This is a photograph of a 20 Tera Ohm Insulation Tester I made around
20 Years Back. It had even Polarization Index for Transformer
Insulation.
A regulated, low ripple, high voltage of very low current capability is
applied by instrument on a DUT (device under test). The leakage
current in nA or pA (picoamps, nanoamps) is measured and readout, The
Innovation i did was to measure the current in Vref and not Vin of
ICL7107, this made the 1/x computation.
Also using the ICL7650 Chopper Stabilized amplifier must be done as
shown in data sheets with grounded guard rings. pA will leak on the PCB
itself if layout is not done keeping this in mind.
Anantha Narayan
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