How to measure whether a solid-state relay is good or bad
The identification of input and output pins and the measurement of their quality are generally marked with the words "+", "-", and "Input" on the housing of AC solid-state relays, while the output end is not divided into positive and negative, but some devices are marked with the words "LOAD". For DC solid-state relays, both the input and output terminals are generally marked with "+" and "-", and some devices are also marked with the words "IN" (input) and "OUT" (output) to indicate differences. When using a digital multimeter to distinguish between input and output terminals, a diode range can be used to perform forward and reverse tests on four pins. Among them, it is certain that the voltage value between a pair of pins conforms to the rules of forward conduction and reverse cutoff, that is, "1.3-1.6V" is displayed during forward measurement, and "1" is displayed during reverse testing.
Based on this, it can be determined that these two pins are input terminals, and during forward measurement, a measurement of "1.3-1.6V" is displayed. The red lead is connected to the positive electrode, while the black lead is connected to the negative electrode. For DC solid-state relays, when the input terminal is found, the positive and negative terminals of the output terminal are usually opposite to each other horizontally. It should be pointed out that some DC solid-state relays have protective diodes at their output terminals. The positive pole of the protective tube is connected to the negative pole of the solid-state diode, while the negative pole of the protective tube is connected to the positive pole of the solid-state relay. During testing, it is important to distinguish correctly.
Detection example: The tested device is a JGTIFA type DC solid-state relay. Its output terminal is connected to a protective diode in parallel. For the convenience of description, label the four pins of the device as ①, ②, ③, and ④. When testing, first distinguish between the two pins on the input end. Using the DT890A digital multimeter diode range, perform forward and reverse measurements on ①, ②, ③, and ④. From the test data, it can be seen that 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 exchanging 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 exchanging the probes for measurement, the instrument displays the overflow symbol "1"; In the other test states, the instrument displays 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 tested device, ① pins are the positive pole, ② pins are the negative pole, and "1.381V" is the forward voltage drop of the internal light-emitting diode of the solid-state relay; ③ and ④ Pin is the DC output terminal, pin ③ is the positive pole, pin ④ is the negative pole, and "0.543V" is the forward voltage drop of the protective 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 measuring the internal light-emitting diodes of solid-state relays using different models of digital multimeter, some instruments may only momentarily flash the reading, and then display the overflow symbol "1". In this case, the probe can be repeatedly exchanged for multiple measurements until a test conclusion is reached.
2. Check the load capacity (1) Using the diode range of the DT899A digital multimeter, first measure the forward and reverse directions of pins ① and ②, and the instrument displays the overflow symbol "1"; Perform forward and reverse measurements on pins ③ and ④. When the red probe is connected to pin ③ and the black probe is connected to pin ④, the instrument displays 1524 (1.524V). When the probe is swapped for measurement, the instrument displays the overflow symbol "1", indicating that pins ③ and ④ are the input terminal, pins ③ are the positive pole, and pins ④ are the negative pole. The pins ① and ② are the AC output terminals of the tested device. (2) Using a DC5V stabilized power supply, set the DT890A digital multimeter to the 2k Ω resistance range to measure the on and off resistance of the output terminal. After closing and powering on S1, the measured resistance value is 1.343k Ω, indicating that the internal bidirectional thyristor is conducting and the load can be connected at this time. When S1 is disconnected, the instrument displays the overflow symbol "1" (the resistance value is infinite), indicating that the tested device is turned off and the load can be cut off at this time. Note that the on state resistance values of the measured output terminals vary depending on the model of the solid-state relay being tested, with a wide range of values ranging from several hours of ohms to several thousand ohms. The on state resistance of the output terminal is related to the input current IS. In the range of 10-20mA, the larger the input current IS, the smaller the on state resistance. The size of the IS value depends on the magnitude of the DC voltage applied to the input terminal, but the input voltage applied must not exceed the rated input voltage value of the tested device. In addition, if the polarity of the input DC voltage is reversed, the solid-state relay cannot function properly. Related information: How to use a digital multimeter to measure the quality of solid-state relays
1. Measure the contact resistance using the resistance range of a multimeter. Measure the resistance between the normally closed contact and the moving point, and the resistance value should be 0; The resistance between the normally open contact and the moving point is infinite. From this, it can be distinguished that is a normally closed contact and that is a normally open contact.
2. The resistance of the coil can be measured using a universal meter R × Measure the resistance of the relay coil at 10 Ω to determine if there is an open circuit in the coil.
3. Measure the pull-in voltage and pull-in current to find an adjustable regulated power supply and ammeter, input a set of voltage to the relay, and connect an ammeter in series in the power supply circuit for monitoring. Slowly increase the power supply voltage, and when you hear the relay closing sound, note the closing voltage and current. For accuracy, you can try multiple times to find the average value.
4. The measurement of release voltage and release current is also connected for testing as mentioned above. After the relay is pulled in, gradually reduce the supply voltage. When the relay releases again, record the voltage and current at this time, and try multiple times to obtain an average release voltage and current. In general, the release voltage of a relay is about 10-50% of the pull-in voltage. If the release voltage is too small (less than 1/10 of the pull-in voltage), it cannot be used normally, which poses a threat to the stability of the circuit and makes it unreliable to operate.
