I have a power supply unit (PSU) of the old type with a 20-pin main power connector for the motherboard. Unlike most of the newer units with the 24-pin power connector this one does not have a green "power on" wire. What is the equivalent to this wire on the old PSU units?

I basically just want to check to see if this PSU is working, and check the voltages, just to rule it out as the cause of problem. But I'm not sure how to do the "paper clip test" where you jump the green and the black (ground) pins, as this one lacks the green wire.

Maybe the power is supplied on this unit as soon as I plug it in? With no need to jump wire any pins? Or is it the grey wire (power good)? What is this "power good" anyway?

How do I even tell what ATX version this PSU is? Number of pins on main connector? Color coding?

Viewing the 20-pin connector from the bottom, with the retaining clip to the right, these are my color coded wires, starting from the top pin on the left row:

  1. Purple
  2. Purple
  3. Black
  4. Red
  5. Black
  6. Red
  7. Black
  8. Orange
  9. Brown
  10. Yellow
  11. Purple (2x)
  12. Blue
  13. Black
  14. Grey
  15. Black
  16. Black
  17. Black
  18. White
  19. Red
  20. Red

The pin # 11 has two purple wires in it.

Here are some additional details from the label of the PSU:

RED +5 V
BLUE -12 V

It looks like grey wire is "power on" and orange wire is "power good".


A typical ATX 20 pin PSU connector should have a "Power On" at pin 14. http://jwenet.net/notebook/2005/1161.html.

Looking at the connector face-on, if all the pin's shielding have their flat side facing to your RIGHT, then pin 14 is the 4th from the top on the RIGHT column.

"Power Good" outputs +5 V after the power supply passes its own self-test. This is typically within a second of being powered on. The +5 V then remains live until the PSU decides something bad has happened, and this tells the motherboard that the voltages may be unsafe. Without the +5 V on "Power Good", the motherboard should not POST.

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  • So I need to place a paper clip (or other conductor) between "power on" pin and a black ground pin? I understand you are looking at the shape of the individual pin insulators of the P1 12V ATX connector. Looking at the connector this way places the connector retaining clip to the right, which corresponds to the color coding I posted above. So my pin number 14 is then grey! It is not green. But the label on the PSU states that grey wire is "PS-ON", and orange wire is "P.G". That means Power Good, I guess, what else? – Samir Dec 30 '12 at 20:03
  • It appears Dell has PSU's with grey PS-ON wires aswell, so you're not alone. See acer5050's post: instructables.com/id/Turn-Computer-Power-Suppy-into-Car-Audio – Bort Dec 30 '12 at 20:21
  • Yeah, some manufacturers apparently use all the colors of the rainbow in their wire coding. Or they used to do so, I should add, as most PSU units supplied today (either pre-installed or off the shelf after market ones) use color coded wires that follow the current ATX standard recommendations. Maybe that's why the PSU units from around 2010 and onwards no longer tell you on the label what wire does what? The labels only state the wattage on the different rails these days. Well anyway, thanks for this confirming piece of information. – Samir Dec 30 '12 at 22:23
  • I have shorted the grey (power on) and a black wire (ground/com), and when I plug in the PSU to the mains outlet the fan inside the PSU starts to spin for a second and then stops. I have tried connecting an Antec stock 120 mm case fan to one of the Molex connectors to create some load, and it also stops when the PSU fan stops. I unplug the PSU and then plug it back in and it's the same thing. It spins for a second and then stops. Is it dead? Or do I have to make more load? Maybe connect a disk drive or something? I will check the voltages now. – Samir Dec 30 '12 at 22:31
  • Alright, now I have the measurements. This unit is pretty much dead. I used a multimeter and I measured all the wires individually. It showed 0 Volts for all but Brown at pin 9 which showed constant 5.06 V. Yellow at pin 10 also showed -3.2 mV but it was decreasing by time, down to -2.0 mV last time I checked after about half an hour. So this PSU is toasted. It's a Macron PSU model MPT-301 that came stock with an old Toshiba desktop from the time when they were still producing desktops. So it was very old, and it served well for many years. I have installed a new PSU now and all systems go. – Samir Dec 31 '12 at 18:09

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