Convert ATX PSU to Bench Supply to Power Circuits
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Convert ATX PSU to Bench Supply
Convert ATX PSU to a Bench Power Supply
Is it possible to use an ATX power supply unit or PSU from an old PC as a bench top power supply to power 5V logic,
but with limitations. The standard computer power supply unit (PSU) turns the incoming 110V or 220VAC
(alternating current) into various DC (direct current) output voltages suitable for powering the computer’s internal
components and with a little bit of imagination it is possible to convert ATX PSU to a bench power supply.
Most computer PSU’s range from about 150W up to 500W so their is plenty of power.
The original ATX standard connector used for Powering the Motherboard
(http://amazon.com/dp/1840786019/?tag=basicelecttut-20) was a single 20-pin Molex
that has all the required +12VDC and +5VDC voltages with huge output currents and
short circuit protection as well as a Power-ON wire that allows the PC’s software to
turn “OFF” the PSU on shut down.
Firstly and more importantly before you start to convert ATX PSU, make sure that the
PSU is unplugged from the mains supply and discharged by letting it sit
unconnected for several minutes before you start. This is important! as it could
result in a potentially dangerous or even lethal situation due to the high voltages
inside the PSU if you decide to dismantle it. Also make sure that the metal box of the PSU is correctly earthed or grounded.
You are responsible for your own safety!.
We can not just simply plug the PSU into the mains supply and expect to get the required 5 or 12 volts output. The standard
PC power supply unit has two safety mechanisms that prevent it from being switched “ON” without the motherboard
attached.
Number 1, the PSU requires a “Power-ON” zero voltage signal to start up similar to the “ON-OFF” switch on the front
of a PC.
Number 2, for the PSU to correctly regulate the +5V output voltage it needs to have some sort of load attached, at
least 5W to trick the PSU into thinking its attached to the motherboard
Unfortunately you can not just have the wires left open, luckily both of these issues are easily fixed.
There are several different coloured wires attached to the 20-pin ATX connector providing several different voltage outputs
such as +3.3V, +5V, +12V, -12V, -5V as well as a number of black ground wires and a couple signal wires as shown in the
following image along with their colour-code and description.
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Convert ATX PSU to Bench Supply to Power Circuits
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20-pin Molex ATX Connector
Pin outs of the 20-pin connector with the colours of the wires used in a standard ATX PSU connector.
2 av 10
Pin
Name
Colour
Description
1
3.3V
Orange
+3.3 VDC
2
3.3V
Orange
+3.3 VDC
3
COMMON
Black
Ground
4
5V
Red
+5 VDC
5
COMMON
Black
Ground
6
5V
Red
+5 VDC
7
COMMON
Black
Ground
8
Pwr_Ok
Grey
Power Ok (+5 VDC when power is Ok)
9
+5VSB
Purple
+5 VDC Standby Voltage
10
12V
Yellow
+12 VDC
11
3.3V
Orange
+3.3 VDC
12
-12V
Blue
-12 VDC
13
COMMON
Black
Ground
14
Pwr_ON
Green
Power Supply On (active low)
15
COMMON
Black
Ground
16
COMMON
Black
Ground
17
COMMON
Black
Ground
18
-5V
White
-5 VDC
19
5V
Red
+5 VDC
2015-09-30 12:44
Convert ATX PSU to Bench Supply to Power Circuits
20
5V
http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supp...
Red
+5 VDC
There are a number of ways to convert a standard computer ATX power supply unit into a usable bench top power supply.
You can keep the 20-pin Molex connector attached and connect directly into it or cut it off completely and group together
the individual wires keeping the same colours together, reds to reds, blacks to blacks etc.
I cut off the connector to have access to the individual wires and connected them into a screw connector strip to give me a
higher amperage output for both the +5V and +12V supplies. You can connect the same coloured wires together using
crimp connectors or posts, is the same thing. Some of the other individual coloured wires we need to keep separate as
detailed below.
