3.3V Relay control



  • I have a 3.3V SSR lying around thats nicely packaged for controlling a mains line that I’d like to use on my printer. I just added a Duex 5 to my Duet and I noticed there is a 3.3V pin on the GPIO header in the top left corner. As I understand it, the fans/LEDs are switched at ground. Would it be unwise to use that 3.3V pin and the ground pin of a fan/LED to drive my relay?

    Thank you in advanced



  • That should work fine but the PS_ON pin on the EXT_5V header on the Duet also switches ground and is designed for just that purpose. You can then control it with M80/M81.



  • @gtj0 I’m using the relay to switch power to my heated bed. Can I do that with the PS On without affecting any other functionality?



  • You say you have a 3.3V SSR ..... most SSR's have a MUCH larger control input range with many going up to the 30V range. If that is what you have then why not just use the bed heater output ? This is how my SSR is controlled for a 120V bed.



  • If you're using it for the bed then no, I'd not recommend M80/M81. Could get confusing.
    The bed connector is also ground-switched so even if your SSR is limited to 3.3v you can still use it.



  • @jens55 well, I should really learn to read lol. Looks like it’s /not/ an SSR, it’s mechanical. ALSO, I did not see that even though the inputs are labeled as 3.3v, the product label states an input range of 3-12V at 3-30mA. Which presents a new problem because my supply is 24V and the jumper on my fans is set to be at VIN



  • @wcj97 said in 3.3V Relay control:

    @jens55 well, I should really learn to read lol. Looks like it’s /not/ an SSR, it’s mechanical. ALSO, I did not see that even though the inputs are labeled as 3.3v, the product label states an input range of 3-12V at 3-30mA. Which presents a new problem because my supply is 24V and the jumper on my fans is set to be at VIN

    Doesn't matter 🙂 You're always switching ground whether you use a fan, bed or PS_ON.


  • administrators

    You could connect it between one of the expansion connector heater outputs (for example heater3) and +3.3V on the expansion header.



  • Oh, don't forget about a flyback diode if the module doesn't already have one.



  • if its a relay module with a transistor to drive the coil, then maybe check the markings on the relay to see if its a 12v coil? it could get a bit toasty if fed 24v


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