ATX PSU voltages

[Adenitz]

Hello guys,

I know that ATX PSU used to power desktop PCs have the following output voltages: 12V, 5V, 3.3V. I know that these voltages are generated only after PSU is powered ON. When PSU/PC is off, there should be only 5V sb (standby) output that powers the mainboard and certain peripherals. When power switch is pressed, pins 16 (green PwrOn) is shorted to pin17 (ground) and all above mentioned voltages are created. When power button is released, psu remain working. What is the voltage on this green PwrOn line?
I know that in laptops, it is 3.3V present on the power switch and this voltage is shorted to ground when power button is pressed.

What happens with ATX switching PSU?

During troubleshoot of one Faulty PSU today, I noticed that if pins 16 and 17are shorted, i.e. psu is jumpstarted it starts working but only until short between 16 and 17 exists. If this short is removed, PSU immediately turns off.
I‘m not sure if this is a correct behavior or not.
Can soneone clarify this start process in more details?

Thank you.

[lotas]

Re: ATX PSU voltages

Yes, everything is correct, this is how the power supply should work, it’s just that on the motherboard this PS-ON pin is controlled by a key (NPN transistor), when power is supplied from the controller to the base of this transistor, it opens the collector-emitter (GND) junction and this pin PS-ON switches to ground and the power supply starts up.

[Adenitz]

Re: ATX PSU voltages

Hi lotas and thank you for the reply.
So, when I use digital multimeter, I won't measure voltage on Pwr_On pin on the ATX 24 pin connector?

I have also one more question. When power switch is pressed and released, shorting pins16 and 17 exists for only about 1 sec or so, but PSU continues to work after that.
When I jumpstarted PSU by using paper clip, it worked only while there was short on pins 16 and 17.

Why is that?

[eccerr0r]

Re: ATX PSU voltages

The motherboard has circuitry to connect the two pins for as long as you need the computer on... That circuit requires power, hence the purple +5VSB power going to the board.

[Per Hansson]

Re: ATX PSU voltages

Hi lotas and thank you for the reply.
So, when I use digital multimeter, I won't measure voltage on Pwr_On pin on the ATX 24 pin connector?

I don't know what PSU you have so I'm just using a Weltrend WT7517 supervisor IC here as an example, it is a common chip used in ATX power supplies.
As you can see the green PS_ON wire is pulled up to 5v by the 5VSB power supply in the ATX power supply.
Then when it is brought low (below 0.8v) the power supply will start up.
This is then held low by the PC motherboard as eccerr0r explained by a transistor.
And when the motherboard wants to turn off the power supply it releases the signal to this transistor.

[Adenitz]

Re: ATX PSU voltages

Great explanation, thank you. Exactly what I needed to know.