On Thu, 05 Jul 2007 21:25:46 -0700, The Phantom
wrote:

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...

This is the same circuit:

http://www.qsl.net/iz7ath/web/02_bre..._esr/index.htm

and here's the schematic:

http://www.qsl.net/iz7ath/web/02_bre..._esr/fig03.gif

The key to understanding how this operates is to note that a square wave is
applied to the bridge. If a capacitor with zero ESR is connected to the
DUT terminals, the output from the bridge will be only some tiny (area)
spikes occurring at the edges of the input square wave. If the DUT has a
non-zero ESR, then the bridge output will have a square wave component
proportional to the ESR value, and this will deflect the meter.

Technically true, except that it measures impedance, not ESR. And it's
a lot more complex than it needs to be. It will tell the difference
between open electrolytics and good ones, at least for large C values.

And it's not "proportional to the ESR value" except under a narrow set
of conditions.

John