Peter Easthope wrote:

On Friday, June 8, 2012 8:25:14 PM UTC-7, Jeff Liebermann wrote:

You might also check the remaining capacitors with an ESR tester.
Just because the look good, doesn't mean they are good.


Do you always disconnect the cap for measuring ESR? I've seen
a mention of checking the ESR of an installed capacitor.

Thanks, ... Peter E.


Doing in circuit testing only works if you have a tool that operates
at low voltage at the test point, one that is below diode drop voltages.

These days, diode drops are in the ~.295 volt area.. So the test
device must be presenting voltages below that to get a fair assessment of
the cap, while in circuit. Also, it makes sense to make sure any test
gear you use does not present a voltage to the circuit above that level,
otherwise, you could damage sensitive components, just testing them.

THe last time I looked, 100 mv. was a good area...

When expecting low ESR readings on a cap, it's pretty much straight
forward, doing in circuit test. Because the value to be expected would
be low to start with. Circuit paths away from that area are more than
likely going to register more.

There are times when in circuit testing fails the ESR test with a
tester that is designed for low voltage sampling.

If you are testing for dielectric break down (voltage ratings), then
yes, you most likely will need to extract the cap.

Jamie