Electrolytic ESR

[John]

I have a faulty switched mode psu. I've tested the ESR of the electrolytics
and one of them may be faulty. I have not used a cap ESR meter until now so
am not sure if the reading I am getting is indicating a good or a suspect
cap. The cap in question is a 500 Volt 33 uF and the ESR measured out of
circuit is one ohm. measuring frequency is one hundred kHz. I've read
that high voltage electrolytics have a higher ESR than low voltage ones.
Does the 1 ohm figure sound ok or suspect for this cap ? I don't have a
similar valued cap to make a comparison test.

Thanks

 

[Jerry G.]

I would suggest this: Compare the reading to another cap at 33 uF that
you know is good. Remove the cap and check it with an ohm meter to make
sure that it is not DC shorted. An analog ohm meter is the best to
check caps for shorts. The ohm meter should give a reading, and then
drop off. It should eventually reach infinite. A digital meter should
go up to a very high value of reading, but if it is pulse sampling, it
may never reach infinite. Most of the higher cost digital meters use
pure DC for ohm readings. You want to know if the cap is shorted in this
case.

The idea is that in theory you want a low reading on the ESR meter, and
an infinite reading on an ohm meter. The cap would have to be removed
from the circuit to use the ohm meter test. Then again, for the low
cost of a cap, I would consider to replace it, if there was any suspect
of it being defective.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
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I have a faulty switched mode psu. I've tested the ESR of the
electrolytics
and one of them may be faulty. I have not used a cap ESR meter until
now so
am not sure if the reading I am getting is indicating a good or a
suspect
cap. The cap in question is a 500 Volt 33 uF and the ESR measured out
of
circuit is one ohm. measuring frequency is one hundred kHz. I've read
that high voltage electrolytics have a higher ESR than low voltage ones.
Does the 1 ohm figure sound ok or suspect for this cap ? I don't have a
similar valued cap to make a comparison test.

Thanks

 

[John]

The cap value measures in tolerance on a capacitance meter.

Analogue multimeter check is fine. (needle drops back fine)

 

[John]

Thanks for the info. The ESR meter is the one described in Poptronics
(formerly Popular Electronics) magazine a few years ago (with the
corrections to the circuit diagram applied). As soon as I built the tester
I grabbed about 20 electrolytics and tested them all. I was surprised that
even old used one's tested good with ESR less than 0.5 ohm. (The only ones
that read higher are low value caps were the capacitive reactance would be
high at 100 kHz as you would expect) I calibrated the meter using low value
carbon resistors. Its just that I heard that high voltage caps have a
higher ESR than lower voltage ones of the same capacitance value. I guess I
will have to get a new 500 Volt 33 uF cap and compare the reading to the
suspect one.

In passing, are any of your cap meters the Poptronics design ? I find that
if you short the probes and zero set the meter then measure a cap the
reading is beyond the end of the scale indicating even lower resistance than
shorted. (there is no DC blocking cap in the Poptronics design....So, I
put a 0.68 uF in series with the probe lead, that fixes the problem. Now
when I short the probes and zero the meter then measure a cap the meter
never deflects to greater than full scale. I believe the readings are now
good based on calibration with the resistors and also when measuring low
value caps and calculating the capacitive reactance value. The meter reads
the correct ohms value.