Hi, Roy.

I don't know if you received my reply in sci.electronics.repair as the
bbs here has been having problems for the past week or so. Here's the
original reply:


"Roy J. Tellason" bravely wrote to "All" (18 Oct 03  20:47:54)
 --- on the heady topic of "ESR meter?"

I've been testing my homebrew adaptor for a dmm to measure ESR.
It uses a common NE555 timer (any mfgr). This is what it looks like:

:   ESR adaptor for DMM
:   -------------------
:
:            o +9V *regulated
:   ,--------|
:   |        |
:   |  .-----------.
:   `--:4    8    3:---------+--3K9----+----o-------> probe
:      :           :         |         |    |
:      :   "555"  6:--.     10K        |    |    set DMM to
:      :  timer    :  |      |       200R (DMM)  200mV AC range
:      :     1    2:--+------'         |    |
:      `-----------'  |                |    |
:            |       === 3n3F*         `----o-------> probe
:           _|_       |                    _|_
:           ///      _|_                   ///
:                    ///

The operating principle of this circuit depends on the DMM being able to
measure "AC" up to about 100K Hz. Many DMM's are able to do so
accurately on the lowest voltage range since this bypasses the
attenuator but you will have to try and see.

The output voltage is about 200milliVolts so it won't bias junctions
(except Germanium). It will thus work well in-circuit as long as the
impedances are much greater than 200 ohms (which is often the case).

The 3n3F capacitor (0.0033uF) sets the output frequency to about 30KHz
which is a medium value that will measure capacitors well between about
10uF to about 100uF.

Larger capacitors may show a false reading because the inner layers are
shunted by the outer ones. So for large capacitors of over 220uF the
3n3F cap should be replaced by a 100nF cap (0.1uF). The lower test
frequency will measure the inner layers better this way.

With capacitors of about 4u7F (4.7uF) and less the capacitive reactance
becomes a significant contribution at 30KHz so a capacitance of 1nF
should be substituted for the 3n3F timing capacitor. The problem of
course is that the DMM now has to measure AC at 100KHz and not all can.

The only critical part is the voltage supply which must be a regulated 9
volts. Otherwise you will have to re-calculate the output divisor ratio.
Sorry about it but aside from that this simple adaptor works well.

Discharge capacitors first!!! I guess a couple of protection diodes
on the input might help in case one forgets.

 Mike
 ****

... Resistance Is Futile!   (If < 1 ohm)
--- Blue Wave/DOS v2.30