Electronic – the correct way to interface 3.3v logic with the PS_ON# 5v signal on a standard PC power Supply

atxvoltage divider

The PS_ON# line of a standard ATX PC power supply needs to be pulled to ground to turn the supply on. The signals on an ATX supply are 5v logic according to the standard. I'm trying to re-purpose an old PC power supply and I need to control it with a 3.3v logic device (GPIO on a Raspberry Pi). What would be the right way to connect the PS_ON# signal to the 3.3v pin so I can use it to turn the power supply on and off.

I've tried a resistor voltage divider but that didn't work because it always pulls the signal low and I'm afraid that hooking the 5v PS_ON# directly to the pin will fry the 3.3v chip. Is there an easy way to do this that I'm missing or do I need a special part to deal with the difference in voltages?

Best Answer

Use a NPN transistor. PS_ON to a 1k resistor at the collector, 1k resistor at the base, emitter to ground. Voltage at the base can be different than the collector.

Update: Further research shows the PS_ON tends to have a pull-up resistor to 5vsb. In that case, adding a 1k to the collector creates a resistor divider. If too high a ratio, then the input signal might not cross to a Input Voltage Low. You can size down the resistor, or omit it.