How to use a digital multimeter to measure the quality of solid state relays
Identify the input and output pins and measure the quality. On the shell of the AC solid state relay, the input terminals are generally marked with "+", "-" and "INPUT", while the output terminals are not divided into positive and negative, but some Devices are marked "LOAD". For DC solid state relays, 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 show the difference.
When using a digital multimeter to identify the input and input terminals, you can use the diode file to test the forward and reverse directions of the four pins respectively, and the voltage value between a pair of pins must be measured in accordance with the law of forward conduction and reverse cut-off , that is, it displays "1.3~1.6V" when measuring in the forward direction, and displays the overflow symbol "1" when testing in the reverse direction. Based on this, it can be judged that these two pins are input terminals, and when measuring in the positive direction, a measurement of "1.3~1.6V" is displayed, the red test lead is connected to the positive pole, and the black test lead is connected to the negative pole. For DC solid-state relays, after finding the input terminal, the positive and negative poles of the output terminal are generally opposite to each other horizontally. It should be pointed out that some DC solid-state relays have protection diodes on their output terminals, the positive pole of the protection tube is connected to the negative pole of the solid-state diode, and the negative pole of the protection tube is connected to the positive pole of the solid-state relay. Pay attention to the correct distinction when testing. Detection example: The device under test is a JGTIFA DC solid state relay. Its output is paralleled with a protection diode.
For the convenience of description, the four pins of the device are marked as ①, ②, ③, and ④ respectively. When testing, first distinguish the two pins of the input. Use the diode file of the DT890A digital multimeter to measure the forward and reverse directions of ①, ②, ③, and ④. It can be seen from the test data that when the red test lead is connected to ① pin and the black test lead is connected to ② pin, the display value of the meter is 1381 (1.381V). When connected to ③, the display value of the meter is 543 (0.543V). When the test leads are exchanged for measurement, the meter displays the overflow symbol "1"; in the other test states, the meter displays the overflow symbol "1".
It is not difficult to draw conclusions 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 solid-state relays without protection diodes at the output end, no matter how the test leads are exchanged to measure pins ③ and ④, 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 test conclusions.
2. Check the loading capacity
(1) Using the diode gear of the digital multimeter, first measure the forward and reverse directions of pins ① and ②, and the meter will display the overflow symbol "1"; , When the black test lead is connected to the ④ pin, the meter will display 1524 (1.524V), and when the test lead is changed to measure, the meter will display the overflow symbol "1", which shows that the ③ and ④ pins are input terminals, the ③ pin is positive, and the ④ pin is negative. The ① and ② feet are the AC output terminals of the device under test.
(2) Using a DC5V regulated power supply, set the digital multimeter to the 2kΩ resistance range to measure the on-off resistance of the output terminal. After S1 is closed and powered on, the measured resistance value is 1.343kΩ, indicating that the internal bidirectional thyristor is turned on, and the load can be turned on at this time. When S1 is disconnected, the meter will display the overflow symbol "1" (the resistance value is infinite), indicating that the device under test is turned off, and the load can be cut off at this time. Note that depending on the type of the solid state relay to be tested, the measured on-state resistance value of the output terminal is also different, and the range of the value is relatively large, some are a few ohms, and some are several thousand ohms. The on-state resistance of the output 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 value of IS depends on the magnitude of the DC voltage applied to the input terminal, but the added input voltage value must not exceed the rated input voltage value of the device under test. In addition, if the polarity of the DC voltage at the input terminal is reversed, the solid state relay cannot work normally. Related information: How to use a digital multimeter to measure the quality of a solid-state relay 1. To measure the contact resistance, use the resistance file of the multimeter to measure the resistance of the normally closed contact and the moving point. The resistance value should be 0; The point resistance is infinite. From this, it can be distinguished which is a normally closed contact and which is a normally open contact.
2. To measure the coil resistance, use the multimeter R×10Ω to measure the resistance value of the relay coil, so as to judge whether there is an open circuit in the coil.
3. Measure the pull-in voltage and pull-in current Find an adjustable regulated power supply and an ammeter, input a set of voltages to the relay, and connect an ammeter in series in the power supply circuit for monitoring. Slowly increase the power supply voltage, and record the pull-in voltage and pull-in current when hearing the pull-in sound of the relay. For accuracy, you can try several times and find the average value.
4. Measuring the release voltage and release current is also connected and tested as above. When the relay is closed, gradually reduce the supply voltage. When you hear the release sound of the relay again, write down the voltage and current at this time, and you can also try Get the average release voltage and release current several times. Under normal circumstances, the release voltage of the 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 will pose a threat to the stability of the circuit. , work unreliably.
