I’m not a big fan of computer power supplies but I had so many extra parts from building my other power supplies, I thought I’d give this a try. The only thing I needed to purchase was a couple of switches. I thought I could pull this off without any assistance but I seem to have run into something that don’t make sense.
I didn’t want to build your typical ATX power supply so I de-soldered all the output wires and only kept the wires I needed – 3.3V, 5V, 12V and ground. Here’s a rough wiring diagram.
My plan was to run the three voltage wires to a 3 position rotary switch then connect the switch output to a terminal on the case front.
The voltages were a little low but I read that I need to add a 10 ohm – 10 W resistor to the 5 volt line. That worked fine as I now have the voltages I need.
The problem occurred when I tried to connect the rotary switch. When I switch from 12V to 5V or 3.3V to 5V the power supply kicks off. If I start at 5V it’s fine but when I switch to 12 or 3.3 it kicks off. If I disconnect the 5V wire and just have 12V – space – 3.3V it works fine. The switch is a “make before break” so I thought that might be the problem. So I opened the gap between the connectors but that didn’t help. The switch is a 4 pole so I moved the 5V wire to another pole and connected the 2 output terminals together. That didn’t help either. I have another switch (4 position – 3 pole). I connected that switch and still get the same results.
I’m out of ideas. Anyone have any suggestion?