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Resistance measurement with Current Source - del20015

U3 LF 356 is used as a constant current source (sink as the current is negative). R4, R5, R7 and R9 set the four resistance ranges by changing the constant current in decade steps. R2 is for calibration of resistance range. The A-B digital control of 4052 selects the range.

Let voltage current and resistance sockets be separate and of different color or use a high voltage electrical rotary switch or relays if you want the same sockets switched. D1, D2 and R8 are to ensure that the FET can be turned off, as the opamp swings from +/- 3.5V only, with some FET it needs to be tweaked.

                        measurement with Current Source
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Resistance measurement with Current Source - del20015

When you keep the current constant, the voltage across a resistor is directly proportional to the Resistor Value. This can be scaled to gat a usable reading on a Digital Voltmeter.

Currents from 10uA to 10mA or even more can be passed thru a resistor, without heating it much and without exhausting your instrument battery. When High resistance is measured you will need more voltage across the DUT (device under test). The current thru it is measured.

In Low Mico-Ohm devices like wires and connectors, we use currents like 10A pulsed to get a quick reading of voltage across. This needs sample-hold, Gold plated four-wire measurements. Excellent instrumentation amps.