MosFETs can generally become unpredictable if they fail.
Generally, there shouldn't be an electrical problem here, because even if the MosFET fails and is 100% on, you still have a fixed resistance (the heater) that will prevent short-circuits.
Plus, pretty much all switching power supplies have over-current protection.
But there can be a thermal problem if your heater is left 100% on all the time. The ideal thing would be something that does not rely on a mictrocontroller or computer to work, for example a bi-metal switch operating a thyristor (mains switch). But I don't know if there are bi-metal switches for these temperatures that you can mount to the heated bed (and that are affordable).
Another thing I would put a certain trust into would be an OP-Amp switch reading a thermistor. Purely analog circuit that will, if done correctly, switch off the mains in every case of failure.
It takes some time to design something like that, but I may have a look at it (I'm designing an UM1 replacement mainboard anyways...).
/edit:
TI has an interesting device: LM27
Guess I'll have to look into that one. It's basically a thermal switch that will activate an output when it measures a temperature above 140°C (there are mulltiple temperature options).
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@polar, exactly my situation.
What I did to counter that:
- Installed a "main" switch that cuts the wall power and has also a fuse. I only power it if I require it.
- as you could read in my experiance, my relay got messed up. I replaced it with a MosFET. To be honest, I have no clue what happens if a MosFet blows (open or closed) but I guess it can be both.
Thermal switch / fuse is a good idea. I will also look into that.
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