(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.
The ATX PS board with 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 only +5 V and +12 V. 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.
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
Wiring coming off an industry standard circuit board will be:
|GRAY||POWER-OK What is this??
|PURPLE||+5 V STANDBY
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 can clip the other wires at the circuit board level.
For the +5 / +12 volt PS, you will need the following combinations:
|GREEN / BLACK||Power on Switch
|RED / BLACK||Load Resistor
|YELLOW / BLACK||+12 volt source
|RED / BLACK||+5 volt source
I use a single common post (GND --
black) for all voltage sources. Our loads are light and we don't
require separate grounds for
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
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. Here are some tips that may
help increase this voltage level, a little theory and 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.