PS_ON driving a normal relay ?



  • So I'm just wondering if the PS_ON pin can be used to activate a normal relay or will the back EMF blow the FET? Current limits, etc ? SSR's are cheap, however I'm still nursing my wounds from last years printer budget. I have about 50 assorted relays of various voltage ratings, sizes etc. and I would really like to turn on and off the main PSU as it keeps fans running, LED lighting running, etc. I also have a few small 5v switchers floating around, looking at one now with 3A rating about 75 x 45mm with screw mounts.

    I realize the PS_ON pulls down to activate so a 5V relay safe to use ? Add a snubber for safety ?

    Thanks,

    Jeff



  • I actually figured out I can answer this question on my own by looking at the schematic for the board. The PMV40UN2 Fet used can more then handle the 50ma I need for a 10A 5 volt relay. The Fet also has a diode built in which might handle the issue of reverse EMF from the relay coil when de-energized. I may toss a fast diode in reverse on the coil just to be safe. FET's are cheap, time is not.

    Jeff


  • administrators

    You definitely need to connect a flyback diode across the relay coil, to protect the PS_ON mosfet from the back emf. Or use a solid state relay.



  • @dc42:

    You definitely need to connect a flyback diode across the relay coil, to protect the PS_ON mosfet from the back emf. Or use a solid state relay.

    Thanks for responding, I was planing on doing so as I'm a bit paranoid on that subject however it's good to get that you better or else feedback. I think an SSR would be the best method, The boss says no more spending so I'm going with a 5V coil 10A relay for now. I'll toss in something fast to snub those spikes. Need to get my standby PSU in a small case now, good thing I have a 3D printer.

    Thanks,

    Jeff


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