How about Double Capacity Heaters & Modulate for Safety?
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Would it be possible to set a maximum modulation of 50% and use an inline fuse to blow at before 100% loading to protect against a mosfet failing closed circuit?
I'm guessing it's far easier to achieve for a DC heater than AC. Essentially if the firmware is in control the heater won't get more than 50% of full current.
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50% PWM would give you half the power that 100% does; however the RMS current (which is what matters to a fuse) would be 1/sqrt(2) times the full current, not half. So the RMS current overload margin when 100% PWM gets applied would only be sqrt(2), which I don't think is enough to blow a fuse quickly.
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Fair enough, you'd have to oversize further. If you dropped an 80w cartridge to 30w at 37.5% duty you'd be looking at the difference between just over 2.0A and 3.3A. Enough marging for a quick blow 2.5 or 3A?
The Up printers use 80W cartridges.
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I like your thinking, thermal fuses offer peace of mind for high powered (or low powered for that matter) beds but I haven't seen anything particularly suitable for hotends. Surely there must be something conductive that melts at 300 degC that could be used as a hotend thermal fuse?
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A short search on Google reveals lead to have a melting point of 327 deg C, why not a lead cored fuse in series with the heater cartridge?
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I'm going to try the 80W at 37% duty at home and see if it can run reliably on a 2.5 or 3A quick blow and fail reliably with 100% duty. Not fool proof testing but better than nowt.
Thermal fuses would be good, but I've struggled to find them too. Other issue is really the heater block would need to be designed to suit or you have the worry of guaranteeing the thermal fuse is held fast and not more likely to fail than the mosfet in a power surge situation.
Guess surge protection in the mains line before these printers would be a good move on that front. Reduce the chance of a electrically damaging event occurring.
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Furthermore why not just solder the leads to your heater cartridge with high melting point solder? At 400 deg C the solder melts, problem solved.
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Just for info, the 5 colour Diamond hot end is supplied with an 80Watt cartridge heater as standard. When I tuned it, the result showed that if unchecked, it could reach a potential temperature of 647 deg C. In use, the PID did it's best but it was impossible to tame the overshoot/undershoot. I ended up running it with 0.6 PWM which tamed it somewhat but I was still concerned that a failure on the control side would be dangerous. I don't know at what temperature PLA is likely to burst into flames but I do know that incipient red heat for metal start at around 500 deg C so to me, the potential to reach 640 plus is downright dangerous. A bit of molten plastic is one thing but flames are another thing all together. I swapped the cartridge for a 40 Watt one which is more than enough and would only reach 340 deg C unchecked (hopefully molten plastic but no flames)
