A Simple Analog ESR Meter

I was always envious of my friends analog ESR meter that allowed him to check and find bad capacitors while they were still in circuit. The particular meter he had is no longer available. It worked particularly well in finding bad electrolytic capacitors in tube amplifiers.

See also My First PCB, which describes the process of designing a PCB for the ESR Meter.

If I was to have my own such meter I would need to do some research to understand how they worked and to come up with my own version. First, what is ESR? ESR stands for the equivalent series resistance of a capacitor. ESR is frequency-dependent, temperature-dependent, and changes as components age. It’s typically important for ‘Wet’ aluminum electrolytic capacitors used in power supplies to have a low ESR. The typical method used for measuring ESR is to supply the capacitor with a known AC current (Icap) at some frequency where the capacitive reactance of the capacitor is very low so that the ESR dominates. By measuring the resulting AC voltage developed across the capacitor’s terminals (Vcap) the ESR can be determined with Ohm’s law: ESR = Vcap/Icap

Next steps will be developing PCB artwork that will make it easy for others to make their own meter. It would also be nice to come up with artwork for the meter instead of using a Post-It® Note.

Projektelemente

Block Diagram (JPG, 40.69 KB)

Most of the designs I found worked along the same lines as this block diagram

Split Rail Supply (PNG, 73.8 KB)

This ESR meter operates of a single 9 volt battery. The plus, minus and ground voltages needed for the operational amplifier are derived using this opamp voltage follower with current boost.

Bread Board (JPEG, 2.2 MB)

To test out the circuit I built up a conventional bread board. BNC connectors allow for easy monitoring of the various waveforms. Initially for testing I simply looked at the signals with an oscilloscope intead of the 500 micro amp meter.

Testing the circuit (JPEG, 2.31 MB)

In this picture you can see the output the oscillator as well as the output of the ESR meter while testing a 50-microfarad electrolytic capacitor.

Waveforms of ESR Test (PNG, 246.3 KB)

In this waveform dump from the oscilloscope you can see the difference between a capacitor with a high ESR in blue and a capacitor with a low ESR in orange.

Practical Test (JPEG, 1.93 MB)

Even before I was able to package the meter into a case I used it to trouble shoot our air conditioner by locating a bad motor capacitor.

Completed Prototype (JPEG, 2.09 MB)

The completed ESR meter is sufficiently portable so that it can be used remotely and away from the work bench. Simple Post-It® Note calibration was accomplished with a handful of 2-ohm resistors. The ESR meter works like an ohm meter. Before measuring a c

Internal Construction (JPEG, 1.96 MB)

The circuit is wired on two prototyping Perfboards. The smaller board on the left is the circuit used to derive the plus, minus and ground references for the op-amps. The larger perfboard is The ESR meter circuit and is held in place by the nuts on the m

Schematic (PNG, 83.3 KB)

In the circuit I decided to build I used several design elements from the DIY examples I found on the Internet.

ESR_Meter_KiCad_15032022.zip (ZIP)

KiCad PCB project

Simple_ESR_Meter.pdf (PDF)