- Damage identification and detection of opto-couplers
The multimeter should be set to resistor R×100 instead of R×10k to prevent the breakdown of the LED due to high battery voltage. Connect the red and black probes to the input terminal and measure the forward and reverse resistance. Normally, the forward resistance is several tens of ohms and the reverse resistance ranges from several thousand ohms to several tens of thousands of ohms.
If the forward and reverse resistances are similar, it indicates that the LED is damaged. Select resistor R×1 for the multimeter. Connect the red and black probes to the output terminal, measure the forward and reverse resistance, and ensure that they are both close to ∞ under normal conditions. Otherwise, the light receiving tube will be damaged.
Select resistor R×10 for the multimeter, connect the red and black probes to the input and output terminals respectively to measure the insulation resistance between the light-emitting tube and the light receiving tube (if possible, use a megohmmeter to measure its insulation resistance, and the rated output voltage of the megohmmeter should be slightly lower than the withstand voltage allowed by the tested opto-coupler). The normal insulation resistance between the light-emitting tube and the light receiving tube should be ∞.
- Damage discrimination and detection of photoresistors
When testing, turn the multimeter to the R×1k Ω position, keep the light receiving surface of the photoresistor perpendicular to the incident light, and the resistance directly measured on the multimeter is the brightness resistance.
Place the photoresistor in a completely dark place, and the resistance measured by the multimeter is the dark resistance. If the brightness resistance is several thousand ohms to tens of dry ohms and the dark resistance is several tens of megaohms, it indicates that the photoresistor is good.
