07.10.2013

Atx power supply power ratings

Computer power supply units (PSU) deliver the power to the PC hardware via a number of cables with connectors. Standard ATX power supplies typically have the main power connector P1, additional 12V connectors, as well as peripheral, floppy drive, serial ATA, and PCI ExpressA® receptacles, which we will describe below.
Under normal operation, PS_ON# is activated when you press and release the computer power button while it is in standby mode.
Also note that many brands such as Apple's Power Mac, Dell (during certain years), Compaq and HP used proprietary boards with entirely different pin designations- see this for info on some brand name power supplies. All voltages are referenced to the same common (if you need to measure any voltage, connect the return lead of your voltmeter to any of the COM contacts).
When the industry began using voltage regulation modules (VRM) running off 12V2 to energize CPU and other motherboard components, the bulk of the wattage shifted to 12 volt buss. Some units may also have an optional 2x3 socket that can be used for auxiliary functions, such as fan monitoring and control, IEEE-1394 power source, and a remote sense of 3.3 V.
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In the revision 2.0 of the ATX PSU specification, P1 changed to a 24-pin part for higher power. For years, Dell used the same connectors as in standard ATX, but wired differently (see the diagram to the right for the pinouts of their Pentium® II and III PCs, Precision 410 and Dimension 8100). Apple's Power Mac G3 and some G4 (APG and PCI) power supplies likewise used standard 20-pin socket with custom pin designations. I suggest that you connect the board with a junk motherboard and harddisk because a single lamp may not enough as a load and the power supply will shutdown. Well, let's take a little care with this test, because many times the SMPS can turn on but no video on Display.
Convert ATX PSU to Bench SupplyIs 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.
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. We can not just simply plug the PSU into the mains supply and expect to get the required 5 or 12 volts output.
Unfortunately you can not just have the wires left open, luckily both of these issues are easily fixed.
Pin outs of the 20-pin connector with the colours of the wires used in a standard ATX PSU connector. There are a number of ways to convert a standard computer ATX power supply unit into a usable bench top power supply. 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.
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.
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.
Pin outs of the 24-pin connector with their respective colours of the wires in the PSU cables. 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).


An old PC power supply unit makes an excellent and cheap bench top power supply for the electronics constructor. 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 current being drawn from the PSU so can get a little noisy. All in all, converting an ATX PSU to a bench power supply is an easy project with many uses. Hello Ray, I built my PSU supply a while back using an old 200W ATX unit with the 20-pin connector and which worked fine.
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Their generic specifications for various desktop systems are defined in Intel's design guides, which are periodically revised. Most of today's motherboards supply their CPU with a separate 12 volt cable, which has 4 pins for ATX style (sometimes called P4) or 8 or more pins for EPS and non-standard high-power systems.
What's worse is some computer brands use in their motherboards standard ATX headers with proprietary non-standard pin-outs.
The designations of the original 20 signals left unchanged for backwards compatibility (see the ATX power connector pinout diagram to the right and also see our guide on interchangeability between ATX versions).
As far as I know, except for Dimension 8100, beginning with Pentium® IV their systems use the standard pin designations.
Although most G4 (such as QuickSilver and Gigabit) towers used 22 pins which are mechanically incompatible with any ATX, Mirrored Drive Doors had 24 pins. From a technical standpoint, GES pin-out makes more sense than any other one because they grouped together the signals of the same name. Some power supply just need back its own motherboard or system in order to function properly. 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. 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. You can connect the same coloured wires together using crimp connectors or posts, is the same thing. Luckily, pin 15 – Black is next to it so I connected a switch between the Pwr_On signal (pin 14) and Ground (pin 15). 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.
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. 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 old 20-pin connector has been replaced by a larger 24-pin Molex connector or even a 20+4pin connector. 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.
Note that some ATX12V power supplies with a 24-pin motherboard connector may not have the -5V (pin 20) white lead. 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.


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. I did not experience the same shutting down issues that you are having so it is difficult for me to comment on this. The unique atx connector on this power supply allows you to use it for both 20 pin and 24 pin atx connector configurations.
Intel® later on introduced so-called ATX12V that differed by an additional 2x2 +12V connector (you can find information on the auxiliary cables here). Note that TRKL refers to a so-called trickle output that is active whenever the computer is plugged in. As a rule of thumb, if you have a branded PC, you should suspect it may be incompatible with the industry standard. If possible place two more harddisk to act as load just incase the single motherboard load not enough to switch it on.
When pin 14 is momentarily connected to ground via the switch, the power supply will turn-ON. 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. 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. Anything below this the power supply may run, but regulation will be very poor less than 50%. In this case use the older ATX power supplies with a 20-pin connector above if you need the additional -5V supply. It's 630 watts and provides needed power when upgrading video cards or adding devices to your computer.
However, in many cases it will, and you can fry your motherboard if you plug a generic ATX PSU into a branded board and vice versa. In reality however, most manufacturers ignored this requirement and wired both +12V1 and +12V2 to the same output with combined overcurrent protection.
Basically, it is just a different name of a standby supply that feeds the power-on circuit.
Alternatively you could try shorting the power supply ON pin to ground (0V) and use the units mains power switch to power up. Accordingly, different ATX-style PSU may use different number of power wires: see the pinout diagram to the right. Here you will find information on the main power connector P1 of both ATX and some branded PCs, which will help you determine the right replacement power supply. By the way, recently Apple "conveniently" removed from their website pin assignments info on most older models.
Well, under certain conditions a new PSU can still be used with an old PC and vise versa- see our guide on connecting 20-pin PSU to 24-pin motherboard.



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