Controlling my heated bed and SSR from DuetWifi



  • I posted this question in another tread and was probably not very good at explaining my self. This time a simple drawing is attached to describe the parts I have in mind.
    Power for the heated bed is delivered from the DuetWifi card, i.e the terminal dedicated for this, NOT from any PSU external. It's 24 volt.
    I suppose this would be more straightforward if a connected switch or button open the SSR with a signal. But here I want, if possible, DuetWifi to do this instead, that after a command from the web interface. How can DuetWifi, while delivering power over the terminal, close or open this, i.e control the SSR? Maybe there is a straightforward way for someone knowing more about the card or this type of wiring. As you probably already notice, my insight is limited.
    PS. I suppose Duet could just cut the power output. Then, why would I need the SSR?

    Here is the SSR I have in mind: http://www.auberins.com/index.php?main_page=product_info&products_id=288



  • SSR is only needed when the power supply for the bed comes from sources other than the DuetWifi board itself.
    Power to the bed (i.e. controlling temperature) is adjusted by sending g-code or using the web interface



  • If your heated bed is 24v and you intend to power duetwifi by 24v and the current of your heated bed is less than 15A then you don't need an ssr just connect the bed to the duetwifi directly.

    If the bed is powered by 220v AC/120v AC then you need a DC-AC ssr.

    If the bed will be powered from a separate power supply or needs more power than 15A then you can use a DC-DC ssr but you can also use external mosfet boards like this http://www.ebay.co.uk/itm/322457707787 or even a mechanical car relay. The duet wifi just switches this external switch using its heated bed output.

    Do a bit more reading around about this technology it seems as though you're not fully grasping what you're doing and wiring this up wrong can be dangerous.



  • @jandan:

    How can DuetWifi, while delivering power over the terminal, close or open this, i.e control the SSR? …........................

    Simple answer. The heated bed terminals are not permanently live. They are switched on and off by the Duet.



  • Thanks for all advice's. I had a small feeling that an SSR was not needed in this case, but would not rely on my own judgment in this case.

    Jan



  • Raising an old thread here but O wondered if someone could answer a quick question. If I have a 220v 600w silicone heat pad it’s draw would be 2.72amps if I wired that to a DCAC SSR I assume the amps become 25 (600/24)? Is this correct? Obviously that would be too much for the board.

    The reason I am trying this is I have a 24v 360w silicone pad currently and I can not get it over 95deg which seems strange to me but I can’t seem to change it. So if anyone has any tips on getting that near it’s supposed 150deg max that would help also!

    Thanks


  • administrators

    The point about using an SSR is that the Duet only needs to switch a tiny current to the control terminals of the SSR, then the SSR switches the much higher voltage and current to the bed heater.

    How large is your 24V 360W bed heater? Have you measured it's resistance? Have you measured the voltage at the bed, to make sure you are getting close to 24V there?



  • It measures 24.1v at the bed so that’s good. The resistance is 1.6ohm. The pad itself is 300x300mm. The supplier tells me it’s natural to get a wattage loss, I agree but it’s surely shouldn’t be so huge.

    Could it also be a PID setting? I can’t get the auto tune to work. It says it can’t reach specified temperature, which I set at 90deg. It can as I have used it at 95 but maybe it’s a time issue?



  • since it has just affected me I would make a warning that if the SSD relay fails and short circuits then the heated bed supply is switched on 100% and not controllable by the Duet. Yesterday my SSD failed and I found the 300mm diameter hotbed heated by my 240v supply to "240 degrees" (maybe it was more than that depending on the thermistor accuracy ). The Duet recognised it as a heating error (temp on the PanelDue was in purple) and no voltage was present on the SSD input. A new SSD fixed the problem, but I am going to put a further safety switch off system into place. Am researching this so looking at either a separate independent system apart from the Duet that will cutoff the 240v supply over a certain temperature, a relay controlled by the Duet to switch the heatbed 240v off when the Duet senses the heatbed error, a thermal fuse on the 240v line next to the heatbed that blows on high temperatures…...
    Not sure how to switch the relay from the Duet which would be fairly simple. ..?via the 5v PSU control.


  • administrators

    @Syko_Symatic:

    It measures 24.1v at the bed so that’s good. The resistance is 1.6ohm. The pad itself is 300x300mm. The supplier tells me it’s natural to get a wattage loss, I agree but it’s surely shouldn’t be so huge.

    Could it also be a PID setting? I can’t get the auto tune to work. It says it can’t reach specified temperature, which I set at 90deg. It can as I have used it at 95 but maybe it’s a time issue?

    Run M307 H0 with no other parameters, and check that it isn't using a maximum PWM below 1.

