With the increasingly stable and widespread use of optocouplers, there are many and various types of optocouplers in the market. Therefore, counterfeit optocouplers have also emerged. In order to enable consumers to buy good optocouplers, Yiguang's agent Chaoyi Electronics will explain how to check the quality of optocouplers: to determine the quality of optocouplers, the forward and reverse resistance of its internal diodes and transistors can be measured in the circuit. There are three more reliable detection methods. 1. Remove the suspected faulty optocoupler using the comparison method, measure the forward and reverse resistance values of its internal diode and transistor using a multimeter, and compare it with the measured values of the corresponding pins of a good optocoupler. If the resistance values differ significantly, it indicates that the optocoupler is damaged. 2. The digital multimeter detection method is illustrated by using EL817 optocoupler detection as an example. The detection circuit is shown in Figure 1. When testing, insert the+terminal {1} and - terminal {2} pins of the optocoupler's internal diode into the C and E sockets of the Hfe of the digital multimeter, respectively. At this time, the digital multimeter should be placed in the NPN position; Then connect the C-pole {5} pin of the optocoupler to the black lead of the pointer multimeter, and connect the e-pole {4} pin to the red lead. Set the pointer multimeter to the RX1k position