"Ross Herbert" wrote in message
...
On Fri, 2 Jan 2009 11:11:21 -0000, "Arfa Daily"
wrote:
:
:"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.
etermining 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
ne, 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
:
I agree entirely. With regard to everyday servicing you only really
require a
"relative" indication. In some cases some intuitive interpretation on the
part
of the user is required to determine whether a cap is "bad" and requires
replacing. Obviously, if the indication is way too high it doesn't take
any
intuition to decide to replace.
A very good example of this occured on my bench last Friday. I had a Sony
DVD / HC that was very 'iffy' at the beginning of a disc. It would freeze
and pixellate pretty much at random, but if you left it and it managed to
get past the intro without totally falling over, it would play ok for the
rest of the disc. On auto setup, it would proceed ok on single layer and CD
discs, but fail every time on a dual layer. I had a laser for the model on
the shelf, so tried it, but it was just as bad. I then turned my attention
to the little servo board under the deck, where there were several surface
mount electrolytics. For no other reason than that similar caps in a similar
location give all sorts of odd problems on many of the Sony CD players from
a few years ago, I got out the ESR meter, and ran it over these caps. Now I
don't know about your experience of ESR and SM electros, but I've always
found that even with brand new ones, the ESR is significantly higher than
you would expect for the same value and voltage of a through-hole type.
There were two 22uF 10v ones on the board, and the first one measured 5
ohms. Now if that was a through-hole one, you'd immediately say it was bad,
but for a SM, that is quite possibly a satisfactory figure. When I measured
the other one, it went 3 ohms, so I found another similar one on another
board, and measured that. It also went about 3 ohms. So, based on the fact
that two of them read one value, and a third read a slightly different
amount, I applied experience, gut feeling and the measurements, and went
ahead and replaced it. Result ? Total cure, and the ESR meter played only a
supporting role in achieving that.
Arfa