Commercial lab power supplies can be pretty expensive. For most of what I do in my little lab I don’t really need that level of sophistication. ATX computer power supplies are cheap and plentiful. I had an old one laying around and I decided to turn it into something that I could use in the lab.
In the old days of the AT power supplies it was easy to turn them on and take power from them. I used a few of these for powering prototypes years ago but the ATX power supplies are a little more complicated and I never got around to figuring out a way to turn them into something useful.
One of the extra features of the ATX power supplies over the old AT versions is that the ATX power supply can be turned on and off by the computer itself instead of a big giant switch that turns the AC power to the supply on and off. This is done by simply grounding one of the wires in the main connector. It’s the green wire. The ATX supplies have a pretty high current capacity for 3.3V 5V and 12V outputs. There are also -5V and -12V outputs, but these aren’t capable of delivering the same current.
The one that I modified had the following specs…
|Total Power Output||230 Watts|
It also has a +5V standby of 2A that is on all the time whether the supply is powered up or not. I had no use for this one so I clipped it off.
I took the power supply apart to figure out where to put the binding posts and the power indicator LED. It made sense to use the same side where the wires come out of the circuit board. The problem with this for me was that this side of the box is on the part of the enclosure that comes off from the rest of it. It worked okay but the soldering was tricky. Other supplies will have different geometry and may be easier. This one is physically very small so I struggled with it a bit.
Radio Shack didn’t have enough of the big binding posts that I wanted so I bought a couple of little ones. I figured that since the negative voltage outputs won’t supply much current that they could stand to be on the smaller ones. It would have also been nice to have a different colored one for each voltage but RS only supplies red and black so I went with those. I didn’t want to spend forever on this little project.
The color code for all the wires…
|Green||Power Supply On|
|Purple||5 V Standby (Not Used)|
|Gray||LED Power Indication|
After I laid out and drilled the holes for the binding posts I started cutting and soldering the wires. The first thing I did was solder the green ON wire to one of the black ground wires. You could wire this to a small switch if you want to but the way mine turned out I have pretty good access to the main switch that came on the power supply. If you want to use the standby 5V power then you’ll have to put a switch between the green wire and ground for powering the supply on and off. You could also build a fancy little circuit that could power the supply off after some time. The sky’s the limit with this.
I put a 220Ω resistor in series with a small LED (check the polarity) and connected it to one of the red 5V wires and one of the smaller black ground wires. This gives me a power on indication. After I finished I discovered that the gray wire is designed to be used for this purpose. Oh well, I’ll remember that next time. Using one of the red wires works just fine.
After that I started cutting and bundling the different colors of wires together. I tried to tie as many of each color to it’s respective binding post as I could but there was no way to get all of those black wires on one post. Since I had extra black binding posts I used a couple for the ground. I tend to have banana plugs hanging 6 inches off of my power supply grounds so this will give me more options anyway.
Once I got them all soldered together I put the power supply back together and turned it on. It just blinked and went back off. It didn’t take me long to figure out what I had missed. This particular supply has a couple of small sense wires coming off of the circuit board. One was orange and the other was black. I took the thing back apart and connected the orange one to the 3.3V post and the black one to ground. When I turned it on this time it stayed one. Success!
In my research I found that there are several different types of power supplies and some have a sense wire that has to be connected and some do not. On some of them it is a different color. It may take a little playing around with whatever power supply you have. There is plenty of information on the internet too.
There also seem to be some power supplies that won’t stay on if there is no load. In this case you will need to get a 10 or 20Ω – 10W resistor and put it between one of the red wires and ground. This will give the supply enough load to stay on. Mine worked without it.
The last thing, was to make some labels with my label maker.
These ATX computer power supplies are actually pretty good switched mode power supplies, and with a little hacking they are easy to convert to bench use. This whole project can be done for less than $15 if you have an old power supply laying around. If not they are easy to find.