Insulation Tester pico amplifier -
del50002
Description.
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 15 years old. 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.
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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.
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.
Data sheets and Information.
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