Friday, June 23, 2017

Server PSU interlock

On my multi-GPU rigs, I use server PSUs like the Dell N750P to provide the 12V power to the PCI-E connectors.  These PSUs do not have power switches, so initially I would just pull the power cord out when I wanted to power them down.  After experimenting with the PSU control pins, I realized they have an active low "power on" pin.  Instead of using a jumper to connect it to ground, I decided to use an electronic switch to power the server PSU when the motherboard powers up.

The switch I used is a common, cheap model 817 optocoupler (pdf datasheet).  When current flows from pin 1 to 2, the optocoupler is turned on, creating a short from pin 4 to pin 3.  For my small circuit shown above, pin 4 is connected to the PS_ON signal, and pin 3 is connected to ground on the server PSU.  Pin 1 is connected to 12V (from the 4-pin 3.5" floppy drive power connector), and pin 2 is connected to ground.  On the back of the board is a 1K current-limiting resistor in series with the red LED which is a power on indicator.

I also made an even simpler interlock using only an optocoupler with the pins straightened and 0.1" header pins:
I connect pins 1 and 2 to the motherboard's power LED pins, which would normally light up a LED  when the motherboard powers up.  The motherboard already has a current-limiting resistor for the power LED, which typically limits the current to around 10mA.

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