Originally Posted by Jubijub
Quick question : I have bought a multimeter, which I plan to use to do the pin outs diagram.
For the from/to diagram for the housings wiring, I just need the continuity mode, no questions there
However, if I want to test with the PSU running to know what type of voltage is supplied by which wire : do you confirm there is no harm for the PSU/the multimeter/myself if for instance I put the two multimeter pins on 2 different 12V connector pins ?
Another example : a "molex" mate'n lock 4 pin has 12v, GND, GND, 5V : what happens if I put the multimeter in 12v and 5v ?
If I remember my physics / "technology" courses, the multimeter measures a difference of potential, so I should read 7 or -7, is that correct ?
First, just work safely. It's really easy to get seriously hurt if you don't know what you're doing when working with the sort of currents a PSU can put out, even if they're DC. It's surprising how many how-to sites and videos don't emphasize that enough; sometimes it seems like we've been using power supplies for so long they've become second nature, making me wonder if some us haven't become complacent about the safety aspects.
As for your questions about measuring voltage across the two live points used in your example, I think you're asking just in case you do so by accident or come across an incorrectly wired cable, but correct me if I'm mistaken. As long as your multimeter is set to measure (e.g., dc voltage) and you're not touching either of the metal parts on the test leads directly, no harm should come to you, the power supply, or the multimeter. That said, the difference noted above (12-5 = 7) by Daggi is similar to what's called a differential measurement, which has its purposes in things like (e.g.) data acquisition. To measure these two voltages properly, you need a ground reference, and you need to measure one voltage at a time.
You might find the following link interesting, though it doesn't go into detail about how to use a multimeter:How To Measure Voltage
Lastly, the proper to check the voltage outputs of your psu is under load, which can be accomplished by hooking it up in your system and turning it on. It helps to first use one of those psu idiot checkers
and ensure your wiring/cabling is correct, if you don't want to blow things up.
Again, be safe (and have fun)!Edited by iamjanco - 5/27/17 at 9:55am