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I need to control two computer power supplies so they turn on and off at the same time. Since the primary power supply doesn't use a standard ATX motherboard connector, I can't just splice the PS_ON lines together.

I am aware of third-party products that can do this like the add2psu - these products take 5V from a Molex or SATA power connector and use it to drive a relay to switch the PS_ON line of the secondary power supply. I'd like to DIY this, but I don't have a 5V or 12V relay handy, so I'm planning to use a MOSFET instead.

My question is - why do these products use a relay instead of a transistor? As far as I'm aware, ATX GND is tied to earth ground, so there is no need for isolation. A continuity test between the GNDs of my two PSUs confirms they are connected. Is the only reason these products use a relay is to account for the possibility of improperly grounded house wiring? If I know mine is properly grounded, it should be perfectly okay to use a FET, right?

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  • \$\begingroup\$ Should be fine but draw a schematic of what you propose, only way to make sure people understand you. \$\endgroup\$
    – winny
    Commented Dec 15, 2021 at 20:00
  • \$\begingroup\$ Something like this. imgur.com/k36D5mZ.png \$\endgroup\$
    – flashbang
    Commented Dec 15, 2021 at 20:13
  • \$\begingroup\$ Gotta ask, What does "at the same time" mean? Same second? Same ten milliseconds? Same microsecond? ... \$\endgroup\$
    – Solomon Slow
    Commented Dec 15, 2021 at 20:35
  • \$\begingroup\$ I think within 100ms would be acceptable. I remember reading something about that being the limit in the PCIe standard for how long a device should wait for its auxiliary power to come online (I'm looking to power a GPU). I haven't confirmed that, but I can at least do it successfully by hand, so I doubt the tolerances are too tight. \$\endgroup\$
    – flashbang
    Commented Dec 15, 2021 at 22:21

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There is no true way to know why products that exist use a relay when it is not needed.

Perhaps when there was a sudden need to control a secondary supply and the circuit was cobbled together in 5 minutes with random components found at home, maybe reused from broken devices - who knows. Then it was turned into a product and all clone makers make similar product now.

Sure, ATX common ground, the black wire, is tied to earth ground, and also the metal case is tied to earth ground. But if both power supplies are not bolted on to the same metal case, and if for some reason, one or both mains plugs are connected to an ungrounded outlet (in some countries this is possible, so you don't even need improper house wiring, buildings may be so old that generic outlets are not earthed), then there will be no continuity between the two power supplies.

At minimum, both power supplies must connect together the black common ground wire. Then you can use a FET, with black ground connected to FET source, drain connected to secondary supply green PS_ON wire, and the gate will be controlled by maybe the red 5V from primary supply.

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  • \$\begingroup\$ If I already know the PSU GNDs are tied together (by checking with a DMM), I still need to add an additional connection tying them together? Also, the PSUs are not both currently bolted to the case, but they are connected to the same power strip. \$\endgroup\$
    – flashbang
    Commented Dec 15, 2021 at 20:08
  • \$\begingroup\$ @flashbang See, that's exactly the reason they use a relay. It might be fine in your case. However, if you power them from a separate power strips or separate sockets, who knows how long the earth wire that connects the power supplies is. So it will just be better to either isolate, or connect the PSU grounds together properly and for sure. If you want, you could use an optocoupler instead of FET, it's not a relay and it still isolates. \$\endgroup\$
    – Justme
    Commented Dec 15, 2021 at 20:11
  • \$\begingroup\$ Right - that's what I figured in my question, but I wanted to make sure there was no additional reason that I was missing. I figured a relay is pretty expensive compared to a FET which made me wonder if there was another benefit to their design. Since this is just for me to use and I know the risks/limitations, I think I'll be okay. \$\endgroup\$
    – flashbang
    Commented Dec 15, 2021 at 20:14

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