Digital megohmmeter circuit and converter functions
Digital megohmmeters generally use a DC voltage converter to convert the battery voltage into DC high voltage as the test voltage. The current generated by applying this test voltage to the object under test is converted into the corresponding voltage value by the current-voltage converter, and then sent to the meter. After entering the analog-to-digital converter, it becomes a digital code, which is then calculated and processed by the microprocessor, and the corresponding resistance value is displayed on the display.
1. DC voltage converter
The DC voltage converter (DC-DC converter) converts the battery voltage into high DC voltage as the test voltage. It is a key part of the digital megger. There are three common test voltages of 250V, 500V, and 1000V (there are also 2500V, 5000V, etc. higher test voltage).
Digital megohmmeters generally use a pulse width modulation (PWM) switching power supply integrated controller to convert the DC voltage of the battery into a pulse width modulation signal, which is converted into high voltage pulses through a step-up transformer and rectified by a voltage doubler (double or triple ) is smoothed into a DC high-voltage test voltage, and its output voltage is achieved by adjusting the pulse width through the output voltage control circuit.
Common controllers include UC3840, TL494, SG3524, etc. This type of pulse width modulation controller is the most popular integrated switching power supply controller in the world. It includes all the control circuits required for switching regulators. For example, error amplifier, oscillator, pulse width modulator, pulse generator, switching tube, overcurrent and overheating protection, etc.
2. Current to voltage converter
The current-to-voltage converter (I-V conversion) consists of the resistor to be measured Rx, the feedback resistor Rf and the operational amplifier A1. It converts the current flowing through the measured insulation resistance Rx into a voltage signal output.
3. Analog-to-digital converter and display circuit
Analog-to-digital converter (A/D) is the link between analog circuits and digital circuits, and is also an important symbol of analog instruments and digital instruments. Digital megohmmeters with an accuracy of no more than 1% usually use ICL7106 (or ICL7107) large-scale integrated circuit. ICL7106 is a 3?-digit double integrating A/D converter with LCD display (ICL7107 is LED display), using its input The ratio characteristics of the voltage and the reference voltage can easily measure the resistance value.
