So, I'd been struggling with a mechanical relay that just wouldn't actuate properly in the printer. I swapped it for the spare (Always have one on hand), which I had pre-tested, and that didn't work either. If you bumped the test button it would stick. I checked contact resistance, and all these other things. So I finally open it up further, and when I go to test the voltage, what do I get but 5VDC, for a 12VCD relay. Ahh, the wonders of ATX power supplies without color coded wiring.
So, here's the ways to avoid fails like this and do it right:
Color code your wiring. Depending on the application, different color codes are used, but I would typically use an ATX scheme with the addition of sky blue for 24Vdc (distinguish this from -12VDC). This will make it easier for you ensure you have the right voltage for each application.
Label wires at each connection: This makes it easier to trace a wire, and be able to sniff test a connection. It works even better if you draw out your circuitry. If you aren't familiar with them yet, I seriously suggest looking at ladder diagram formatting for anything you intend to wire by hand rather than put in a PCB. Consider an alphabetical prefix for each voltage with a number for the wire itself, possibly with a postfix for the device or connection as well. This means that every wire within a printer, or even every connection should be identifiable by a 3-4 digit code.
Test points! If you can't check a voltage with the appropriate panels dismounted to access it, you've probably designed something wrong, especially if it's something like a mechanical relay, even more so if it's part of a thermal fuse circuit as mine was.
If anyone else has minor electrical derps like this, feel free to share. They are far from unique, and printers are complex electromechanical beasts.
A place to post your fails. We know you got 'em, now share what you think caused 'em
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