On Jan 2, 7:13 am, "Leonard Caillouet" wrote:
"Arfa Daily" wrote in message

...





"Ross Herbert" wrote in message
.. .
On Wed, 31 Dec 2008 09:30:11 -0800 (PST), wrote:

:On Dec 30, 6:27 pm, "Leonard Caillouet" wrote:
: wrote in message
:
:
...
:
:
:
: I have a question about ESR measurements.
:
: I have been doing some reading about ESR and the literature
indicates
: it can be a strong function of frequency. In the design a SMPS I
: assume the ESR of importance is at switching frequencies (at least
: for the line side capacitor).
:
: I have also noticed that often the measurement made in trouble
: shooting are low frequency measurements (step function - looking at
: the abrupt change in voltage).
:
: Is the above accurate and if so when a cap goes bad do the low
: frequency measurements catch the majority of the high frequency ESR
: failures. Are there significant failure modes where a low frequency
: ESR measurement would miss the higher frequency ESR failure?
:
: Thanks Much,
:
: Most of the ESR meters that I have seen test in the 50-200 kHz range.
This
: pretty much covers the switching frequencies of most power supplies,
at
: least in consumer equipment that I am familiar with.
:
: Leonard
:
:Thanks everyone for your replies, you have answered my questions.

Now to throw in a curve ball...

In designing his Tan-Delta meter, Cyril Bateman used 100Hz sine. His
theory was
that using 100KHz could cause the measuring lead inductance to exceed the
self
inductance of the capacitor under test and thus complicate the accuracy
of the
result.

The thing is with a bog-standard ESR meter, it's all about relative
measurements, 'feel', experience, and intuition rather than accuracy.
Determining whether or not an electrolytic is faulty by way of its ESR, is
a bit of a black art, and is with any ordinary 'in-circuit' ESR meter. The
reading just gives you some help and 'feel good' backup. This is why I
have contended on many occasions that such an instrument is not one of
absolutes, and cannot be just picked up and used to give 'go / no go'
definitive answers about the state of a cap, by anyone who chooses to buy
one. It's just a helper that gives you one more pointer, allbeit a mostly
pretty good one, without having to remove the cap (generally) from the
circuitry that it's in.

The thing is with the inductance argument, I would have thought that given
that the self inductance of a 'standard' electrolytic is quite high due to
the way they are constructed, the inductance of the test leads would have
been pretty insignificant in comparison ??

Certainly, in practice, if it is a 'problem', it has never caused me any
trouble with using my Bob Parker to help identify bad 'uns ...

Arfa

Ditto Arfa's comments. I have three ESR meters and they all read
differently, but they giv me enough info to sort out which caps are bad and
which not. And when there is doubt, change them. Caps are cheaper than the
time to worrry about sorting out the accurate measure, which is somewhat
meaningless anyway, as circuits vary so much. What might be bad for one
application may be ok for another. Regardless, if the ESR seems high I just
change them.

Leonard

Just out of curiosity, has anyone tried using an infrared
thermometer to try and find bad or failing caps. I did a quick search
on the on the internet and found units you can get for as little as
$20. The spot size is very small and I believe the top of the cap is
an ideal target. Could use a black sharpie to make a small black spot
for even better measurements. A SMPS cap which had a High ESR should
read high in temp, might be more sensitive than using your finger to
test for temperature. An open or failed cap should read low. Of
course the unit being tested couldn't be completely dead. If it
worked one nice feature would be being able to identify bad caps
before even removing the PC board.

The power dissipated in the cap should be proportional to the
increase in ESR ,the temperature increase should be on the fist order
equal to the power dissipated in the cap. So if the original
temperature rise due to the new caps ERS was 1 degree C and the ESR
increased by a factor of 10 the old caps temperature should be about
10 deg higher, easy to measure.

Might be able to find bad IC's as well. Perhaps the increase in
sensitivity using this device would help find other failing or failed
components as well. Might also be useful in identifing failing or
failed components in very high voltage circuits. Those components you
would not want to touch with power on.

Don't know if the idea has any merit but thought I would mention it.

Gordon ,