    Ar 24.1V your bed heater should produce 363W power when it is cold. If it is a PCB bed heater then the power will reduce as it heats up. If it's a nichrome/silicone one then the reduction in power when hot won't be significant.

    How large is the bed? I recommend a heating power of around 0.4W per square cm, although you can get away with 0.25. Insulation below the bed will help, if you don't have that already.


  • administrators

    @Paulh:

    since it has just affected me I would make a warning that if the SSD relay fails and short circuits then the heated bed supply is switched on 100% and not controllable by the Duet. Yesterday my SSD failed and I found the 300mm diameter hotbed heated by my 240v supply to "240 degrees" (maybe it was more than that depending on the thermistor accuracy ). The Duet recognised it as a heating error (temp on the PanelDue was in purple) and no voltage was present on the SSD input. A new SSD fixed the problem, but I am going to put a further safety switch off system into place.

    It's unusual for DC-AC SSRs to fail when driving resistive loads, but when they do they fail short circuit as you say. What make/model was it?

    I recommend using one of the following two approaches to protect against failure in the heating control system:

    1. Passive protection. Choose the bed heater power such that if the bed heater is left full on, the temperature doesn't rise above a safe limit, e.g. 150C. Most silicone heaters can withstand 200C; although the glue holding them to the bed plate might soften first, so the cork os other insulation below the heater should be capable of supporting it.

    2. Mount a thermal cutout on the bed plate and connect it in series with the heater.



  • @dc42:

    @Syko_Symatic:

    It measures 24.1v at the bed so that’s good. The resistance is 1.6ohm. The pad itself is 300x300mm. The supplier tells me it’s natural to get a wattage loss, I agree but it’s surely shouldn’t be so huge.

    Could it also be a PID setting? I can’t get the auto tune to work. It says it can’t reach specified temperature, which I set at 90deg. It can as I have used it at 95 but maybe it’s a time issue?

    Run M307 H0 with no other parameters, and check that it isn't using a maximum PWM below 1.

    Ar 24.1V your bed heater should produce 363W power when it is cold. If it is a PCB bed heater then the power will reduce as it heats up. If it's a nichrome/silicone one then the reduction in power when hot won't be significant.

    How large is the bed? I recommend a heating power of around 0.4W per square cm, although you can get away with 0.25. Insulation below the bed will help, if you don't have that already.

    The actual bed size is around 340x 320 so it works out around 0.3w? If my math is correct?

    It is indeed rated at 360w so it seems to be matching up with what you’ve said I just can’t get the temp out of it. When the current print finished I will do what you’ve said and run the tuning with no parameters.


  • administrators

    It's not tuning you need to run, it's M307 H0 which just reports the heater parameters:

    10:53:41
    M307 H0
    Heater 0 model: gain 90.0, time constant 700.0, dead time 10.0, max PWM 1.00, calibration voltage 0.0, mode bang-bang, inverted no, frequency default

    So it's safe to do while printing.



  • Gain 141 time constant 620.4, dead time 0.7, max PWM1.00 mode PID

    So I ran the bed tuning at 75deg. Put a little tin foil beneath the bed. Now I just managed to reach 130deg!



  • SSD that failed was bought from ebay:

    Solid State Relay Module SSR-25DA 25A /250V 3-32V DC Input 24-380VAC Output GT
    "futek" brand
    ( 182110092170 goodgadgetsa )
    Planning to put Thermal cutout under the heatbed, in series with the 240V supply to the bed.
    The cork insulation worked well and worthwhile. Limiting the maximal temperature by choosing appropriate heater wattage sounds wise to me but is only known after it is fitted.


  • administrators



  • If anyone here is interested, I have a pile of AC switching SSRs that were pulled from retired aerospace repair equipment, that I would be more than happy to share for the cost of postage.

    Omron, Crydom, and I think one other brand, all rated for 30A or better.

    No intent to profit on this, just thought to offer as I've seen the discussion about fake Fotek SSRs a couple of times now.



  • Hey_Allen.
    Iwas just fixing to order one.

    I would love to get one from you. I am sure they are probably better than what I could buy.
    Tell me how you would like to arrange this.



  • I was on here looking for an answer as to why my T0 heater will not cut off when it reaches the set temperature. I have ran the Auto Tuning and put in the PID's. Played around with them for a couple of hours and I still get voltage to the heater after it reaches set desired set point.

    Any Ideas?


  • administrators

    When using PID control, the heater power will not be cut off when it reaches the set temperature. Instead it will reduce the PWM to a suitable value to maintain that temperature. You may get a small amount of temperature overshoot after it reaches the set temperature, but it should be less than 5C if the heater is well-tuned.


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