Electronic Engineering Schematics
High Resistance Indicator - del50004
Measuring Tera Ohms and Giga Ohms needs 100V and above for good
measurement. This circuit can measure or indicate the rough
magnitude of the resistor. 100, 10 or 1 Mega Ohms.
When 10V is applied on a resistor like 1 Mega Ohms. The currnet
is 10 Micro Amp. 10 Mega gives One microamp. If this goes away
in the bias and leakage of the circuit. There is no measurement.
We need to use FET input Amps. The bias current can cause an
error too. Offset voltages also need to be low.
Description - I don't remember if this circuit worked properly.
But a few were made and i might not have shown the modifications
that were done to make it work. This was meant to be a
portable, low cost, insulation tester for an electrician. If you
try it out and debug it it may work well.
resistance & capacitance without A/D
A negative voltage is derived by shifting gnd with two diodes, i
feel this did not work very well. Two pins of CD4028 pins are
also used to boost the reference to get two extra ranges as 4051
has a 100E on resistance.
Theory of Operation -The 555 clock makes 4029 counter count. But
the clock can be clamped to gnd by a TL062 window comparator.
The clock is frozen when the input value to comparator pin 5-2
is within a lower limit and upper limit "window" pin 3-6.
The 4029 counter BCD is decoded to decimal by 4028 which drives
the LEDs, keep LED drive within 3mA or chip will be loaded. Use
high efficiency extra-bright LEDs.
Meter with LED Analog Bar
The 4029 BCD also controls a shunt resistor array with CMOS
switches 4051. The voltage across shunt is a sample of leakage
current. This is compared in the window comparator to freeze the
Clock and LED display to give a reading of the leakage current
or Insulation Resistance.
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20th Mar 2020