Measurement principle and system block diagram of multi-contact insulation resistance microcomputer test system

　　The insulation resistance between certain contacts of electronic equipment or multi-core cables is usually measured using a megohmmeter. The megohmmeter has a narrow measurement range (100 kΩ～500 MΩ), low accuracy (10%～20%), and complex operation, especially for multi-contact measurement, which is difficult to automate.
　　
　　This section designs a bridge zeroing measurement method, which uses a single-chip microcomputer system to achieve a wide range (20 kΩ～1275 MΩ), high accuracy (error is ±2.5%) and automatic measurement (automatic switching of range and automatic switching of detection points).
　　
　　I. Measurement principle and system block diagram
　　
　　1. Measurement principle
　　
　　This system uses the bridge zeroing method to measure the insulation resistance when the external voltage is 500 V. The circuit schematic is shown in the figure below.

　　
　　From the figure, we can get:

　　
　　When Rx is very large, Ux is very small. In order to ensure the accuracy of A/D conversion, Ux needs to be program-controlled amplified. Assume that the amplifier outputs U XK and the amplification factor is K, then equation (3-6) can be written as

　　
　　Formula (3-7) is the mathematical model of measurement. From the model, it can be seen that the measured value of Rx has nothing to do with the external voltage U (500 V). When K, Ri and R2 are given, the bridge is balanced and only U2 and UXK need to be measured. Then the insulation resistance Rx can be obtained by formula (3-7).

　　2. System Block Diagram
　　
　　The measurement system consists of two parts: the main control circuit and the sampling circuit. The following figure is its system block diagram. The main control circuit is an 8031 ​​single-chip microcomputer system. The sampling circuit consists of a relay array, a high-voltage power supply detection bridge, and a programmable amplifier.

　　
　　The system working process is: according to the key operation, the single-chip computer system controls the relay array according to the given program, so that the detection point is connected to the Rx end of the measuring bridge; the high-voltage power supply provides a 500V DC voltage; the voltage output by the bridge is amplified and A/D converted by the programmable amplifier and read by the single-chip computer; Rx is calculated according to formula (3-7) and recorded by LED display or printer.