Three detection methods of optocouplers

Optocouplers are now widely used in various control circuits. Since the light-emitting diodes and phototransistors inside the optocouplers only convert the voltage or current changes of the circuit before and after into light changes, and there is no electrical connection between the two, they can effectively cut off the potential connection between circuits and achieve reliable isolation between circuits.

Optocoupler detection:
To determine whether an optocoupler is good or not, you can measure the forward and reverse resistance of its internal diode and transistor in the circuit. The more reliable detection methods are the following three.

Comparison method: Remove the suspected problematic optocoupler, use a multimeter to measure the forward and reverse resistance of its internal diode and transistor, and compare it with the measured values ​​of the corresponding pins of a good optocoupler. If the resistance value is significantly different, it means that the optocoupler is damaged.

Digital multimeter detection method: The following uses PC111 optocoupler detection as an example to illustrate the digital multimeter detection method. The detection circuit is shown in Figure 1. During the detection, insert the + terminal {1} ​​pin and the - terminal {2} pin of the diode connected to the optocoupler into the c and e jacks of the Hfe of the digital multimeter respectively. At this time, the digital multimeter should be set to the NPN block; then connect the c pole {5} pin of the phototransistor connected to the optocoupler to the black test lead of the pointer multimeter, and the e pole {4} pin to the red test lead, and set the pointer multimeter to the R×1k block. In this way, the deflection angle of the pointer multimeter pointer, which is actually the change in photocurrent, can be used to judge the condition of the optocoupler. The greater the deflection angle of the pointer to the right, the higher the photoelectric conversion efficiency of the optocoupler, that is, the higher the transmission ratio, and vice versa; if the needle does not move, it means that the optocoupler is damaged. [/align][align=left] Photoelectric effect judgment method: Still taking the detection of PC111 optocoupler as an example, the detection circuit is shown in Figure 2. Place the multimeter in the R×1k resistance block, and connect the two test leads to the output terminals {4} and {5} of the optocoupler respectively; then connect a 1.5V battery in series with a 50-100Ω resistor, connect the positive terminal of the battery to the {1} pin of PC111, and the negative terminal to the {2} pin, or the positive terminal to the {1} pin and the negative terminal to the {2} pin. At this time, observe the deflection of the pointer of the multimeter connected to the output terminal. If the pointer swings, it means that the optocoupler is good. If it does not swing, it means that the optocoupler is damaged. The larger the swing deflection angle of the multimeter pointer, the higher the sensitivity of the photoelectric conversion.