I while I wait for my LCR meter to come, I read about how you can test capactiors basic with a DMM. One way is the resistance measurement which I read, if you set to ohm setting and then connect the leads, it should failry quickly read a resistance and get to OL.
I have a 10uF 400v capacitor and I did this, the DMM ran quickly throught he KOhm but then progressed to the MOhm setting and just slowly rises, and takes about 2 min to get to OL.
Does this mean the capacitor is bad?
I have a 10uF 400v capacitor and I did this, the DMM ran quickly throught he KOhm but then progressed to the MOhm setting and just slowly rises, and takes about 2 min to get to OL.
Does this mean the capacitor is bad?
It depends entirely on the test voltage and current output of the meter for a given resistance range...they are all different.
Mike
Mike
Not sure if anyone knows but also, I am trying to understand the basic premise of this test. Is it just applying voltage to let it charge and thus build opposing charge, to attract each other but never touch each other? Effectively becoming an open circuit?
That isn't really a good test. You can't properly test a capacitor, except for a short, with a DVM,
unless it has a capacitor test function built-in. Even my Fluke is not very good for testing capacitors.
unless it has a capacitor test function built-in. Even my Fluke is not very good for testing capacitors.
So you mean, just apply the leads under the continuity test? That is really the only test to do with a DMM?
Yeah, unless it has a dedicated capacitor value test built-in. And I don't really trust that either, since my Fluke
disagrees a lot with my dedicated RLC impedance tester, and I know which of those I trust more.
disagrees a lot with my dedicated RLC impedance tester, and I know which of those I trust more.
Okay the 87V does have a built which I will for fun check against my LCR when I get it. I was just playing around in the meantime 🙂
I suspect you will find significant differences between them, even if it's a cheap RLC tester.
I figured. I am starting out with the aliexpress type of LCR meter that has the Kelvin leads. After that, down the road I will be looking at something more sophisticated but from what I was told and can tell, this one, for 75 dollars is a really great start.
It depends on how quickly your meter acquires a reading and displays it. By that time, the cap could have charged enough to add significant error to the ESR reading attempt. The larger the cap and the sooner you snap the reading up to a point, the lower that error would be. With ESR, it is difficult to get accurate readings without specialized equipment made just for that purpose, since ESR is typically just a small fraction of an ohm in resistance. As for your capacitor reading attempt, what you saw your meter doing is just what I would expect a good capacitor to do in that setting. The ohmmeter sources a current and reads the resulting voltage drop with the voltmeter section in the meter, and then calculates & displays an ohms value. As the test current from the meter charges the cap, the cap charges, the meter up-ranges to a smaller test current, and this goes on until the meter has run through all its upper ranges. The cap finally reaches full charge at the meter's test voltage, stops drawing current, and the meter sees this as an open circuit and displays the OL message. So in short, you would need an LCR meter to read the actual capacitance value and dissipation factor (DF) of your cap. From the DF, you can get a close approximation of the ESR value.
An ecap can have 3 different and not necessarily related problems:
1) Partial loss of capacitance
2) High ESR
3) Excessive leakage current corresponding to a parallel leakage resistance.
The test descibed in post #1 adresses the leakage only.
If the Ohmmeter finally reads OL, this means that the leakage resistance is larger than the limit of your meter. (Meter polarity matters.)
If this is good/high enough depends on the application.
High voltage, low current (tube) circuits are most demanding.
Also leakage might get worse at higher voltage.
Nevertheless this test makes sense given that cap meters typically don't provide leakage testing.
A dedicated ESR meter can be useful because it allows in-circuit measurement.
Just don't expect accurate readings below maybe 0.1R with a 2-lead meter.
DMMs providing capacitance measurement tend to give high readings when a cap is leaky due to the measurement principle.
LCR meters are most accurate (when testing caps out-of-circuit).
1) Partial loss of capacitance
2) High ESR
3) Excessive leakage current corresponding to a parallel leakage resistance.
The test descibed in post #1 adresses the leakage only.
If the Ohmmeter finally reads OL, this means that the leakage resistance is larger than the limit of your meter. (Meter polarity matters.)
If this is good/high enough depends on the application.
High voltage, low current (tube) circuits are most demanding.
Also leakage might get worse at higher voltage.
Nevertheless this test makes sense given that cap meters typically don't provide leakage testing.
A dedicated ESR meter can be useful because it allows in-circuit measurement.
Just don't expect accurate readings below maybe 0.1R with a 2-lead meter.
DMMs providing capacitance measurement tend to give high readings when a cap is leaky due to the measurement principle.
LCR meters are most accurate (when testing caps out-of-circuit).
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You can estimate ESR with a square wave signal generator and a series resistor. The ESR and the series resistor form a divider that controls the voltage step at the capacitor terminal on the edges of the square wave. So with a 100 ohm series resistor and 1V peak-to-peak square wave if you get 10mV peak-to-peak on the capacitor you know its about 1 ohm ESR.
Yes, the ESR can be roughly found from the square wave/scope method.
Using an ESR or LCR meter is quicker and potentially (depending on meter) more accurate.
But ESR has nothing to do with leakage which in tube amps typically causes more problems than the ESR.
Using an ESR or LCR meter is quicker and potentially (depending on meter) more accurate.
But ESR has nothing to do with leakage which in tube amps typically causes more problems than the ESR.
Which can easily be measured by nothing more than a power supply, resistor and a DMM. The capacitor should be charged to it's rated voltage at no more than a couple of milliamps - pick the resistor value from something convenient that will limit the current to this. Measure the voltage across the capacitor and, once it stops rising, measure across the resistor. Use Ohm's law (I=V/R) and you know your leakage current. Nothing precision needed as only a rough value is required.
NOTE: Leakage is NOT something typically measured by modern equipment so don't look for the function on your shiny new ESR meter.
Usual and regretfully necessary disclaimer: Probably high voltages involved so be careful - not only can it kill you, it will hurt the whole time you are dying.
Hal
I often roughly check the capacitance of an electrolytic capacitor with the resistance measurement function of a cheap digital multimeter and a watch. First discharge the capacitor, then connect the meter set to a high resistance range and measure how long it takes until the reading reaches OL. Then do the same with a known-good capacitor of the same capacitance.
The times should be roughly equal. If the capacitor under test takes much shorter to reach OL than the known-good capacitor, it is probably dried up and has almost no capacitance left. If it takes much longer, it is probably due to leakage current. You can then try to reform the capacitor, slowly discharge it and do the test again.
The times should be roughly equal. If the capacitor under test takes much shorter to reach OL than the known-good capacitor, it is probably dried up and has almost no capacitance left. If it takes much longer, it is probably due to leakage current. You can then try to reform the capacitor, slowly discharge it and do the test again.