Maybe an odd question here, and I have a theory of what the answer will be, but figured I would throw it out here.
Is it ok to test a component that is mounted on an existing PCB? For example, I picked up a printer/scanner combo at Goodwill and ripped it apart. It has a bunch of surface mount LEDs and I would like to see how bright they are, but I don't want to remove them from the boards right now. Can I just hook them up to 5v (+ a resistor) to check, or is this a bad idea?
Can I just hook them up to 5v (+ a resistor) to check, or is this a bad idea?
It really depends on how the rest of the circuit is wired. I can't say it is a "bad" idea, but I wouldn't say it is a good idea, either. What may happen is that another component(s) might "get in the way", and they might cause a problem (short circuit or something else) that would cause them to fry or make the LED not operate.
If you don't care about the circuit, see if you can "cut the traces" on either side of the component, so you can isolate the device and then test it in-situ.
A similar issue can be found when testing resistors or caps (especially SMT components) on a PCB - because of the surrounding circuit and depending on how it is wired, the value you may see on your multimeter may or may not match the actual value of the component being tested...