On Fri, 06 Jul 2007 07:04:58 -0700, John Larkin
wrote:

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

I agree. It's rude, it's crude and it "sorta" works. I think this other
one: http://www.ludens.cl/Electron/esr/esr.html might work somewhat better.

It occurred to me that if one were to connect a "known good" capacitor
(low ESR, in other words) of about the same value across the other bottom
leg of the bridge (and maybe drive the bridge with a sine wave), then you
would get a fairly good indication of ESR. This could be a cheap fixture
when testing a bunch of caps for ESR, say on incoming inspection.

It also occurred to me that if one's purpose were to test electrolytics to
be used in 60 Hz power supplies, perhaps another technique might be
workable.

Use a sine wave source of, say, 1kHz to 10kHz. Wire up an opamp based
impedance converter to synthesize a negative resistance. Let an ordinary
potientiometer be the resistance that is converted (one-to-one) to a
negative resistance. Place the capacitor under test in series with the
negative resistor and drive the combination with a low impedance sine wave
source. Measure the current through the combination and dial the
potentiometer around until the current is maximized. The resistance of the
potentiometer is then equal to the ESR of the cap.

A negative resistance could be synthesized in the kilohertz range easily.
This could even work at higher frequencies if the impedance converter is
made to work ok.