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February 1st, 2026 03:10
Dell XPS 420 problem
Has been trouble free for many years until today when it wouldn't power up this morning. Quick troubleshooting and the power supply was good. It finally started after I cut the power supply green wire going to the motherboard, grounded the green wire side leading to the power supply, and then depressed the power button at the front of the desktop. Only this way is it starting, just pressing the front power button has no effect. Turning off, I depress the power button on the desktop. Then, when the motherboard seems to be disabled (and the power supply fan speed greatly increases), I then unground the green wire going to the power supply. What is the problem, is the main power switch partially faulty somehow? Seems like its activating the PC, but only after I have grounded the green wire manually. Thank you in advance for any help and solutions.
anne_droid
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February 1st, 2026 11:42
Hi
Dunno.
Perhaps the green wire is routed via the Main Board and needs to be grounded either via the switch or a component on the MB.
However the internet has a better explanantion, naturally...........
On an ATX-style power supply like the one used in a Dell XPS 420, the green wire is not a ground; it is the PS_ON control signal that tells the PSU to turn on its main outputs when pulled low.
What the green wire does
The green wire on the main motherboard connector is the PS_ON signal, generated by a small standby circuit inside the PSU.
When the PC is “off” but plugged in, the standby 5 V rail is active and holds PS_ON at a logic high level via an internal pull‑up resistor.
The motherboard’s power button circuit works by momentarily shorting PS_ON to ground, and the motherboard then holds it low when it wants the PSU fully on.
Why grounding it starts the PSU
The PSU’s control logic is designed so that when PS_ON is pulled to ground (i.e., logic low), it enables the main DC rails (+12 V, +5 V, +3.3 V).
If you manually connect the green wire to any black wire (0 V return), you are imitating what the motherboard does when you press the power button, so the supply starts up and the fan spins.
When you disconnect the green wire from ground, PS_ON floats back high via the internal pull‑up, and the PSU shuts its main rails back off.
Why Dell behaves this way
The Dell XPS 420 PSU follows the standard ATX control scheme, so this behaviour is expected rather than a Dell‑specific quirk.
It is provided so that the motherboard can implement soft power (ACPI, standby, software‑driven shutdown) instead of using a hard mains switch in series with the AC input.
So, grounding the green wire “enables it to start” because you are asserting the PS_ON signal low, which is the PSU’s designed method of turning on the main outputs.