Introduction: Another Benchtop Power Supply From PC Power Supply

This instructable will show how I built my benchtop power supply from the power supply unit in an old computer. This is a very good project to do for a number of reasons:

- This thing is very useful for anyone who works with electronics. It supplies nice, clean DC power in a number of voltages with overload and short circuit protection built right in!
- It is a very easy project. Most of the work is already done for you inside the computer. It's just a matter connecting a few wires and you're done.
- It's very cheap. I got the old computer for free and the rest of the parts were under $10. A commercially built benchtop power supply like this could run you more than $150!
- It's somewhat environmentally friendly since your recycling old parts to make something new.

I should mention that this is not an original idea of mine. I learned everything I know about this project from other Instructables about power supplies (there are dozens). My project is unique only because of the enclosure I built for it. The guts are the same as any other one.

My particular unit is capable of suppling +12, +5, +3.3 VDC and -12, -5 VDC. These 5 rails along with the Ground rail can be mixed and matched to provide many different voltages eg. the voltage between the +12 and -12 rails is 24 volts). There is also a handy on/off switch in the front with lights that indicate how the unit is operating.

Since I don't have any electronics projects on the go yet, I am only able to demonstrate a simple relay circuit. Here you can see the relay powering different combinations of indicator lights based on the state of the pushbutton.

Step 1: Gather Your Tools and Get Yourself an Old Computer

The things you'll need for this project vary greatly depending on your own design but you'll definatly need:

- A multimeter
- A pair of wire cutters / strippers
- A screwdriver with a Phillips head and flat head
- An electric drill with a set of drill bits

Other Materials / Tools I used that you might want to consider:

- A sheet of 1/4'' craftboard
- Carpener's glue
- Clamps of various sizes
- Table saw
- Carpener's square
- Measuring Tape

Electrical Drvices:
- An on/off toggle switch
- Red 5mm LED
- Yellow 5mm LED
- 330 Ohm resistors
- Solder iron and solder

Connectors and Rails:
- Machine screws
- Washers
- Hex nuts
- Ring terminals
- Zip ties

The washers, hex nuts and ring terminals should be sized appropriately to fit the machine screws. The ring terminals should be able to accept 16 to 14 guage wire (this allows several wires from the power supply to fit in at once).

Finally, you're going to need a computer. I put a wanted ad for old computers in the local online classifieds. A week later I had 3. Or perhaps you already have one lying around. A lot of schools will throw away a bunch of computers once in a while too. People should be happy to give them away since it costs them money to dispose of them. Either way, when you get your hands on one you'll be ready for the next step.

Step 2: Extract the Power Supply

It shouldn't bee too hard to remove the outer case of the computer. Usually there is nothing more than a couple of thumb screws holding it on. Once you have removed the fasteners, it should slide right off.

The power supply shouldn't bee too hard to identify either. From the outside of the box, you can tell where it is because of the big fan and the socket into where the computer cord plugs in (Photo 1). In most cases there's also a rocker switch near the socket and fan. Once you're inside, you'll also be able to see that the power supply is a large grey box with a big bundle of multicolored wires sticking out of it (Photo 2).

The wires coming out of the power supply have white plastic plugs on the ends of them called molex connectors. There should be several of them connecting to the hard drive, CD drive, floppy dive, motherboard, fan, etc. (Photo 3). You'll want to unplug all of these. Make sure you have them all and that they're pulled free of all the brackets and cables inside (Photo 4).

Once that's done you'll need to remove the screws that hold the power supply to the case (Photo 5). In some cases the power supply may be rivited to the compter case. You can make short work of the rivets with an electric drill and a metal drill bit.

After that the power supply should lift right out of the case (Photo 6). For this project you won't need the rest of the computer but keep in mind that it still contains many useful parts like fans, motors, ribbon cables, capacitors and resistors just to name a few. Also, the pins on the CPU are made out of gold.

Step 3: Get Out Your Multimeter

Each color of wire coming out of the power supply box supplies a different voltage. All wires of the same color supply the same voltage. It's your job to use the multimeter to find out what color supplies what voltage. Now I SUPPOSE I COULD tell you the colors and their respective voltages (which I will later) but you should test them anyway just to be safe.

You should start by using your wire cutters to cut all of the wires from the molex connectors. Cut as close to the connector as possible since you may want to have the wires as long as possible(Photo 1). You should also remove the zip ties that are bundling them together. Later, you may want to make your own bundles of each wire color.

Take one wire of each color and strip a little bit of the insulation from the tip. If you have a terminal strip handy you should connect the wires to that, otherwise try to bend them away from each other so that the tips do not touch each other.

Now let's fire this thing up. You should get the power cord and plug it into the back of the power supply. Plug the other end into a wall outlet. If there's a rocker switch on the back of the power supply flip that to the on position. Next you should take the green wire and touch the bare end off of the metal casing of the power supply. It should spring to life with the telltale "whirr" of the fan. In order to keep the power supply running while your working you should secure the wire to the case with one of the mounting screws (Photo 2). All of the wires are live now so don't let them touch each other! If they do the power supply will shut off and you'll have to unplug it and plug it back in.

Now set your multimeter to DC volts. Keep the black probe touching off the black wire and touch the red probe off of each color of wire. The reading you get from each color is the voltage supplied from that particular color. They should read as follows:

Yellow +12V (Photo 3)
Red +5V (Photo 4)
Orange +3.3V
Blue -12V
White -5V

Your reading may differ a little since the power supply won't put out a stable voltage unless there's a minimum load on it. Since your multimeter isn't enough of a load you might get slightly different readings.

In addition you'll have these wires:
Black - Ground
Green - Power On signal
Grey - Power OK Signal
Purple - Standby Signal

As you test the wires you should write down the results. You will need them when labeling your posts later. Also, find the decal on the side of the power supply case and write down the maximum amperage rating of each rail (Photo 5). Try not to exceed these ratings as the power supply will overload and shut off.

Step 4: Build an Enclosure...Maybe?

While most people seem to add a few binding posts to the case and stuff everything inside I don't recommend it since the wires could interfere with the rotation of the fan. You also increase the risk of shorting one of the wires to the case or other components. I think it would be a lot easier to wire everyhting up too, since you can have a lot more room inside an enclosure.

My enclosure is pretty simple. It's nothing more than five pieces of craft board glued together - all butt joints (Photo 1). My woodworking skills are very limited but I still found this to be extremely easy.

The enclosure does not have a back side. Instead, the back of the power supply case makes it up. This allows access to the socket for the power cord and to the master switch. It also allows the fan to expel exhaust air. Small openings above and below the case allow fresh air to enter the enclosure (Photo 2). The power supply case is hot-glued into the enclosure.

Once side of the case is not glued on. Instead, there are small blocks glued into each corner to keep it place. The side fits snugly into the rest of the enclosure so that additional fasteners such as screws or latches are not needed to keep it in place. This allows access to the inside of the enclosure (Photo 3).

Step 5: Making the Connections

All wires of like colors will need to be gathered and connected together. You should remove the zip ties that bundle the wires from each molex connector. Next, cut and splice all of the wires to the same length and use a ring terminal to hold them together. I bought ring terminals that were rated for wire bigger than the power supply wires. This allows several wires to fit into one connector. It cuts back on materials and time.

Using a ring terminal is easy. Just slip the bare end of the wire into it and use the wire cutters/strippers to crimp the terminal to the wire (Photo 1). Do this for all of the wires except for the grey, purple and green ones. There ary typically a lot of red and black wires coming from the power supply, you'll probably need to use several ring reminals for these (Photo 2).

Now, all of the ring terminals for each color can be slipped over a bolt and put through the front of the enclosure (Photo 3). The ring terminal and he head of the bolt should remain inside while the washer and hex nut are used to secure it from the outside (Photo 4).

Step 6: Adding the Switch and LED's

If you don't know how to solder than there are plenty of instructables on it. In any case, you'll need to break out the soldering iron and the solder for this one.

Wiring up the switch is simple enough. Remember that green wire you secured to the case? You just need to interrupt that wire with a toggle switch. Just cut and splice the green wire somewhere along the middle and solder the two new ends to the terminals of the switch (Photo 1). The switch should be easy enough to mount simply by drilling an appropriately sized hole in the enlosure and popping it in (Photo 2 ).

The LED's can get a little trickier. If you're using a red and a yellow LED then the forward voltages are roughly the same and you can use the same value of resistor for each. Since the voltages of the grey and purple lines are 5 volts, you'll need two 330 Ohm resistors.

Each LED has a short and a long lead. The short end is the Cathode (-) end and a short piece of wire should be soldered to it. The long end is the Anode (+) and the 330 Ohm resistor should be soldered to that. It helps to prepare the LED's in this way before mounting them inside the enclosure. The LED's can easily be mounted in an appropriately sized hole with a drop of crazy glue.

Now all you have to do is solder the purple wire (Standby) to the resistor end of the yellow LED and solder the grey wire (Power OK) to the resistor end of the red LED. Next, the wires coming from the cathode should be connected to the Ground rail (Photo 3).

Step 7: Finished!

And there you have it, a cheap benchtop power supply out of recycled parts. In the near future I will try to start prototyping some elecronic circuits that I will need for a much bigger project that I have in mind. I believe that this nifty piece of equipment will be invaluable for that process.

As always, I hope you enjoyed reading and hope you found all of the information you need for building your own benchtop power supply. As I said, there are many other instructables out there on virtually the same thing so you should check them out too.

If you have any questions or comments feel free to leave them in the "comments" section.