Linking PC PSUs for a dual rig build.

[jeremy]

I am going to try to help, but I’m not 100% sure that I understand the problem. It seems to me that you have two power supplies that can be switched on independently (P1 and P2). If either P1 or P2 are on, then you also want to have an extra supply P3 automatically switch on, is that correct?

If so, you need to do two things. The first thing to do is to determine whether one of the supplies is on. To do this, you can make an OR gate using two diodes and any positive voltage rail on P1 and P2, kind of like the schematic in this question: https://electronics.stackexchange.com/questions/130932/switching-between-battery-and-usb-using-diode-or-logic . This is what wraper is talking about.

Next, you can input this to a small circuit which will set the green wire on P3 to ground. Something like an SPDT relay as mentioned earlier would work fine, but you could also use a transistor. There is indeed a relay on that PCB you posted. For an SPDT relay, you would connect the diode or gate to the coil, connect 0V to the normally open (NO) and connect the common to the green wire on P3. Some more info on relays here: https://www.electroschematics.com/9598/spdt-relay-switch/

[Crysis1982]

That is exactly what I want.

Thank you for the drawings.

It was either that or using a switch on the front of the case to change the circuit depending on what system I was going to be using (P1 or P2) to send the current to P3 to jump the PSU.

You have been rather helpful.

[jeremy]

Cool. Don’t forget to also join the 0V of all three supplies together (although this probably wouldn’t be necessary given they should all be earth referenced).

[wraper]

If so, you need to do two things. The first thing to do is to determine whether one of the supplies is on. To do this, you can make an OR gate using two diodes and any positive voltage rail on P1 and P2, kind of like the schematic in this question: https://electronics.stackexchange.com/questions/130932/switching-between-battery-and-usb-using-diode-or-logic . This is what wraper is talking about.

Next, you can input this to a small circuit which will set the green wire on P3 to ground. Something like an SPDT relay as mentioned earlier would work fine, but you could also use a transistor. There is indeed a relay on that PCB you posted. For an SPDT relay, you would connect the diode or gate to the coil, connect 0V to the normally open (NO) and connect the common to the green wire on P3. Some more info on relays here: https://www.electroschematics.com/9598/spdt-relay-switch/

You either use relays with contacts attached in parallel and coil powered from 12V, or 2 diodes so current can be sinked from green wire of 3rd PSU to either of motherboards. NOT BOTH  . There is no need for diodes or any other circuit if relays are used.