On Mon, 24 Jan 2011 19:20:58 -0600, Jeffrey Angus
wrote:

On 1/24/2011 6:40 PM, William Sommerwerck wrote:
Set the audio gen to about 100kHz and connect the output
across the electro under test -- then connect the scope probes
directly to the same cap (not the generator). You should see a
small voltage at 100kHz on the scope.

If one takes that literally, the scope probes /will/ be connected directly
to the generator.

Did you leave something out?

Nope, it IS connected directly to the generator.
And the capacitor across both.

Retch.

You see a reduced value of signal due to the ESR of the capacitor
vs the source impedance of the signal generator.

That would work if you knew the exact source resistance of the
generator and that the generator is not going to try and protect
itself from what it considers to be an AC short circuit load.

If you have 1 volt of output swing, and an assumed function generator
output impedance of 50 ohms, the short circuit current will be about
20ma. Shove that into a 0.5 ohm electrolytic cazapitor, and you'll
measure maybe 10mv across the cazapitor. That's kinda hard to see. Of
course you could supply more than 1 volt but then you will have some
problems trying to use this technique without removing the cazapitor
from the PCB.

For what it's worth, I did it this way until I bought an ESR meter.
Later, I decided to actually measure the output impedance of my
function generator. The data sheet said 50/75 ohms, but it was
anywhere between 30 and 100 ohms depending on the position of the dial
and output step attenuator. So much for accuracy.

If you look carefully at the schematics supplied below, most of them
put a known series resistance at the output of the generator section
to the cazapitor. You could do the same thing with the function
generator. 100 ohms in series with an alleged 50 ohms will not
magically make the method more accurate, but it will reduce the error
to a tolerable level.

Pretty simple.
Not direct reading, but accurate comparison.
Jeff

Nothing is simple, but these examples might help:

Measuring Capacitor Self-inductance and ESR
http://www.emcesd.com/tt020100.htm

Oscilloscope ESR Tester
http://electronics-diy.com/electronic_schematic.php?id=948

99 Cent ESR Test Adapter
http://octopus.freeyellow.com/99.html

More links at the bottom of this page:
http://en.wikipedia.org/wiki/ESR_meter

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558