Final Notes
Most projects hit a snag or two along the way, and so
did this one. If you look carefully, you may spot a small
inconsistency between the photo of the printed circuit
board in Figure 3 and the ExpressPCB layout file included
in the online files. This is the result of an initial design goof
on my part, which required me to cut a couple of PCB
traces and re-locate components R7 and C4. I revised the
PCB layout after the fact, and the ExpressPCB layout file at
the article link has these corrections and agrees with the
schematic.
This meter is — in principle — suitable for checking a
capacitor’s ESR without removing it from the equipment
it’s connected to. The impedance of the surrounding
circuitry is normally much higher than the ESR being
measured, and the voltage developed across the CUT is
quite small: less than 100 millivolts — much too low to
switch on any semiconductor junctions in the vicinity.
Power to the equipment should be off, of course, and the
ESR meter should probably be running off an isolated
power source like a 9V battery. I have not tried this type
of measurement myself, but I see no reason why it would
not be successful.
At this point, I would like to mention some limitations
of this instrument, or of almost any ESR meter:
1) This meter is not suitable for testing capacitors less
than 30 microfarads. If the CUT is too low, the reactance
at the measuring frequency becomes significant, resulting
in an excessive ESR reading. The solution to this problem
is to redesign the system to use a higher frequency. If the
need arises, I may try this as a future
project.
2) A capacitor with an internal
short circuit will appear to have a
misleadingly low value of ESR, so
don’t be fooled (as I have been).
Check with a DC ohmmeter if
there’s any doubt.
3) Because an ESR meter is
essentially a low range ohmmeter,
long test leads from the CUT can
contribute errors to the ESR reading.
4) ESR can depend on external
factors such as temperature or
applied voltage, so a capacitor may
behave a bit differently in a real
circuit than when it is being tested
all by itself.
5) Although this unit has some
protection built into it, applying a
fully charged high value capacitor to
the test terminals could damage the
circuitry. It’s always a good idea to
manually discharge a capacitor before testing.
One final remark: ESR measurement does not usually
require a high degree of accuracy, and the meter
described in this article should be adequate for routine
troubleshooting. In my case, it was very helpful in
identifying questionable components, possibly saving me
some hair-pulling/teeth-gnashing frustration on a future
project. NV
January 2016 29
■ FIGURE 7. Rear panel of the ESR meter.