To start up a stand alone PSU for either testing purposes or as a bench power supply, we need to short together pin 14 –
Green (Power-ON) to one of the common black wires (ground) which is how the motherboard tells the power supply to turn
“ON”. Luckily, pin 15 – Black is next to it so I connected a switch between the Pwr_On signal (pin 14) and Ground (pin 15).
When pin 14 is momentarily connected to ground via the switch, the power supply will turn-ON.
Next we need to provide a small load on the +5V (red wires) output to trick the PSU into thinking its attached to the
motherboard and to keep the power supply in the “ON” mode. To do this we have to connect a large resistor of 10 Ohms or
less, with a standard power rating of 5W to 10W across the +5V output using just one set of the red and black wires, pins 3
and 4 will do.
Remembering from Ohms Law that the power (P), developed in a resistor is given by the equation: P = I2 × R or P = V2 / R,
where: P = power developed in the resistor in watts (W), I = current through the resistor in amps (A), R = resistance of the
resistor in ohms (ohm) and V = voltage across the resistor in volts (V). The voltage will be +5V and the power required is 5W
or above. Then any standard power resistor below 5 Ohms will do. Remember though that this resistor will get HOT! so
make sure its out of the way.
One other option we have is to use pin 8 – Grey (Pwr_Ok) as a visual indication that the PSU has started up correctly and is
ready to operate. The Pwr_Ok signal goes high (+5V) when the power supply has settled down after its initial start up, and all
the voltages are within their proper tolerance ranges. I used a red LED in series with a 220 Ohm current limiting resistor
connected between pins 8 and pin 7, (ground) for this power ready light but anything similar will do, its only indication.
Testing the Power Supply
Once assembled you should end up with something like this.
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When you plug the PSU into the wall socket and turn the switch “ON” at the back of the power supply (if it has one), only two
voltages should be present at the connector. One is pin 14 the Pwr_ON green wire which will have +5V on it. The second is
pin 9 the +5V Standby (+5VSB) purple wire which should also have +5V on it. This standby voltage, is used for the
motherboard’s power control buttons, Wake on LAN feature, etc and typically provides about 500mA of current, even when
the main DC outputs are “OFF”, so it can be useful as a permanent +5V supply for small power uses without the need to turn
the PSU “fully-ON”.
Some newer ATX12V power supplies may have “voltage sense” wires that need to be connected to the actual voltage wires
for proper operation. In the main power cables you should now have three red wires (+5V) all connected together and three
black wires (0V) connected together as the others have been used for the switch and LED. Also connect together the three
orange wires to give a +3.3V output if you require it.
If you have only two orange wires, you may have a brown wire instead which must be connected with the orange’s, the
+3.3V for the unit to be able to power up. If you only have three red wires, another wire (sometimes pink) must be
connected to them. But check this first.
If everything looks ok then we are good to go and the PSU should switch “ON” giving you a very cheap bench top power
supply. You can test the output voltages using a multimeter or connecting a 12V bulb into the different sockets to see if the
PSU works. The voltage combinations that can be outputted by the PSU are 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5,
-5), 7v (+12, +5), 5v (+5, 0) which should be sufficient for most electronics circuits.
You could also connect a LM317 Adjustable Voltage Regulator, a 5k adjustable potentiometer, a 240 Ohm resistor for
biasing and a couple of smoothing capacitors across the +12V supply to give a separate adjustable output voltage from
about 2.0 to 12 volts but this is an additional feature.
The 24-pin Molex ATX Connector
In newer desktop PC’s, version 2 ATX power supplies are used called ATX12V. The old 20-pin connector has been replaced
by a larger 24-pin Molex connector or even a 20+4pin connector. The four additional pins are: two additional pins
numbered 11 and 12 are +12v (yellow), and +3.3v (orange) and the two additional pins numbered 23 and 24 are +5v (red),
and ground (black) respectively. The newer ATX12V pin outs and colours are given in the following table for reference.
