Can a capacitor have low ESR and still be bad?

[amelbye]

Can a capacitor have low ESR and still be bad?

I'm wondering, because I just swapped out a bunch of high ESR caps from a power supply, and the unit went from faulty to dead...

I checked the ESR on all the parts I put in (didn't have enough new parts around, so I took some from a project I've given up on)

Could it be that some of the replacement capacitors have other problems that doesn't affect their ESR readings?

[PCBONEZ]

Re: Can a capacitor have low ESR and still be bad?

If a cap is shorted it will read low ESR.
-
The way to tell is to also check DC resistance with a DMM.
If it reads low on both it's a shorted cap.
-
Most caps won't have to be pulled to check resistance.
Only need to pull the ones that read low to make sure it's not a parallel resistance issue.
If they read high in circuit they will still read high out of circuit.
-
Also check that you didn't make a solder bridge or a cold joint somewhere.
.

[b700029]

Re: Can a capacitor have low ESR and still be bad?

The capacitance could also be too low.

[Agent24]

Re: Can a capacitor have low ESR and still be bad?

the unit went from faulty to dead...

What was the original fault?

Do you know what ESR value the old capacitors _should_ have been? If your new ones had ESR much lower compared to what the originals had when they were brand new that can also cause issues.

(Though I believe that the PSU will still work in such a case, but you will get worse ripple than there should be)

[999999999]

Re: Can a capacitor have low ESR and still be bad?

How do you define "faulty" as it was previously? Were the caps visibly vented? I am wondering if you simply caught a PSU fault in its early stages and the existing damage got worse. Electronics like to do that the *next* time they're cold powered on.