A DC power supply can be hard to find and expensive. With features that are more or less hit or miss for what you need.
In this Instructable, I will show you how to convert a computer power supply into a regular DC power supply with 12, 5 and 3.3 volt outputs. For about $10!
Why use a computer (ATX) power supply? Well, they're available everywhere, and they can output tremendous amounts of power in a small form factor. They have overload protection built right in, and even a 500W model can be reasonably priced with high efficiency. The voltage rails are incredibly stable. Giving nice, clean DC current even at high loads.
Plus, it's likely that many of you simply have an extra one lying around doing nothing. Might as well get the most value for your investment.
Step 1: Getting Started
The first order of business is that of safety. While I'm reasonably sure that there isn't enough residual energy to stop your heart, those capacitors can still bite, and that can cause significant pain and maybe even burns. So be paranoid when getting close to the internal circuitry. It would probably be a good idea to put on some insulating gloves. Also (obviously) make sure the thing is unplugged. You are responsible for your own safety!
Here are the tools/parts needed:
3 x "Banana Jack" Insulated Binding Post sets
1 x bag of "#6" Ring Tongue Terminals (16-14 gauge)
Small bit of heat shrink.
Ok, let's get to voiding some warranties!
Step 2: Opening Up
Open the PSU and make an assessment of the space you have to work with. Make sure that there won't be any clearance issues for the binding posts or wires.
Once you have decided how your PSU will be configured, mark with pencil where you want to drill the holes later on. This will help you in cutting the wires to the appropriate length.
Step 3: Wires, Wires Everywhere
You will be met with the daunting task of sorting through a hundred wires of different colors. The only colors we care about are Black, Red, Orange, Yellow and Green. Any other colors are superfluous and you can cut them at the circuit board.
The green wire is what tells the power supply to turn on from stand-by mode, we want to just solder it to a ground (black) wire. Put some heat shrink on this so it won't short out on anything else. This will tell the PSU to be constantly on without a computer.
Cut all of the other wires down to about a foot, and remove any zip-ties or cable organizers. You should have a forest of wires with no connectors.
The colors represent:
YELLOW = 12 Volts
RED = 5 Volts
ORANGE = 3.3 Volts
BLACK = Common Ground.
Now, theoretically, you could be done. Just hook the wires to 4 large alligator clips (one for each color set) or some other terminals. This might be handy if you're just going to be powering one thing, such as a ham radio, electric motor or lights.
Step 4: Grouping Wires
Group the 4 wire colors together and cut them to length to where you marked where the posts would go. Use the wire strippers to take off the insulation and stick about 3-4 wires into one tongue terminal. Then crimp them. The exact number of wires per voltage rail depends on the wattage of the PSU. Mine was a 400W and there are about 9 wires per rail. You need all these wires so that you can get all of the current rated for that rail.
Step 5: Holes
Now we come to the drilling. With most power supply units, you won't be able to completely remove the circuit board from the chassis. But you should be able to remove it partially and wrap it in plastic so that it doesn't get contaminated by metal shavings.
Onces you have the holes drilled, file away any rough spots and wipe down the chassis with a damp cloth.
This might be a good time to figure out something for that hole the old wiring harness used to go through. I used a washer and the head of a bolt to make a cap, and epoxied it in there. But this is purely cosmetic and unimportant.
Step 6: Putting It Together
Now comes the fun bit. Install the binding posts while using a small screwdriver to make sure they're all orientated right when you're tightening them down.
Install the tongue terminals onto the back of the binding posts and tighten them down good and snug with the pliers. This might be tricky if you have a high-wattage PSU as you will have more wires. The most the posts shown in these pictures can take is 4 tongue terminals.
After that's done, close up the power supply.
I had some clearance issues with mine- the 90mm fan just wouldn't fit. I figured since it will not be acting as the exhaust fan for a computer anymore, it wouldn't be needed anyway. So I removed it.
Step 7: Make It Pretty
You need some way of clearly marking which post is which voltage. You could go super polished and make a color-coded decal in Illustrator and print it at your local print shop, but I'm lazy... and cheap. So I used some permanent markers.
You could also take some plastic or vinyl paint and color each post. Whatever puts a bee in your bonnet.
Lastly, stick on the rubber feet on what you want to be the bottom.
Step 8: Conclusion
My 400 Watt power supply can deliver 23 Amps through the 12V rail, and 40 Amps through the 5V. That's very good for something that, aside from the initial cost of the PSU, cost about $10.
Step 9: Updates
This project is not necessarily original and has been done by many people.
The most "together" project is that of this guy: http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply
There are a multitude of other projects, but I feel mine and his are the best I've seen so far.
Issue of the Resistor
Power supplies need a certain minimum load to work properly. The min. load for mine is around 0.8 amps. Thus if you plan on powering LED's or other such low-power device exclusively, you'll need a resistor to provide a load. Otherwise you will damage the PSU.
A meaty 10-Ohm, 10 watt resistor from Radio Shack is a good choice. Wire it across 12 volt and ground.
-12V and -5V lines
It has been brought to my attention that the -12V and -5 lines are pretty handy for diversifying the voltages this thing can produce. These are the white and blue wires I told you to cut earlier.
Of course, adding them is simple, it's just a matter of getting two extra binding posts and connecting the wires to them. It's just a question of "Do I need these?"
I didn't, all I really needed was the 12V line. But as I said, if you need them, they're easy to install.
Still going strong! This little PSU has been super handy.
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