Testing electrolytic caps

[punkykatt]

Hey Guys, I was checking through my notes on testing electrolytic caps and one of the tests I have is to: Disconnect the negative lead from ground and check for any DC voltage going to ground, there should be none.  Connect (+) meter probe to lifted end of cap under test and (-) meter probe to ground.  Is this correct?   

[stratele52]

Hey Guys, I was checking through my notes on testing electrolytic caps and one of the tests I have is to: Disconnect the negative lead from ground and check for any DC voltage going to ground, there should be none.  Connect (+) meter probe to lifted end of cap under test and (-) meter probe to ground.  Is this correct?   

This is only good for can capacitor , not for insulated ( from the chassis ) capacitor. And this is for leaking  ( ? ) test only . If no voltage do not mean cap is good.

[jjasilli]

That test would work for any cap, electrolytic or not.  That is, the "output side" of any cap should not give a DC voltage reading - which can be measured from the "output" side of the cap to ground (chassis). If the "output side" of the cap is already connected to ground, then you must lift its ground connection, as you say, to take the measurement.

Connect (+) meter probe to lifted end of cap under test and (-) meter probe to ground.
OK, but note that some electrolytic caps are wired "backwards", such as in the bias supply.

[punkykatt]

The electrolytic cap being tested is a filter cap in the power supply with a B+ of 262VDC at that node. I unsoldered the negative lead from ground, connected the (+) MM probe to that lead and the (-) MM probe to ground.  Im getting  250+ VDC from that negative lead to ground??  I tried several new electrolytic caps and I get the same readings???   Am I doing something wrong?   Shouldnt I be reading zero VDC? I know those new caps are not shorted.

[jjasilli]

I just lifted the ground from a B+ filter cap in an amp on my bench, and (to my surprise) get the same result as you -- about 240VDC on both sides of the filter cap. 

[Willabe]

Try putting a 1 ohm R in series with the ground side of the cap and put your meter across that. Just like  on a tubes cathode when measuring for current.

What you got now?

                  Brad        

[PRR]

All electrolytics will leak.

With a 10Meg meter you will read nearly full voltage.

With a 1 ohm shunt you should read about nothing (if you do, the cap is WAY bad).

Leakage in power supply electros is not a problem, unless the supply can't handle the drain, or the leakage current makes the cap hot. For example: 300V supply, 3mA leakage in cap. Most supplies can tolerate another 3mA. 3mA at 300V is about 1 Watt. 1 Watt in a small cap is significant heat. 3mA may not be bad today, but it will only get worse. Order a new cap from Doug.

So if 3mA is "interesting" we want to meter in that range. Try a 1K resistor. Read voltage. If under 1V (1mA), the leakage is fine. (Does not prove the cap is fine.) If 10V (10mA), then it's pretty sick.

[punkykatt]

Thanks for the replies Guys.  Using a 1k resistor as a shunt on the lifted ground side of the cap and measuring voltage across that resistor , voltage reading was less than 1 volt.  I did the same test to all of the filter caps in the power supply and got various readings but all were under 1 volt.  This is good as far as any caps shorting to ground. Correct?