UVT is an undervoltage fault. It is believed that many customers will still encounter such problems during use. The common undervoltage detection points are the voltage on the DC bus side, and a low voltage value is sampled after being divided by a large resistance resistor. After the voltage value is compared, the output voltage is normal, the overvoltage signal or the undervoltage signal. The sampling value of the voltage signal of Mitsubishi A500 series inverter is obtained from the switching power supply side and is isolated by the optocoupler. During the maintenance process, it is found that the damage of the optocoupler occupies a large cause of the undervoltage fault. proportion.