Is this the correct way to turn an ATX PSU into a timed power supply?

Thread Starter

s355

Joined Apr 13, 2021
32
WHen the button is pressed, the PSU should be on for a certain amount of time then shut off. Is this the correct way to do so?
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sghioto

Joined Dec 31, 2017
2,441
Looks correct if you complete the ground connections on Q2 and the relay coil.
This also assumes the PS_ON to GND is correct to turn on the supply.
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Irving

Joined Jan 30, 2016
2,322
You need a diode, cathode to +ve, across the relay coil to protect Q1 from back emf when the relay drops out.

Actually you don't need Q1 and the relay at all.... PS_ON is spec'd as a TTL level signal, with an internal pull-up to 5vSB. You only need to pull it to TTL logic 0 (<0.4v) to turn the PSU on. Just connect the collector of Q2 to PS_ON.

I'd put a 1k resistor between 5vSB and the switch.

The timing will be quite crude - how long were you expecting?
 
Last edited:

Irving

Joined Jan 30, 2016
2,322
There is no 'unknown' voltage to be present. There's no magic about PS_On, it's a TTL input with a (typically) 50k pullup, designed to be driven by an open-collector output, which is exactly what we have here...

Here is the description of PS_on from the data sheet of a common control chip used in ATX psus:

"Remote On/Off logic input for CPU or controller. Turn on/off the PWM Output after the 7 msec
/ 25 msec delay.
PSon = 0 means that the main SMPS is operational.
PSon =1 means that the main SMPS is off and the latch is reset."
 
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