Resettable Thermal Fuses Recommendation

  • Any recommendation on Resettable Thermal Fuses for a 500W, 240V mains heated bed controlled with an SSR?

  • If your in the UK these are good (i would use them on the input side of the SSR Tho as that is low power DC) They may well send abroad if your not in the UK.

    Manually resettable Thermal Fuse/Switch

    HTH Doug

  • @dougal1957 Perfect, thank you. I am in the UK.

    You have confused me a bit saying use it on the low power DC side of the SSR though. Surely if the SSR fails open you want to cut the mains voltage going into the SSR. On the line going into connector 2 on the SSR.

    Top right in the picture below

  • @richardmckenna You are correct, you need to break the AC side after the SSR to prevent a thermal run away.

    I'm not familiar with the one in the link above but all of the ones I have used need to be near or against the heat source to work correctly.

    Keenovo will install them right in your bed heater if you order them that way.

  • @richardmckenna You are of course perfectly correct I must have had a blonde moment so to speak (Not to offend blondes of course) Don't know what I was thinking.

    I did consider having one in there to control a AC Relay on the input to the PSU have a push button to start it up and if the thermal fuse popped then it would kill power to the whole machine but that was for the auto resettable switches I have, the manual ones don't need that level of complexity!

  • @timcurtis67 said in Resettable Thermal Fuses Recommendation:

    Keenovo will install them right in your bed heater if you order them that way.

    Had mine done that way. Very easy.

  • @dougal1957 said in Resettable Thermal Fuses Recommendation:

    I did consider having one in there to control a AC Relay on the input to the PSU ...

    Non-ideal design. Thermal fuses (one-shot or reset) are very close to 100% reliable. Relays are not. The thermal could go and the relay could hang...

    Adding complexity to a system to make it "more" failsafe often has the exact opposite effect.

  • @danal Agreed the KISS effect has a lot going for it

  • @danal Can you back that up with spec sheets? I'm not being sarcy, I'm curious.

    Guided contact relays are often sold with mean time to dangerous failure figures to allow the perfomance level calculations (edit: corretion I think the information is required for the manditory document detailing when critical parts of the circuit should be replaced regardless of working or not) for safety interlocks. That's what I'd look for on the snap switches spec sheets.

    I'd guess the ones with the data are low current devices designed to be included in a safety interlock circuit rather than snap switches/fuses that can handle 10-20A.

    If using a relay with fuses before the coil and contacts, flyback/freewheel diode across the coil and don't use it past it's speced life time you'll struggle to beat them for reliability.

  • Excellent point on the specs. I am not, in any way, claiming that relay failure is likely. Only possible, even if extremely rare, and/or distant in time/cycles. Angles and heads of pins and all that.

    Phrased another way:

    Are you willing to bet your house on those specs, by running a relay and NOT running a one-shot thermal device? I will be the first to say, It is a statistically safe bet, per the spec sheet. Probably MANY other things in life to worry about.

    On the other hand, if there is also a one-shot thermal device, what is the point of the relay?

    Phrased yet another way:

    Given a device that has a spec sheet that shows the extreme rarity of failure (or very large amount of real time and/or cycles prior to failure) vs. a device that works 100.0000000% of the time when temperature is exceeded (it literally just melts), and is MUCH cheaper and easier to set up, I know which one I'm picking.

  • It's certinally better than nothing. I just like being able to verify the function of a safety circuit rather than trust the manufacturer hasn't had a tollerance slip up on the part. That said I blindly trust an electrical fuse when I drop wire guage from one that cam safely take enough current ftom the PSU to trip it into overload protection.

    Back to topic the resettable devices are a current concern for me. My planned interlock architecture uses a relay/contactor that latches on with user input and holds on so long as PS_ON remains on, e-stop isn't hit, and any additional in loop thermal switches don't trip. As I'm only breaking relay coil currents the following seems a neat solution with a convenient bolt hole:

    Best answer would probably be a combination of resetable switches and permanent thermal fuses.

    Edit: For reference to the original post this relay system is interlocking one power supply. This same interlocked supply would feed the SSR.

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