24-pin Molex ATX Connector
Pin outs of the 24-pin connector with their respective colours of the wires in the PSU cables.
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Pin
Name
Colour
Description
1
3.3V
Orange
+3.3 VDC
2
3.3V
Orange
+3.3 VDC
2015-09-30 12:44
Convert ATX PSU to Bench Supply to Power Circuits
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3
COM
Black
Ground
4
5V
Red
+5 VDC
5
COM
Black
Ground
6
5V
Red
+5 VDC
7
COM
Black
Ground
8
Pwr_Ok
Grey
Power Ok (+5 VDC when power is Ok)
9
+5VSB
Purple
+5 VDC Standby Voltage
10
12V
Yellow
+12 VDC
11
12V
Yellow
+12 VDC
12
3.3V
Orange
+3.3 VDC
13
3.3V
Orange
+3.3 VDC
14
-12V
Blue
-12 VDC
15
COM
Black
Ground
16
Pwr_ON
Green
Power Supply On (active low)
17
COM
Black
Ground
18
COM
Black
Ground
19
COM
Black
Ground
20
-5V
White
-5 VDC
21
+5V
Red
+5 VDC
22
+5V
Red
+5 VDC
23
+5V
Red
+5 VDC
24
COM
Black
Ground
The newer type ATX12V PSU’s are a little more tricky to convert as they use a ‘soft’ power switch function and require a
much larger external load resistance. To get them to start-up, or switch-ON, the supply must be loaded to at least 20W or
10% of the rated power for the larger 600W+ PSU’s. Anything below this the power supply may run, but regulation will be
very poor less than 50%.
Again the voltages that can be output by this unit are the same as before 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5,
-5), 7v (+12, +5), 5v (+5, 0). Note that some ATX12V power supplies with a 24-pin motherboard connector may not have the
-5V (pin 20) white lead. In this case use the older ATX power supplies with a 20-pin connector above if you need the
additional -5V supply.
An old PC power supply unit makes an excellent and cheap bench top power supply for the electronics constructor. The
power supply unit uses switching regulators to maintain a constant supply with good regulation and short circuit protection
cause the unit to shutdown and be re-powered immediately if something goes wrong.
The only downside with using an ATX PSU as a bench power supply is that the cooling fan’s rpm responds to the amount of
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Convert ATX PSU to Bench Supply to Power Circuits
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current being drawn from the PSU so can get a little noisy. Also the ATX PSU requires a certain amount of fresh air to keep it
cool inside which may not be possible when laid onto a bench.
All in all, converting an ATX PSU to a bench power supply is an easy project with many uses. Not bad for something that
would otherwise get thrown away.
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34 Responses to “Convert ATX PSU to Bench Supply”
6 av 10
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Stafylidis Vladimiros
Hi. Very very helpful guide ! I have a couple of questions…..if you pleased
1) what is the watt capability of a 250 watt psu e.g. 4pin peripheral connector ?
2) is it safe to connect a TEC1-12706 peltier and which connector is better ? 12V from
20pin or one of the 4pin peripherals ?
3) why don’t you also connect the wires from the peripheral connectors with the other
coloured wires together ?
4) I’d like to connect a 3 contact illuminated switch to pin14 and pin 15 and the switch
to light when on but I’m not sure how …. Is this the correct way to switch on/off the
psu (mine haven’t an on/off) ?
thanks in advance
February 28th, 2015 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=6113#respond)
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2015-09-30 12:44
Convert ATX PSU to Bench Supply to Power Circuits
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Wayne Storr (http://www.electronics-tutorials.ws)
The 250W power rating relates to the maximum output power capability of the PSU
at room temperature. It is generally the sum of the all the +12v and +5v connectors.
There is no reason why you can not power a TEC from the 4-pin 12v peripheral
connector. Connecting the power-ON (pin 14) to ground is enough to turn the PSU
ON, but as it is an active low output it can not be used to illuminate the switch. You
would need to illuminate the switch separately.
