of circuit.

I think you can get digitial multimeters that also have a cap setting built in. Mine doesn't have that, so I bought a separate cap meter.
 
I have a Fluke 179 that will test the caps. It works in circuit as well...pretty well. If the reading is good, it is good. If not, I test it out of the circuit to make sure it is really bad. I don't think most DMMs will test them though.
 
If you're going to test a cap out of circuit, you may as well just replace the cap altogether. Caps are cheap.
If you are talking about a low voltage common cap, I agree. I don't have every cap value - especially high voltage filter caps which aren't usually that cheap.
 
Bad caps are supposed to read as shorted and good caps are supposed to have a very high resistance or OL if you are using the ohms setting.
 
theres actually a capatience meter out there somewhere that could test em.

If i go through tor trouble to pull a cap and suspect it, i just replace the damn thing...
 
Cap Checker

I use an ESR meter. By EDS, Electronic Design Engineers. Cost is around $180.00
Well worth the investment if you work on alot of switching power supplys.Sort open in circuit with an ohm meter is hard to do. Especialy when there are resistors coils other caps invloved. I don't mind shot gunning caps on a monitors, but why do it on a board with 20 caps on it?.

I also picked up there Leak Detector (another $180.00). It beats pulling leads and cutting traces when you are trying to find a shorted component on a large board that is drawing the power supply down. Both of them have payed for them selves in the first 2 weeks I bought them.
 
You can check caps with a DMM if thats all you have. You can check them by using the resistance setting (Ohms). You put a lead on each leg of the cap (assuming it's discharged) and you should see a sharp increase in resistance then a slowly tapering increase. Reverse the leads and you will see a quick dip in resistance then it will start to rise again. This is best done with an analog meter such as a Simpson 260 (at least that is what I prefer) because you can see the needle dip then rise upon each reversal. But, if you ask me, the best way to test a cap is to apply a voltage, say like 9V from a 9V battery. Connect the battery to the cap (be mindful of polarity) + to + and - to - and then remove it. Test the cap on DC Volts setting and at first you should see a pretty close reading to what ever the 9V batt is reading. If you leave your meter connected you will see the Voltage slowly dropping this is a good way to see if it's charging. To test for a leaky cap, do the same except let the cap sit for a while (15, 30 min to an 1 hr or more) and then re test. It should hold pretty close to the original charge. Keep in mind though, this test method is really only good for caps around 10 Uf and above anything less is too quick to charge and discharge. An ESR is your best bet but you can make due with what you've got.
Greg
 
Back
Top Bottom