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Insulation Tester pico amplifier - del50002


The leakage current in an insulation or high value resistor is measured using ICL7650 and the amplified milli-volts is applied to Vref of ICL7107 to get its reciprocal value.

Guard rings in strain gauge measurement

With these circuits you can make an insulation tester going upto two tera ohm, hence currents will be in pico amps, great care required in design. The rotary switches for voltages and ganged interlocked range switches must not be phenolic  but  industrial epoxy based molded switches.
The parts list which is not in the circuit, is listed below, the circuit is many years old, 80s. but you may get some idea on high resistance measurement. Use 1% MFR for all Resistors and low leakage plastic caps for low values. These instruments i used to calibrate with Victoreen Resitors and a Electrometer from Princeton Research, I faintly remember.

IC1 ICL7107 A/D converter 31/2 ICL7107 Device Information
IC2 ICL7650 chopper stabilized amplifier ICL7650, ICL7650B Chopper-Stabilized Op Amps
IC3 CA3140 any high imp opamp CA3140 Device Information
IC7 IC9 NE555 timer LM555 - Timer

Pico Amplifier - Insulation Tester

Theory of Operation.

The Device Under Test DUT say a transformer is placed in a Metal tray connected to the Guard SK3 terminal. The metal tray has a 3mm glass sheet on which DUT is kept. For 2 Tera ohm  make sure that Humidity and Dust do not affect measurements. 
Instrumentation and Automation - 01

Now  1000V is applied on a DUT terminal from the high voltage supply, ( in reference to Guard SK3 at earth and 0V). Then the point where the leakage is to be measured is connected to SK4 via a BNC short, scope, shielded cable. The current  goes thru a shunt selected by S5 and voltage across the shunt is measured by ICL7650 a Chopper stabilized amplifier, with ultra low offset and bias. The amplified output is fed to Vref of ICL7107 which displays the Insulation Resistance.

The full range is not valid in this circuit and for low values, change to a range where the reading is more number of counts. The above circuit does not include some upgrades and changes i did later using ICL7135 etc.. If i find it i will add it later.