http://ludens.cl/Electron/esr/esr.html
I changed the opamp to high voltage version. It's all I had in stock that was cheap. I have a small stock of opa2134 but was not willing to sacrifice one to this experimental project.
It runs well at +9Vdc single supply but the output justs start to be corrupted @ 8.5Vdc.
So I set the regulator to 8.9Vdc. This means I cannot run the tester from a 9V battery. So I use the output of my adjustable lab supply set to around 11.5Vdc.
This results in a higher output voltage from the oscillator. So instead of increasing the primary turns to more than 400T, I reduced the secondary turns from 20T to 16T. It gives 250mVpp instead of 200mVpp as the square wave test signal.
The resolution of the meter near the Xc value of the capacitor under test is not good. Difficult to measure the difference between 1ohm and 1.2ohms. This makes an enormous difference to the actual ESR if the Xc were 0.9ohms.
But the meter clearly shows faulty caps and is capable of showing esr that has degraded to say 1.5times Xc for values over around 0.5ohms
I am thinking of changing the 10r in series with the DUT to 1r or 2r to improve resolution at the low ohms end and maybe a little bit more gain in the second opamp.
Xc = 1 / {2 Pi F C) in my case the F is 87kHz (11.5us from the scope screen).
The meter reading is {esr + Xc}.
This is a bigger value than esr alone that some meters can read.
I have just bought an Ebay component tester to do esr and many other component measurements. But these tools cannot check esr when the cap is in circuit.
The Ludens meter's main advantage is in circuit testing.