As detailed in the sidebar
(“What a Capacitor Really Looks
Like”), a number of factors
contribute to power loss in a
capacitor. These losses can be
lumped together as ESR, which
looks like a small resistance in
series with an ideal (lossless)
A simple technique for measuring ESR is to supply the
capacitor with a known AC current (Icap) at some
frequency where the reactance of the capacitor is very
low so that the ESR dominates. Measure the resulting AC
voltage developed across the capacitor’s terminals (Vcap)
and you can find the ESR by dragging out Ohm’s Law:
ESR = Vcap/Icap
This is the basis of the ESR meter I describe in this
article. A glance at the equivalent circuit model shown in
the sidebar should make this clear.
All capacitors have an inductive component which
can possibly interfere with the ESR measurement. In some
ESR meters, a square wave or pulsed source is used to test
the capacitor, and the
resulting inductive spikes
can cause an abnormally
high ESR reading.
Accordingly, I have
incorporated a sine wave
source into the design to
avoid this possibility.
The block diagram in
Figure 1 shows that the
ESR meter is made up of
four fundamental sections:
1) a sinusoidal oscillator to
supply an AC current to
the capacitor under test;
2) an ESR detector to
sense the AC voltage
developed across the
capacitor; 3) a meter
amplifier and rectifier to
display the ESR on a panel
meter; and 4) a power
converter and voltage
regulator section similar to
that found in many
The full electrical
schematic diagram of the
ESR meter is shown in Figure 2.
This supplies the necessary AC signal for driving
current through the capacitor being tested. The circuit
here runs at approximately 100 kHz, which is an industry
standard for making ESR measurements. One section of
dual op-amp U1 functions as a phase-shift oscillator in this
application. I like this circuit and have used it in several
projects. It’s simple to implement and gives a pretty good
approximation of a sine wave. It’s ideal for generating a
fixed-frequency signal through audio frequencies and
beyond, if the requirements are not too demanding.
The other section of U1 acts as a buffer and amplifier.
January 2016 25
■ FIGURE 1. Block diagram
of the ESR meter.
■ FIGURE 2.
schematic of the