Electronic Engineering Schematics
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.
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
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
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.
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.
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20th Mar 2020