February 28th, 2015 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=6114#respond)
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Tom
hi, i have a problem with high noise of the atx when i just wire pin14 and gnd. but
when i connect the 20pin molex to a motherboard i can switch the atx on just by
connecting two pins on the mobo (pwr+-, its just for start they dont need to be
connected after that) and the atx is absolutly silent (except the fan). Is it possible to
start the atx like the mobo does? Or do you have any suggestions what the noise could
be and how to avoid it?
February 10th, 2015 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5870#respond)
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Wayne Storr (http://www.electronics-tutorials.ws)
The high frequency noise is probably due to the switch mode circuit constantly trying
to adjust itself as with the molex connector disconnected there is voltage but no
current to sense. Once connected to the motherboard there is a load and therefore
current to control. As explained above, a suitable resistive load across the output will
trick the psu into thinking its connected to the motherboard and keep the psu in the
“ON” mode. Pin 14 only needs to be momentarily connected to ground (0v) to start it.
February 11th, 2015 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5877#respond)
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Tom
thank you for your reply, but i am still unable to kill the noise. i have found that for
the atx are important only pins 2 (0.25A) and 9 (0.34A). But when i replace one of
them with some device with even higher comsumption, the atx is noisy again.
February 12th, 2015 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5902#respond)
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Mark Primavera
Very good tutorial I just completed my power supply unit – I am mainly using it to
supply 12V to charge my R/C airplane batteries. Going to share my project with my R/C
flying club.
Thanks again!!!
January 20th, 2015 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
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Kiberu Ivan
thanks for the informative post. ..
I have an ATX power supply 400W and a PWM solar regulator 30A idle. is there a way
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5360#respond)
2015-09-30 12:44
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to feed the regulator with a DC Source other than the solar panels to make an
automatic quick 12V battery charger. The regulator accepts 16V – 25V at the input.
December 25th, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
/comment-page-2#comment-5360)
jon
Definitely try grounding the green wire before buying anything. My old ATX from
2003ish works just fine without a load.
December 21st, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5315#respond)
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Rick
I have a couple of old AT power supplies. They actually switch on right at the 110/220
volt line input (yes these are really old). My question is this: do I still need to keep a
constant load on this type of PSU because it seems to work fine without it?
December 18th, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
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Wayne Storr (http://www.electronics-tutorials.ws)
If your AT supply works without a load attached then no.
December 18th, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5289#respond)
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Cliff Jones
It’s still a good idea to put the 5W power resistor in place. The voltage regulator
operates best at the middle of it’s range. Without the resistor whenever you connect
any load you are going from a no-load to a -load condition. There is a potential for
ripples, spikes etc.
Plus the 5W resistor can help with absorbing any transient short circuit you might
encounter, maybe saving your project and obviating the need to open the power
supply to replace a fuse.
Of course “condensers” or “smoothing capacitors” mentioned above are really what
you’d prefer for dealing with transient shorts you might encounter in your projects.
I’ve used 1/4 farad, 600V non-polar capacitors out of a dead guitar amp with good
results. But be careful, a capacitor can hold a charge for years, and pack enough
wallop to hurt someone if inadvertently grounded. (plus never ground a large
capacitor with itself, you can damage them)
December 26th, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=5378#respond)
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g mustafa
I have a supply. I want to convert it into battery charger. When I try to rise voltage
throw TL 431.over volt protection shutdown the supply. Supply model is HP
2507fwp.pl help me 4 this problem. Sorry 4 my English.
September 20th, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
Reply (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html?replytocom=2105#respond)
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2015-09-30 12:44
Convert ATX PSU to Bench Supply to Power Circuits
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SAMER AL-HAKEEM
hi G Mustafa, please send me PWM ic number(if TL-494 or the like) on my email:
samer5004@hotmail.com (mailto:samer5004@hotmail.com)
to explain how u can convert it .
if u want , i will write in arabic.
October 5th, 2014 (http://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html
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