SSR and Mech Relay for Mains bed Controlling, Need a doublecheck



  • Right wiring up the keenovo heater
    Bed wireing
    Sorry for large image i hope it scales in the forum...
    I just want to be sure I have this right before I do a power test...

    I have the bed pins wired to the SSR INPUT...
    Mains Earth Live and Neutral are all wired to PSU, Using o ring connectors, ive also wired Live out from the PSU, that thick red wire is going into the mechanical Norm Open (ie off) pin. The switch pin then goes to the SSR 1 (short brown), through SSR to the braided bed cable. Ive assumed the bed any polarity as theres no markings...
    Through bed and back up through the other braided cable and thats terminated on the neutral pin of the PSU.

    Now one last thing. PS_ON has 3 pins. 5V PS_ON and GND.

    Is the middle PS ON pin a ground sink or active high?

    And also do I need to edit anything in config that triggers PSON when heating the bed?

    I hope this post makes sense :D, I just want to be sure I have this all right before i risk starting a house fire 😛


  • administrators

    PS_ON is an open drain output. It is turned on (pulled low) by M80, and off by M81. If you use it to drive a mechanical relay, you will need a flyback diode.

    Rather than an SSR + mechanical relay to control the bed heater, it's more usual to use an SSR + thermal cutout.

    Check that you have the + and - wires from the Duet bed heater output to the SSR control terminals the right way round.



  • the way i had it in my head, the master mechanical relay is less likely to fail On... so its a good master failsafe for software based runaway protection to kill.
    By thermal cutout, do you mean a thermal fuse? I have one on the live side of the bed heater wire (thats why its shorter :p)

    A flyback diode, you mean any old shotty?

    You say that PS_ON is pulled Low when on, so its off high? Bit confusing... Though also back to front to what i thought it would be, but also a bit of an issue as if software crashes that would mean PS_ON would go high and the mechanical would stay powered, so wont protect thermal runaway in that instance...


  • administrators

    The relay coil should be connected between PS_ON and either +5V or VIN depending on the coil voltage, with a flyback diode in parallel with it.

    It is most unlikely that a properly-rated SSR with a resistive load will fail always-on, but if it does then the thermal cutout/thermal fuse will protect against the bed overheating.



  • @dc42 said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    The relay coil should be connected between PS_ON and either +5V or VIN depending on the coil voltage, with a flyback diode in parallel with it.

    I am planning something similar. Can you give a recommendation/part no. for such a diode?



  • @mangy_dog said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    the way i had it in my head, the master mechanical relay is less likely to fail On...

    If the relay coil is fails... the relay will fail open... but this is VERY rare... because the coil is not a moving part. A much more common relays failure is "Welding", which happens when they turn on. Therefore, I would not bother with the relay at all. Just my opinion.

    I would do the SSR, AND a one-shot or self-reset thermal cut device as the actual safety net.


  • administrators

    @wilriker said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    @dc42 said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    The relay coil should be connected between PS_ON and either +5V or VIN depending on the coil voltage, with a flyback diode in parallel with it.

    I am planning something similar. Can you give a recommendation/part no. for such a diode?

    It's not critical. 1N400x (x = any digit in 1 to 7) is one possibility.



  • @dc42 Thanks. Ironically after your response I realized that I do not really need this diode because I do not attach the mechanical relay directly to the Duet but use one of these boards and as far as I can tell they already have the diode in place. 🤦

    But still that part number will make it to my "you might need it some time" list. 👍



  • @danal said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    @mangy_dog said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    the way i had it in my head, the master mechanical relay is less likely to fail On...

    If the relay coil is fails... the relay will fail open... but this is VERY rare... because the coil is not a moving part. A much more common relays failure is "Welding", which happens when they turn on. Therefore, I would not bother with the relay at all. Just my opinion.

    I would do the SSR, AND a one-shot or self-reset thermal cut device as the actual safety net.

    +1 on this. Mechanical relays can fail shut due to contacts welding together. I have seen it a few times (not on Printers).

    SSR is the easiest way to go, adding a thermal cutoff is the safest way. It's also safer for the duet board. No extra diodes to add.



  • Well it didnt exactly work...

    Need some advice, regarding the mechanical relay.

    I have 1 pin of the coil wired to 5v on the duet3d and one pin wired to PS_ON, when I send M80 nothing happens. The relay is a 5v DC trigger type...

    what am i missing? (other than a brain or a clue)



  • @mangy_dog which 5V pin are you using? The one next to PS_ON is an input. You need to use one from any of the extension headers.



  • The PS_ON pin sinks to ground when M80 is called up.

    Basically it completes the circuit to ground.

    You are using a 5V relay right? The coil is rated at 5 volts?



  • @wilriker said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    @mangy_dog which 5V pin are you using? The one next to PS_ON is an input. You need to use one from any of the extension headers.

    You sure? Looking at the wireing diagram its just a pin to the 5V rail on the board. So when the boards powered it should be able to provide 5v, alternatively can be used to give 5V off a supply...

    PS_ON sinking to ground makes sence tim, thats how i thought it should work...



  • Well circuit tester out... ok that pin doesnt read any voltage.



  • Ok, So that 5v is the external 5v input...

    Am i right to assume if i put a connector on the EXT 5v EN jumper that 5v pin is then connected to the 5v rail on the duet?

    So the normal use of using an external 5v PSU would be to disconnect the internal 5V header so external can be used...
    But If I connect both headers am I right to assume that the 5V pin on the PS_ON header is just directly conencted to the 5V rail so just turns into a simple 5V pin for me to run the coil on?

    Or is there some other circuitry going on that might mess up things?



  • OK last thing last night I found the circuit diagram... And there's a zener between the 5v in pin and the 5v rail... That's why it didn't work....
    I'm not prepared to void my warranty and solder jump that Zener....



  • @mangy_dog Just take any of the three (or actually four) available 5V output pins on the expansion headers. There is one on the Panel Due header, one on the large expansion header and on on the CONN_SD header. And in the unlikely case of all these pins used by something else you could also take the 5V from the VFAN select jumper header.

    P.S.: the latter might be protected by the 1A fuse but I hope your relay won't use that much current. 😂



  • With mech relay working, having to manually do m80 m81 to trigger it... but my keenovo heater is heating up nicely 😄

    Now all i need to do is tie M80 to trigger when the bed is set to active... and M81 when the bed is set to off and in thermal runway detection...
    How 😄



  • @mangy_dog said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    Now all i need to do is tie M80 to trigger when the bed is set to active... and M81 when the bed is set to off and in thermal runway detection...
    How 😄

    +1 on that



  • @mangy_dog said in SSR and Mech Relay for Mains bed Controlling, Need a doublecheck:

    Now all i need to do is tie M80 to trigger when the bed is set to active... and M81 when the bed is set to off and in thermal runway detection...
    How 😄

    The part for thermal runaway protection is already in place while doing a print AFAIK but not in idle (see this thread for a current discussion on this topic).

    EDIT: I just checked the source code and the PSU is shut down on a heater fault while printing or while a print is paused.

    I am not sure if there is a method to automatically connect M80|M81 to bed heating.


 

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