External Relay with PS_ON - Which line to switch?

  • Hello,

    I've been trying to set up an external relay on my CoreXY printer to make use of the PS-ON function, but have been having issues with where to put the relay. I'm using the Duet 2 Wifi (fantastic piece of kit!)

    My aim is to keep the 5v supply to the Duet as 'always-on' (hence it's own supply) so that I can still log in via DWC, and only provide the motors, fans & heaters power when necessary. I like the idea of being able to upload files, play with the config etc whilst knowing everything else is off. It will allow me to power down the heaters etc when a print is finished, but also to give a level of protection if the Duet recognises a fault (and lets me still see what fault it found, whilst keeping everything else powered down). I know relying on firmware detection isn't the perfect safety answer, but is a good starter-for-ten

    My issue is where to place the relay in the power wiring. Below is a diagram of how I originally intended to impelment this, using one of these relay modules which I think are very common place (I actually had an Ebay equivalent, rated at 10A):
    I put this in (one coil switching the live, other switching neutral) and all worked fine for a few cycles, but then the contacts fused closed...

    Reading some more into it, I believe this was due to the inrush current on the power supplies. Both of my 24V supplies are cheap LED ones (PS2 is 250W, PS3 is 480W) as normally used on 3D printers. I originally expected a max current draw of ~5A @ 240Vac (assuming some losses). However from looking at datasheets for various power supplies, I believe the inrush currents can be as high as 10x max operating currents (as the power capacitors have very low impedence at startup). This means I would need a relay rated at ~50A to be safe and these seem hard to come by! I've since managed to find a 30A module (https://www.ebay.co.uk/itm/2-Way-30A-High-Low-Level-Trigger-Relay-Module-Smart-Home-Automation-Control-L-6/184077472003?ssPageName=STRK%3AMEBIDX%3AIT&var=691772179715&_trksid=p2057872.m2749.l2649)

    As I see it my options are:

    • Find a 50A relay and retry as before
    • Use two of the 30A modules, one for PS2 and one for PS3
    • Put the 30A modules on the 24V lines coming out of either power supply
    • Any combination thereof

    Does anyone have any experience or advice on which might be the best option? Also, if going down the mains-side relay, should I switch both live and neutral, or just live?

    Duet Power Wiring.png


  • I am facing same dilemma. Got this relay https://www.amazon.com/gp/product/B0117FB276 and plan to use it to switch the +24V to the two heaters. Duet will be powered all the time by 24V. Haven't tried it yet.

    Will consider later to add a +5V PS and cut the the +24V Vin to the duet to reenergize also steppers and fans. Not sure if this is necessary though.

  • It really depends on how "safe" you want to be. If you switch the mains feeds to the power supplies then there could be some delay between the input going off and the 24V dropping since the capacitors will keep the 24v going for a second or two. On the other hand, if you leave the power supplies always powered and switch the 24v going to the duet and external mosfet, then you may not be able to turn off the mains quickly in the event of a PSU short, fire, etc.

    In practice, I think most folks switch the mains (as I do) since the time between you recognizing an issue and hitting your emergency stop button will probably be longer than the time it takes for moving kinematics to drain the PSU caps.

    You only need to switch the line/hot wire to the PSUs and an SSR might be better than a relay. SSRs when they fail, they usually fail cosed but as you've discovered overloaded relays will also fuse closed. 🙂

    You can control the SSR or relay from the PSU_ON pin on the duet and you can also wire your emergency stop button (if it's normally closed) in series with that line so the button will trigger the SSR/relay as well as the PSU_ON signal.

  • Thanks for your input @gtj0 & @zapta

    I think what I'll do is get one of the 30A electro-mechanical relay modules to switch the 24V side of both power supplies and also get an SSR to cut the live wire going in to them. I'll drive all of them via the Duet's PS_ON. That way I can get the best of both worlds.

    My physical E-stop button is already wired to cut the mains supply to both 24VPSUs so I'll keep that separate and up-stream of the SSR. I generally keep that depressed unless printing (and especially will do with this setup) in case I accidentally activate the PS_ON when I didn't mean too. You can see it in DWC too as it reports the input voltage 🙂

  • @engikeneer, be aware that PS_ON provides protection only during printing. If for example your printer is in idle mode, you set the temp manually from PanelDue and the system has a thermal runaway, e.g. due to a shorten mosfet, duet will detect it but will not disable PS_ON.

    (your plans sounds reasonable to me, I don't have a suggestion).

  • @zapta It still provides "manual" protection in that you can send M81 via the DWC "ATX Power" buttons or any other means at any time.

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