The insulation resistance tester mainly consists of three parts. The first is the DC high voltage generator, which is used to generate a DC high voltage. The second is the measurement circuit. The third is the display.
（1） Measurement Circuit
In the previously mentioned megger (megohmmeter), the measurement circuit and the display part are combined into one. It is completed by a cross-coil ohmmeter head, which consists of two coils with an angle of 60° (approximately) between them. One coil is connected in parallel across the voltage, and the other coil is connected in series in the measurement circuit. The deflection angle of the pointer is determined by the current ratio in the two coils. Different deflection angles represent different resistance values. The smaller the measured resistance, the greater the current in the coil connected in series in the measurement circuit, and thus the larger the deflection angle of the pointer. Another method is to use a linear ammeter for measurement and display. In the previously used cross-coil ohmmeter head, due to the non-uniform magnetic field in the coil, when the pointer is at infinity, the current coil is exactly at the place with the strongest magnetic flux density. Therefore, even though the measured resistance is large and the current flowing through the current coil is small, the deflection angle of the coil will be relatively large. When the measured resistance is small or zero, the current flowing through the current coil is large, and the coil has already deflected to a place with lower magnetic flux density, so the resulting deflection angle will not be very large. In this way, nonlinear correction is achieved. Generally, the resistance display of a megohmmeter head needs to span several orders of magnitude. However, when a linear ammeter head is directly connected in series in the measurement circuit, it will not work. At high resistance values, the scale marks are all crowded together and cannot be distinguished. In order to achieve nonlinear correction, nonlinear components must be added to the measurement circuit. Thus, a shunting effect is produced at small resistance values, while no shunting occurs at high resistance values, allowing the resistance display to span several orders of magnitude.
（2） DC High Voltage Generator
To measure insulation resistance, a high voltage must be applied to the measurement terminal. According to the national standard for insulation resistance testers, this high voltage value is specified as 50V, 100V, 250V, 500V, 1000V, 2500V, 5000V…
There are generally three methods to generate DC high voltage. The first is the hand-cranked generator type. Currently, about 80% of the megohmmeters produced in China use this method (the source of the name "megger"). The second method is to step up the voltage through a mains transformer and rectify it to obtain DC high voltage. This is the method generally used by mains-powered megohmmeters. The third method is to use a transistor oscillator or a dedicated pulse width modulation circuit to generate DC high voltage. This is the method used by battery-powered and mains-powered insulation resistance testers.
