ESR Meter for Testing Capacitors

Update: It took me 8 years but I have uploaded a video showing how well this works.

 

Update: Schematic showing parts values available on request.

I always had a feeling I should have an ESR meter but the high cost of commercial kits kept putting me off. I wanted something that was more than a go / no-go indication. However, I didn't want a digital display. As time passed I tucked away some of the circuits I came across.

One day, when looking at some air conditioning controller boards I had been given, I noticed an IC marked LMV324. Turns out this is a 3V single supply version of the LM324. Since I had recovered several of these, and faced with a task where an ESR meter might be useful, I set about amalgamating some ideas I had seen.

The result was a resounding success. I could discriminate between shorts, 1 ohm and 0.33 ohm resistors with ease. Testing on capacitors showed that for the same value and type of capacitors I could easily detect which of a group had a higher ESR. This was true whether the capacitors were in circuit or not.

Here is a brief overview of how it works. The IC contains four op-amps. One op-amp is configured as an oscillator around 50-100kHz. After buffering by another op-amp, the heavily attenuated waveform is applied to the capacitor under test. The capacitor is in parallel with 5 ohms of resistance. The third op-amp, used as a differential amplifier, amplifies the difference in voltage between:

• the parallel resistance of the 5 ohms of resistance and the capacitor under test versus
• a second resistor string of just 5 ohms.

 So, the better, or lower, the ESR of the capacitor at the oscillation frequency the greater the output of the differential amplifier stage. Finally, a precision rectifier stage is used to drive the meter.

There is minimal meter deflection when no capacitor is being tested so the coin cell lasts for ages. When I'm finished using the meter I pop the battery out and reinsert into the holder upside down for storage.

I was going to make up a table to calibrate the meter but I never got to it. It was just so intuitive in use that it seemed unnecessary.