Testing Optoisolator IC PC123
Some optoisolator ICs do not have any reading when you test on the collector and the emitter pins with analogue meter set to X 10k ohm range. You may have thought that it can be a bad opto since it does not show any reading. One good example is the PC123 optoisolator IC which is found in many SMPS will not show any reading when tested using the analog meter. In order to solve this problem just connect two analog multimeter in series (just like batteries in series) and set to x 10 k ohm range and test the opto. A good PC123 opto should show one reading on both meters. If you still did not get any reading then the optoisolator IC is said to be open circuit. Hope this trick helps!