12V, High Current ATX Power Supply Mod, Part 2

This is part 2 of a three part post on turning a PC PSU into a high current, 12V power source for a hobby charger. This was work done on the morning of the 15th, but I did not have time to post about it at that time, so I’m doing it now.

To begin with, I used the following site article as a guide to the conversion. In the article, they put outputs for +5V and +12V, as well as -5V and -12V. However, I really have no need for the other outputs, as I am also building a lab-quality, adjustable, dual output, linear power supply for my miscellaneous projects. So, I simply cut all the other output wires, so that I could add them in the future, if I ever wanted too. Also, there wasn’t a whole lot of room, so I probably would have needed a bigger case if I wanted additional outputs anyway.

What I did in part 1 was cut all the output wires I didn’t need, and solder some banana jack output terminals to the +12V (red terminal) and 0V (black terminal) wires. I had a purple and gray wire, which I was going to use for a “plugged in” indicator and an “power on” indicator, but decided not too, at this time.

If you look, you can see a gold heatsink hidden in the back, this is a 5Ω resistor, which provides a load on the 5V necessary for proper operation of the power supply. I just tucked in it back there because there isn’t a lot of room anywhere else.

I used a drill and a grinder cut a spot for the terminals in the lid, and mounted them in place with some silicon caulk I had handy to help keep them in place, but mainly to keep the mounting nuts from loosening over time. I found a power switch in my junk box and ran a wire from ground (black) to the switch, then from the switch to the green wire. This turns on and off the outputs, but doesn’t shut off the mains power. I then used liquid electrical tape on everything, as there were a bunch of solder joints and lots of cut wires.

To be double sure, I put a coat of liquid electrical tape on the side of the case next to the wires, as well.

More modern supplies have voltage sense wires, which are connected at the motherboard connector, to monitor the voltage of the outputs. These are smaller wires of the same color as the wires they monitor, and must be connected together. Mine had them for 3.3V (orange), 5V (red), and I can’t remember now if it had it for 12V (yellow), but I doubt it, as 12V is not critical on a computer. This is the 5V voltage sense wire, connected.

I put it all together (quite a tight fit now), and measured the output. It looked pretty good, at 12.09V, according to my $5 multimeter.

Here’s what it looks like, completed.

Now, I didn’t get a chance to test it much, but the initial results seemed disappointing. I put it on a half amp charge on my three cell Lipo quad pack, and I heard the fan slow down as soon as the charge started. I used the charger’s menu to find out the input (PSU) voltage, and it was around 10.4 volts. Wow, I thought, that’s disappointing, to drop that much under that little of a load. I then tried to up the charge to 6A, which I figured it should be able to handle no sweat, and the charger stopped the charge because the input voltage dropped too much (it can function properly down to 10V). Since it’s rated at 18A, I was honestly very disappointed. I mean, I know the charger won’t be very efficient, as it has to have a buck/boost converter, but even at 50% efficiency, at 12V input it should not have drawn more then 12A input, and I doubt it’s only 50% efficient, I can see 75%, but not 50%. But even then, I should have 18A, right?

Well, I figured I would, but it appears I don’t. I’m not quite sure why. I did think it might be because power supplies sometimes have split rails, and I might have only been pulling from one rail, so I might only have had half the power available. All in all, I’m not quite sure why I don’t have the power I should. I’ll test it more tomorrow, and see just how much current I can charge at before my power supply lets me down.

I’m also going to keep my eye out on a high current, 13.8V power supply on eBay, the ones designed to power radios and other high power electronics that are designed to run off vehicle power. They are generally pretty pricey though. I might be able to make my own, as well. We’ll see.

Testing results will appear in part 3.


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