Purpose of D1 and D2

[abdulbadii]

Would anyone please help out explain or show what is the purpose of D1 and D2 in circuit:

[bob91343]

They clip the wave to about 0.7V peak, or 1.5V p-p.

[TurboTom]

They are just for ESD protection and to safely discharge the DUT if someone attaches a charged cap to the tester. Voltage across the capacitor is unidirectional and very low (0.1V max). It's a very simple but still interesting instrument. In a practical setup, one of the voltage dividers (probably ideally R9/R10) should be adjustable to zero out the reading while no DUT is attached. The device will only work for (comparably) large capacitors (electrolytics) since it doesn't have a phase-sensitive rectifier and requires Zc to be much smaller than ESR to produce an accurate result.