Electrical fail
Posted: Sun Jan 01, 2017 6:04 pm
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.
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.