Measuring the resistance of the coil: the universal meter R×10Ω can be used to measure the resistance of the low-power relay coil, so as to determine whether the coil has an open circuit phenomenon. Measuring contact resistance: use the resistance level of the universal meter to measure the resistance of the normally closed contact and the moving point, and the resistance value should be 0, (the contact resistance value can be measured within 100 milliohm in a more accurate way); The resistance of the normally open contact and the moving point is infinite. So you can tell that one is normally closed and that one is normally open. Measure the release voltage and release current: it is also connected to the test as above. When the low-power relay is pulled off, then gradually reduce the supply voltage. When the low-power relay is heard to release sound again, note down the voltage and current at this time. Select five steps under normal circumstances, the release voltage of the low-power relay is about 10~50% of the pulse-in voltage, if the release voltage is too small (less than 1/10 of the pulse-in voltage), it can not be used normally, which will pose a threat to the stability of the circuit, and the selection of work is not reliable. Measure pull-in voltage and pull-in current: find the adjustable regulated power supply and ammeter, input a set of voltage to the low-power relay, and connect the ammeter in series in the power supply loop for monitoring. Slowly turn up the power supply voltage and note down the pull-in voltage and pull-in current when you hear the pull-in sound of the low-power relay. To be accurate, try the average several times. When selecting a low-power relay, the power supply voltage of the general control circuit can be used as the basis for selection. The control circuit should be able to provide sufficient working current to the low-power relay, otherwise the low-power relay pull-in is unstable.