Contact resistance measurement circuit

The contact resistance measurement circuit is shown in the figure. It mainly consists of constant current circuit and amplifier circuit. The operational amplifier 1C1 and the transistor VT form a
follower, and the voltage at both ends is equal to the rated stable voltage of the voltage regulator tube VS1. Since the resistance of Ra is fixed, a
constant current flows through Rs. The resistor under test is connected in series between terminals A and B. At the connection point between the resistor under test and the resistor under test, the fractional current is very small, so
the current through the resistor under test and the wind can be regarded as the same. In this way, the voltage between terminals A and B It is only related to the resistance measured at terminals A and B (proportional relationship). The measurement of resistance
is converted into a measurement of voltage. The operational amplifier IC2 and its peripheral resistor-capacitor components form an inverse amplifier, which amplifies the voltage between A and B.
The amplification factor is brother/Rz. c], Cz is the memory capacitor required by IC2. The millivoltmeter is connected to the output of IC2. Measure the output voltage of IC2. Assume
that the output voltage of 102 is Vo and the voltage amplification factor is K. , the rated stable voltage of VD1 is Vw, and the measured resistance is feet. , then there is
    /Ct=VwR4Rn/R3RZ
    R =Rz Ra V/R4 Vw
    in the figure. T, VD3-VD6, 1C3 and other components constitute a DC regulated power supply, which outputs a stable 12V voltage when the measuring instrument uses AC mains power
.