[ Español ]

ESR meter

When I first started doing repairs of electronic devices I realized dry electrolytic capacitors were a common cause of failures. Their measured capacity is almost correct but their internal resistance rises causing many problems. This is common in capacitors running hot for example in switching power supplies.

I shopped around to buy an ESR meter but nobody knew what I was talking about, so I decided to build one from common components. A friend pointed me to this site and told me it worked fine:


(Courtesy of Gary Woods)
And it did! Once calibrated it works great. To calibrate it requires nothing special, just connect some resistors of known value and adjust some presets to have the meter show the right value at various scale points.

My surprise however was some time ago when I found out Gary Woods page no longer existed. So far I could not find another web with its schematic and instructions so I have put them here:

ESR meter schematic

Important notes:

  1. IC1 and IC2 are National LM324N or compatible.
  2. Resistors marked with "F" are 1%; others are 5%
  3. R7 must be 0.5 W
  4. IC1 must be capable to oscillate at 100 kHz.
  5. R1 sets the frequency: from 1K to 3.3K ohm. (I've set it to 1K2) ~ 100 kHz
  6. R10 optional, can improve scale linearity.
  7. R15 optional; helps IC pulldown in some cases.
  8. R21 sets linearity at midscale: from 330 to 2.2K~ ohm.
  9. R24 optional; corrects DC offset with infinite ESR.
  10. R26 sets zero: from 68 to 240 ohm.
  11. D6 and D7 set correct reading at 50 ohm.
  12. Calibrate at 24°C. Wait 20 minutes after soldering.

1N34A are germanium diodes, normal silicon diodes will not work. The meter is a 1mA needle ammeter.

This is the scale to replace the ammeter scale:

ESR meter scale

And this is the explanation that came with these pictures: