Desktop Power Supply from a PC

Updated March 23, 2005
(See the narrative and disclaimer at the bottom of the page)

A completed 145 watt ATX power supply with switch, binding posts, labels and feet. Notice the zip ties in the ventilation slots
that hold the load resistor.

This ATX PS board has leads for +5 (RED), -5 (WHITE), +12 (YELLOW), -12 (BLUE) volts, Ground (BLACK) and switch (GREEN). Dell power supplies manufactured between 1996 and 2000 do not follow the industry standard pinout and color codes. The fan has also been unplugged for better viewing. Since this PS was converted for use in the logic and robotics labs, the selected voltages were tapped. Other users may want combinations of +3.3 V (ORANGE), +5 V and/or +12 V if they are converting one of the newer supplies. For R/C applications, the 5 volt output can also serve as a desktop source to drive receivers and servos. If used as a power source for the micro and sub-micro servos, you must be careful not to drive the servo to either endpoint to avoid stripping the smaller gears in these units. Most standard servos have sufficiently robust gear trains and will simply stall if pushed to the mechanical stops..

Measured voltages on this particular PS (1996 P5-100 MHz Gateway) were about 5.15 and 11.75 volts. The remaining leads have been clipped off at the circuit board.

View of the case top with fan, binding posts and switch. The switch (SPST) and binding posts are available at Radio Shack or other electronics suppliers.

Power supplies in today's computers are known as SWITCHMODE or Switching Mode power supplies and require a load to continue to operate after being switched on (the term switching mode actually applies to the technique of A/C to D/C conversion and not to the power up action). This load is provided by a 10 watt, 10 ohm wire wound load resistor (sandbar - about $0.80 at Radio Shack) across the +5 volt supply. Some inexpensive power supplies may fail if forced on without a load. The sandbar has been zip tied to the case with a small amount of heat sink compound applied. Without cooling, the resistor will get very hot and may fail prematurely. With this arrangement, the resistor will remain barely warm to the touch. If you are using a high wattage supply, it may be necessary to double up on your load resistors to avoid overheating.

Additional comments

Disclaimer: The information presented should not be considered a "HOWTO" article, but merely a documentation of my conversion process. Modern PC Power Supplies can produce high output current levels that may cause internal overheating in the PS or damage to devices connected to them. Any individual attempting their own conversion is cautioned to carefully research their PS specifications and to be mindful of the associated voltages and power. DO NOT work on your opened power supply with it plugged in!!!!

The PS in the picture is a 145 watt ATX salvaged from a 1996 P5-100 MHz Gateway -- I salvage all usable parts from the older PC's before dumping them. This one is set up for a logic lab, hence the +5, -5, +12, -12 volt taps. We also use the +5 to drive servos in the robotics lab. This supply does not have a 3.3 V source, but the newer supplies do. INTEL has continued to modify the ATX specifications to include additional power connectors to support the increased power requirements of the newer motherboards. Before any modification is attempted, you should be sure of the type of power supply you are working with and the output currents being produced at each voltage level. Higher wattage supplies can generate fairly hefty levels of current and may overheat or damage devices attached to them. See the Table of Representative Current Levels for other power supplies.

Wiring coming off an industry standard circuit board will be:

WHITE-5 V     (May not be present on recently manufactured supplies)
GRAYPOWER-OK     What is this??
BROWN+3.3 V REMOTE SENSING     Design Guide Update

*** Note that the 1996-2000 Dell's did not completely follow this color coding -- check your voltage levels with a meter before wiring ***

The yellow, red and black wires will likely be grouped together with a clip. Some of the PS's will have a detachable plug for the fan and some will have the fan permanently attached to the circuit board. If the fan is attached, I usually clip the wires then re-solder and cover with heatshrink tubing -- this gives more working room while modifying the PS and allows me to lube the fan.

If you are going to use only the +12 and +5, you may clip the other wires at the circuit board level. For the +5 / +12 volt PS, you will need the following combinations:

GREEN / BLACKPower on Switch
RED / BLACKLoad Resistor
YELLOW / BLACK+12 volt source
RED / BLACK+5 volt source
ORANGE / BROWNSee the Design Guide Update

I use a single common post (GND -- black) for all voltage sources. Our loads are light and we don't require separate grounds for each.

Leave 3 black wires -- switch, load resistor and common (GND) binding post

Leave 2 red wires -- 5 volt binding post and load resistor

Leave 1 yellow wire -- 12 volt binding post

Leave the green wire -- power on switch

If sense wires are present, refer to the Design Guide Update

If you expect to place high current demands on your powersupply, it may be prudent to run two wires to each binding post -- while it is very unlikely that the 18 AWG wire will overheat, there have been some instances of melted wires and connectors occurring on high demand motherboards.

As an aside, you can get 7 volts from the +5 V and +12 V outputs -- the +5 V is considered the negative (GND) and +12 the positive -- some geeks will use this combination to run their fans at a lower speed to reduce noise.

I've followed all the instructions, but the output voltage on the +12 V side is still low -- what can I do?? Many of the R/C folks are converting power supplies for the purpose of driving field chargers and are finding that voltage levels below 12 volts are sometimes insufficient to power their chargers. Read these TIPS for some options that may help increase this voltage level, provide a little theory and identify the connector pinouts found on most PC supplies.

Cut everything else off even with the board. I usually cut the power harnesses so I can keep as much together as possible. The wires remaining in the power supply should be left long and cut to length as needed. If you leave them too long, they will get in the way when boxing it up, especially if the fan is internal rather than external. Be sure that they stay out of the way of the fan blades. Also be sure to reattach the fan -- some supplies will not function without the fan attached - in any event, you need the cooling. This PS in the pictures has the fan mounted on rubber shock mounts and is extremely quiet. I will also disassemble the fan and lube the bearings while I have the PS open. Since these are salvaged, the fans have been in use for some time and normally the bearings are dry -- I use a high grade sewing machine oil from SINGER. Any light oil will work, just don't use WD40 --

These power supplies are called SWITCHMODE or SWITCHING MODE power supplies and must have a load to function -- hence the 10 ohm 10 watt load resistor on the 5 volt line. These resistors are known as wire wound or sandbar resistors and can be purchased from Radio Shack for about $0.80 each. This resistor will get hot and should have some sort of heat sink. The technique I use keeps them amazingly cool and is easy to do -- just pick the flattest side of the resistor, apply some heatsink compound and attach to the case. I will usually hit the inside of the case with a file to remove any stamping flash on the ventilation slots. The switch (single pole, single throw) and binding posts can also be found at Radio Shack or other electronics supply houses.

I usually deal with on-line suppliers such as Jameco, Digikey, Mouser, etc. because we are buying in larger quantities and Radio Shack is too expensive for large numbers of items. However, you should be able to convert your PC supply for $5.00 or $6.00 dollars -- less if you have a junk box of parts. I suppose you could add a LED indicator with dropping resistor to show the PS is turned on, but the fan is a pretty good hint. We have had supplies running 24/7 for months without problems -- just electricity consumption.

The PS has some fairly hefty electrolytic capacitors and can still give a bit of a shock immediately after being unplugged -- let it sit a couple of minutes before poking around inside. Obviously, you can get whacked if you are inside the case with it still plugged in -- probably won't kill you, but you WILL turn it loose (never mind how I discovered this bit of information).

If you have any questions, comments or corrections, feel free to mail me.

Updated March 23, 2005