The layout below shows the pinout connections for the ATX power connector.
| 11 | 1 |
| 12 | 2 |
| 13 | 3 |
| 14 | 4 |
| 15 | 5 |
| 16 | 6 |
| 17 | 7 |
| 18 | 8 |
| 19 | 9 |
| 20 | 10 |
The pins correspond to the following functions:
1 = 3.3V
2 = 3.3V
3 = GND
4 = 5V
5 = GND
6 = 5V
7 = GND
8 = PWR_OK
9 = 5VSB
10 = 12V
11 = 3.3V
12 = -12V
13 = GND
14 = PS_ON#
15 = GND
16 = GND
17 = GND
18 = -5V
19 = 5V
20 = 5V
Signal Definitions
Please see section 4.2 in [1] for a more detailed description of the above signals as well as color coding.
PS_ON# - an active low signal that delivers power to every main power rail when grounded. When held high or left open circuited, the power rail outputs should not pass any current and should be held at 0V with respect to ground. This signal should be maintained at +5VDC by a pullup resistor that is internal to the power supply.
5VSB - a standby voltage that is used to provide power for circuits
during the powered-down state of the power rails.
This could be used to power a small logic circuit
that controls the power to a much larger circuit. i.e. as a means of lowering
power consumption when the larger circuit is not needed...
PWR_OK - a "power good" signal that should be set high by the power supply to indicate that the +5VDC and +3.3VDC outputs are above the undervoltage thresholds of the power supply. PWR_OK is deasserted to a low state when either of the +5VDC or +3.3VDC output voltages falls below the undervoltage threshold or when the mains power has been removed for enough time so that power supply operation is no longer guaranteed..
Usage
In order to utilize an ATX power supply in your project all that you need to do is ground the PS_ON# signal to turn on the main power rails.
Some ATX power supplies also have a main power switch which must also be turned on.
If section 3.2.1 in [2] is looked at it can be seen that to meet the ATX specification all positive rails must be within +/- 5% "under all line, load, and environmental conditions."
An ATX power supply was meaured in the lab under no-load conditions and was found to have +3.27V on its +3.3V rail and +5.14V on its +5V rail. This is well within +/- 5%.