The English name of solid state relay is SSR. The single-phase SSR is a four terminal active device. There are two input control terminals and two output terminals. The input and output are optically isolated. When the input terminal is added with DC or pulse signal to a certain current value, the output terminal can change from off state to on state. The principle is shown in Figure 1 below.

In general, the multimeter can't judge the quality of SSR. The correct way is to test the circuit: when the input current is zero, the voltage measured by the voltmeter is the grid voltage, and the light is not on (the bulb power must be more than 25W);

When the input current reaches a certain value, the SSR conduction voltage drop measured by the voltmeter (above 2V) increases

Next). (please note for the first time that the leakage current is caused by the RC circuit in SSR, so it can't be equal to the ordinary contact relay and contactor.)