This circuit senses the impedance of the capacitor at 100kHz test frequency and converts it to 1V/ohm dc voltage. So ESR can be sensed with ordinary multimeter.
Because electrolytics are mostly resistive at 100kHz this voltage is approximately the ESR of the condenser.
The measurin range is about 10mΩ...1Ω.
1.Circuit needs stable 12 V and this comes from the 7812.
2. 555 genarates 100 kHz square wave. It goes thru the 220Ω current limiting resistor and DC-blocking capacitor to the test propes.
2. Q1 amplifies signal
3. Q2 amplifies and filters signal
4. Schottky-diode D4 rectifies signal. Trimmer R13 fine adjust sensitivity to 1 V/Ω. Use 0,1 Ω resistor.
Procedure: Zero meter, measure resistor, adjust trimmer, zero meter, measure resistor, adjust trimmer... until reading of 100mV is achieved
Correct lenght for test leads is 10...20 cm (=5...10 inch). Leads must be bundled together as shown in photo below.
This is to minimize
test lead inductance variation.
Short test leads. (eg. connect both leads to the same leg of capacitor)
Wait until reading is stabile.
Select correct range and press the zeroing button on multimeter(REL Δ in meter shown in photo).
Meter is ready for measurement
Zero point shifts if RF-interference level changes eg. you bring meter near compact fluorescent light.
Because of this please check zero point by shorting the leads together and noting the meter shows zero.
Press the zeroing (REL Δ) button if needed.
Schematic diagram, here
Improved version with display.
You need to add amplifier and zero adjust for the panelmeter
Schematic diagram with amplifier
Meter installed in a box
Printed circuit board. Operational amplifier is TSH22. ACHTUNG! PCB is missing pulldown resistor.
Add to IC2B pin 7. About 100 Ω likely works, I used 22Ω one. Or use CMOS op-amp
Eagle-files + CAM-data