How to use a digital multimeter to measure the quality of a solid-state relay
Solid state relay, abbreviated as SSR. It is a new type of contactless switch composed of electronic components, which has the characteristics of reliable operation, fast switching speed, and long service life. It replaces conventional relays in control circuits.
The INPUT two terminals are input terminals, connected to control signals, and the other two terminals are output terminals, connected to relays. Solid state relays can be divided into DC and AC solid state relays according to load types. The former uses power transistors as switching elements, while the latter uses bidirectional thyristors as switching elements.
Communicate the principle of zero crossing solid-state relay. The so-called zero crossing refers to triggering the thyristor when the AC voltage is zero or just crosses zero, which can reduce the impact on the power supply when the thyristor conducts. The zero crossing voltage is generally defined as ± 25V. In this region, as long as an input signal is applied, the SSR will conduct. When the power supply voltage is greater than ± 25V, the SSR will not immediately conduct when an input signal is applied. It will only conduct when the power supply drops to the next zero crossing region.
When detecting SSR, it is possible to measure whether there is DC voltage at its input terminal. If there is DC voltage, it indicates that the input signal is normal, and then measure whether there is AC voltage at the output terminal. If there is, it indicates that the SSR is normal. If there is none or the voltage drop is very low, it indicates that the solid-state relay is damaged
Using the diode mode of a digital multimeter, perform forward and reverse measurements on ①, ②, ③, and ④. According to the test data, when the red probe is connected to pin ① and the black probe is connected to pin ②, the instrument displays a value of 1381 (1.381V). When swapping the probes for measurement, the instrument displays an overflow symbol of "1"; When the red probe is connected to pin ④ and the black probe is connected to pin ③, the instrument displays a value of 543 (0.543V). When the probe is swapped for measurement, the instrument displays the overflow symbol "1"; In the remaining test states, the instrument displays the overflow symbol "1". It is not difficult to draw the conclusion that: ① and ② are the DC input terminals of the tested device, ① is the positive terminal, ② is the negative terminal, and "1.381V" is the forward voltage drop of the internal light-emitting diode of the solid-state relay; ③ and ④ The pin is the DC output terminal, pin ③ is the positive terminal, pin ④ is the negative terminal, and "0.543V" is the forward voltage drop of the protection diode connected in parallel to the output terminal of the solid-state relay. Note that for solid-state relays without protective diodes at the output end, no matter how the probes are exchanged to measure pins ③ and ④, the instrument will display the overflow symbol "1". When using different models of digital multimeters to measure the internal light-emitting diodes of solid-state relays, some instruments may only display a momentary flashing reading, followed by the overflow symbol "1". In this case, the probes can be exchanged multiple times until a test conclusion is reached
