How to use a digital multimeter to measure the quality of solid state relays
Solid state relay is referred to as SSR. It is a new type of non-contact switch composed of electronic components. It has the characteristics of reliable operation, fast switching speed and long life. It replaces conventional relays in the control circuit.
Among them, the two terminals of INPUT are input terminals, which are connected to control signals, and the other two terminals are output terminals, which are connected to relays. Solid state relays can be divided into DC and AC solid state relays according to the load type. The former uses power transistors as switching elements, and the latter uses bidirectional thyristors as switching elements.
Principle of AC zero-crossing solid state relay. The so-called zero-crossing refers to triggering the thyristor when the AC voltage is zero or just crossing zero, which can reduce the impact on the power supply when the thyristor is turned on. Generally, the zero-crossing voltage is defined as ±25V. As long as the input signal is applied in this area, the SSR will be turned on. When the power supply voltage is greater than ±25V, the SSR will not be turned on immediately when the input signal is applied. Only when the power drops to the next step The SSR is only turned on when it is in the zero region.
When detecting SSR, you can measure whether there is DC voltage at the input terminal. DC voltage indicates that the input signal is normal, and then measure whether there is AC voltage at the output terminal. If there is, it means that the SSR is normal, if there is no or the voltage drop is very low, it means that the solid state relay is damaged
Use the diode gear of the digital multimeter to measure the positive and negative directions of ①, ②, ③, and ④ from the test data. When the red test lead is connected to the ① pin and the black test lead is connected to the ② pin, the display value of the meter is 1381 (1.381V), exchange the test leads When measuring, the meter displays the overflow symbol "1"; when the red test lead is connected to the ④ pin and the black test lead is connected to the ③, the meter displays a value of 543 (0.543V), and the meter displays the overflow symbol "1" when the test leads are exchanged; In several test states, the meter will display the overflow symbol "1". It is not difficult to draw a conclusion from this: ① and ② pins are the DC input terminals of the device under test, ① pin is the positive pole, ② pin is the negative pole, "1.381V" is the forward voltage drop of the light-emitting diode inside the solid state relay; ③, ④ pins It is the DC output terminal, the ③ pin is the positive pole, and the ④ pin is the negative pole. "0.543V" is the forward voltage drop of the protection diode connected in parallel at the output terminal of the solid state relay. Note that for a solid-state relay without a protection diode at the output end, no matter how you exchange the test leads to measure its ③ and ④ pins, the meter will display the overflow symbol "1". When using different types of digital multimeters to measure the internal light-emitting diodes of solid-state relays, sometimes the display value of some instruments just flashes the reading momentarily, and then displays the overflow symbol "1". Draw the test conclusion
