On Sun, 8 Jul 2007 12:27:49 +0100, "john jardine"
wrote:


"John Larkin" wrote in message
.. .
On Fri, 06 Jul 2007 02:54:06 GMT, "Lord Garth"
wrote:


"John Larkin" wrote in
message
.. .


It doesn't measure ESR, it measures a sort of twisted/nonlinear/offset
version of total impedance. It probably will tell open electrolytic
caps from good ones, but the thing the needle indicates isn't ESR. For
very big C's with low ESL (ie, when Z is mostly ESR, and low ESR at
that) then it mostly measures ESR.

For a similar level of complexity, one could measure true ESR. Or for
a lot less complexity, one could measure what this thing measures.

John


Circuit please...


Something roughly like this, maybe. This is a 2-minute hack, so don't
jump all over it right away; a real design would take more time.
Basically, it's an oscillator that dumps current into the cap, and a
synchronous detector to remove most of the orthogonal component. It
ain't perfect, but should be a lot better than a scalar measurement.

John


This was built along similar lines. The synchronous rectification is not
perfect due to phase shift in the forward signal path but the arrangement is
useful for reducing the annoying capacitive quadrature component from Q's up
to about 3 (at the 100kHz). It's usable down to the milliohm area and proved
the (cheap) 1000 off 10uF cap's I bought, had an ESR of between 1 and 3
ohms!.
OTOH, the very expensive "BC" 10,000uF 63V caps I used on a job had an ESR
of 5m ohm. 'Spose you get what you pay for.


The problem with all these circuits is the simplicity of the meter
needle: it can't untangle the inductive, capacitive, and resistive
components. If I were routinely repairing stuff that may have bad
caps, I'd build a scope probe that injected a square wave current into
the tip and displayed the result on the scope, with switchable
frequency between, say, 1 KHz and 100 KHz. The scope vertical gain
switch adds a lot of rangeability. This could be useful for all sorts
of things, not just caps. Diodes, too!



--- esl
/\ --------
/ \ /
/ \ ------- slope = 1/C
/ \ /
/ ------ -- esr
-----------


John