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DiagramElectronic Circuit Design
ESR Meter (Equivalent Series Resistance Meter)The ESR
(Equivalent Series Resistance) of an electrolytic capacitor is
normally just a low voltage capacitor (such as a 1000uF, 16V cap),
small fraction of anOhm for a high capacitance, and can be as high
as two or three Ohm for a low capacitance, high voltage cap
(1uF,450V). This resistance increases when thecapacitor ages, and
it often does do such a dramatic way that the equipment completely
ceases to function or even blows up semiconductors. To
findcapacitors that have degraded to 100 times their normal
resistance while their capacitance remains fine is very common.
An additional beauty of an ESR meter is the capacitor can be
checked in almost all cases while they are in circuit. This is so
because a good capacitor wouldmeasure like a short circuit, so
while measurement, any other parts connected in parallel will have
minimal influence. This features are the reason why anESR meter is
irreplaceable tool for troubleshooting electronic equipment.
The circuit below works by applying a 50kHz, 200mV square wave
to the capacitor under test, in series with a 10 Ohm resistor. The
AC voltage occurringacross that resistor is measured and displayed
on a meter. So the whole thing is just a simple ohmmeter that uses
ultrasonic AC for measurement instead ofusual DC used by every
common ohmmeter. It does not make semiconductor junctions enter
conduction, which further helps to make this meter suitable
forchecking capacitors mounted in a circuit since the AC voltage
used is so slow.
The transformer is the only part that could pose problems to
inexperienced builders. This transformer is made using an Amidon
ferrite core, type EA-77-188,which is a tiny double-E core having a
cross section of 22mm2, and external dimensions of about 19x16x5mm
total. The nylon bobbin that Amidon deliverswith it is used, wound
a primary winding consisting of 400 turns of AWG #36 wire. As
secondary, 20 turns of AWG #26 wire is wounded. You can adjust
theturn numbers in inverse proportion to the cross section area if
you have a larger or smaller core. The wire size is not important,
the gauges I used are about 3or 4 numbers thicker than necessary,
while at the same time this bobbin has room for wire at least two
numbers thicker than the ones used. Thus, for eachwinding, you can
choose from about 6 different wire gauges with negligible impact on
the performance.
Adjust its set screw for accurate zero position using the
galvanometers original scale. Short the test leads together with
the circuit powered up, and adjust R11precisely for a full-scale
reading. Now, take the meter front cover off, get a pencil and a
few resistors in the range of 1 to 22 Ohm or so. Mark
thecorresponding deflections on the meter scale using the resistors
as test objects. You keep this crude hand-drawn scale indefinitely,
or you use it as a templateto draw a definitive scale on the
computer, print it and install it in the meter is depend on your
choice. [Source: Homo Ludens Electronicus]
May 26, 2010 | Testing and Measurement | Tags: ESR Meter
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