How to test capacitors for shorts

[qguy]

I need to test a couple of capacitors for shorts, i have a dmm, the values are from
ceramic 10p to 470p
ceramic 0.047 uf
mylar 0.2 uf
electrolytic 100uf

Are there different ways to test each type for shorts ?

also, please be specific on providing instruction like where to set the DMM.

Thanks

 

[gmphadte]

Ohmmeter test. Do not touch the test leads as your body will add resistance.

Capacitors rarely go short and electrolytics may short. I have seen only two in 30+ years. Dried electrolyte is a common problem.

The electrolytic will show low ohms when u connect the leads and gradually will show increase in resistance. Can be better seen/felt on an analog multimeter.

Gajanan Phadte

 

[FdW]

You could add a resistor in series with the capacitor and then connect it to its nominal voltage. Then you can measure the voltage across the resistor to determine if the capacitor is shorted or leaking.

 

[qguy]

I tried a 2.2 uf capacitor on the DMM set to 2M, the figures started at 0.6 up to 1.9 and then it stopped, this means its good, right ?

Thanks everyone

Ohmmeter test. Do not touch the test leads as your body will add resistance.


The electrolytic will show low ohms when u connect the leads and gradually will show increase in resistance. Can be better seen/felt on an analog multimeter.

Gajanan Phadte

 

[DF96]

Yes. A good large cap will start low and move up and off scale on a resistance range. A good small cap may just be off scale from the start.

Bear in mind that this DMM test only uses low voltage, so a cap may pass but still be leaky at a higher voltage. However, you asked about testing for shorts and the DMM is fine for that. Cap shorts are rare.

 

[qguy]

Ok thanks,

How about for larger caps, I tried a new 2200uf, 35v and when I put the red DMM to the positive side of the caps and the black to the negative, nothing happens, but when I reverse the polarity, I get an increasing resistance, but its negative in value., I tried it with 2 new caps, does this mean that when testing larger caps, I should reveres the polarity ?

 

[sofaspud]

, I get an increasing resistance, but its negative in value.,

Negative resistance? Mhos? Siemans?
Some meters, in resistance mode, may have the + voltage output on the black lead.

 

[DF96]

Some meters measure resistance by putting a fixed current through the resistor and measuring the voltage. A big capacitor will retain some charge so if you reverse the leads you may get a negative reading for a while.

You can check the polarity of the leads with another DMM switched to a voltage range.

 

[qguy]

Negative resistance when dmm is set to 2M and when Red is on negative lead of cap

Negative resistance? Mhos? Siemans?
Some meters, in resistance mode, may have the + voltage output on the black lead.

 

[sofaspud]

When I've tested caps this way (which hasn't been for a while) the resistance would rise to infinity on my DMM. I'm surprised I haven't seen the same thing mentioned in this thread.

 

[qguy]

for smaller caps, on the 2m setting, Mine just just goes from 0.XX and stops at 2.00 when the leads are installed correctly

When I've tested caps this way (which hasn't been for a while) the resistance would rise to infinity on my DMM. I'm surprised I haven't seen the same thing mentioned in this thread.

For the larger caps, I need to reverse the leads to get a reading on the 2M setting, reversing the polarity of the leads, the effect on the display of my DMM is a negative resistance, which is actually correct since the leads were reversed, the resistance actually is increasing since the value say starts from -0.719 to -0.718 etc. What bugs me is that why is the meter is unable to display a positive figure when the leads is correctly placed ??? Maybe its my DMM

 

[JMFahey]

It's not "negative resistance" at all (which exists but is a *very* different concept) but simply, as said before, that you pre-charged the cap one way with your multimeter and when reversing you measure negative *voltage* for a while.

 

[Enzo]

If your meter tries to read resistance, and there is some voltage present in the cap, the meter will interpret this voltage as "negative resistance." Your meter tries to push a volt thropugh the part, and when it sees not only that volt on the other side, but even more, that is how it gets confused.

leakage at working voltage is an important consideration. Especially for high voltage caps as used in tube amps. Your test cap might measure fine at a volt or two, or even 50v, and still act like a wire at anything over 200v. The only way to test for that is to apply that much voltage to the cap. The old "magic eye" cap testers from Heathkit, Eico, etc were pretty good at that. My old Eico will put up to 500v on the test cap.

 

[FredsBands]

On film capacitors you can add series resistance to make the RC time constant slow enough to read on the meter - 37% of full scale is one time constant. that should give you a very rough indication of the capacitance when you do the math.
On electrolytics, when good the resistance should approach infinity, but most electrolytics esp the large ones will have leakage resistance which will limit the reading to less than full scale. Now if the mfd's are large, your meter will only slowly fill the cap with charge, so the voltage will rise very slowly (big RC time constant). Shorted will be obvious, as the resistance will be maybe only a few ohms; bad leakage will show maybe only a few hundred or thousands ohms.