AC Powered Heater Protection

  • Hi Everyone,

    I am awaiting the delivery for my Duet Ethernet so I can get my CoreXY built. I have a question about the heated bed and ways to stop it from running if the SSR fails. I understand that you can get a heater fault if the temperature exceeds the maximum amount. Is there anyway you can get it to trigger an output to switch a relay to cut the power? The relay obviously needs to be closed under normal conditions and then trip when an over heat is detected.

    Kind Regards,
    Sam Logan

  • administrators

    My recommendation is not to rely on firmware to protect against a heater fault. Use one of the following two mechanisms instead:

    1. Choose your bed heater power carefully so that in the event of the heater remaining permanently on, the bed temperature will not exceed the safe limit for the heater and the materials it is in contact with. For example, most silicone heaters can tolerate 200C, so you could choose the power so that the maximum temperature that can be reached is 180C. This power will still allow you to reach 100C or even 120C for ABS printing.

    2. Mount a thermal cutout on the bed plate and connect it in series with either the bed heater or the power to the whole printer. Thermal cutouts are readily available because they are used in microwave ovens, tumble dryers and other domestic appliances. You can get ones that have to be replaced when they pop, and once that self-reset when the temperature has dropped sufficiently. The self-resetting ones have the advantage that they can be tested.

    Although DC-AC SSRs (for AC mains bed heaters) can in theory fail short circuit, when they are used to control resistive loads such as heaters they are very reliable - assuming of course you don't choose one with too little rated current.

  • Thanks for your reply. Yeah I am building a 400x400 bed and have 2 400w silicone heaters for them. I think I might look for the thermal cutouts to protect it. I use SSR's for my brewery and have not had any issues with them but I thought I would see what else I could do.

  • Self-resetting thermal cutoff. Keenovo will build them into heater(s), if you ask.

  • @samlogan87 I would add a thermal fuse to protect the bed from overheating even if the SSR fail shorted. Like the example on the photo that is rated to shut-down at 180C.

    they are very common components used with coffee machines and other home appliances that use heaters.


  • @danal
    Self-resetting is not a good idea. There are two main reasons for excessive heating in the bed. Either the controller has lost its mind or the device switching power to the bed has failed. If you use a self-resetting thermal fuse it will keep cycling the power on and off, and will keep heating the bed excessively, possibly damaging the bed an other components, possibly leading to fire.

  • @mrehorstdmd self resetting will just act as a poorly tuned bang-bang.

    Temperature will rise above the fuse threshold, open the circuit, this will allow the temperature to cool below the reset threshold, the fuse will close allowing current to flow and the bed to resume heating until the temperature again rises above the threshold and the cycle repeats endlessly maintaining a temperature close to the two thresholds.

    It is of course less than ideal as a dead stop in case of a fault is preferred, but depending on how the fuse is wired changing it may not be an easy task.

  • @dino I wouldn't order a heater with a self resetting thermal cutoff. They are not thermostats- they are intended to be safety devices. I doubt that the self resetting devices are meant to operate over many cycles before the device fails (hopefully they fail open).

  • @mrehorstdmd my example was also worst case, if there's a polyfuse or other latching type resetting thermal fuse it will continue to leak a small amount of current when tripped, maintaining its temperature and preventing itself from resetting until the current is removed. Like you said they are safety devices and are designed as such.

  • That thermal fuse at the picture is not self-resetting 😄 When you go over the specified temperature it just cuts the circuit for good. You need to replace them to work again. It is a last resource option for great safety.

  • I'm the one with the self-resetting device, installed by Keenovo. Everyone's points are valid... it would cycle... and it could fail as well...

    Nonetheless, given that it is a second or third layer of protection, I'm OK with that. You may not be.

    Here is yet another layer of protection:

    0_1528753944585_BFD Fireball Crop .jpg

  • @danal What type of self resetting fuse is it if you know? I kind of regret not getting one built into my bed, Even though I have redundant safety measures as well an extra one wouldn't have been bad.

  • @dino said in AC Powered Heater Protection:

    @danal What type of self resetting fuse is it if you know? I kind of regret not getting one built into my bed, Even though I have redundant safety measures as well an extra one wouldn't have been bad.

    I don't know, it is/was embedded in the silicone by Keenovo, so I've never seen it.

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