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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.
Measure 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.
                      Resistance Indicator

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.

Microohm 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.

Data sheets and Information
TL062 - Dual Low-Power JFET-Input Opamp - They feature high input impedance, wide bandwidth, high slew rate, and low input offset and input bias currents.
Voltage Comparator Information And Circuits - The circuits shown are based on the LM339 Quad Voltage Comparator chip or the LM393 Dual Voltage Comparator chip. The LM311 Voltage Comparator can be used for these applications as well but it also has a number of unique features.

Here is a professional instrument - High Resistance Meter - Model HR2 - The Model HR2 meter measures resistance in nine ranges from 1 ohm to 1.9999 Tera ohms (1.9999x1012 ohms), and conductance in a single range of 0.00 to 199.99 pS.