Mains heated bed suggestion How many watts is appropriate
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Hello David,
Some good advice here: https://duet3d.dozuki.com/Wiki/Choosing_a_bed_heater
They recommend 0.4W per cm^2 for quick heating which would be around 350W for your bed size.
If you got a 350-400W bed at 24VDC you could run it from the internal bed output instead of using 120VAC (on Duet 2)
With proper precautions it is OK to use mains heaters, the things you want to make sure of:
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Make sure the heater will not rise to dangerous temps if left on at full power. If bed power is limited enough, even a 'stuck on' bed heater will stay within reasonable temps due to temperature loss into the environment.
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If the bed heater does not meet #1 and is powerful enough to overheat dangerously under a 'stuck on' condition, you should provide thermal cutoff of some nature. One option is to use thermal fuses on the bed itself, these normally disconnect when overtemp. One option might be use these thermal cutoffs inline with the SSR signal to ensure the relay is deactivated if the bed goes over temp. You could also use a second safety circuit which could cut off main power to the machine in the same type of situation.
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With mains AC power you absolutely must ground any metal frame elements that an AC wire could come into contact with. This means metal frame, aluminum bed parts, etc. It sounds like you've done this already. For additional safety you can connect the machine to your mains through a GFI device which would provide additional protection against electric shock.
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@nhof said in Mains heated bed suggestion How many watts is appropriate:
hey recommend 0.4W per cm^2 for quick heating which would be around 350W for your bed size.
If you got a 350-400W bed at 24VDC you could run it from the internal bed output instead of using 120VAC (on Duet 2)
With proper precautions it is OK to use mains heaters, the things you want to make sure of:Make sure the heater will not rise to dangerous temps if left on at full power. If bed power is limited enough, even a 'stuck on' bed heater will stay within reasonable temps due to temperature loss into the environment.
If the bed heater does not meet #1 and is powerful enough to overheat dangerously under a 'stuck on' condition, you should provide thermal cutoff of some nature. One option is to use thermal fuses on the bed itself, these normally disconnect when overtemp. One option might be use these thermal cutoffs inline with the SSR signal to ensure the relay is deactivated if the bed goes over temp. You could also use a second safety circuit which could cut off main power to the machine in the same type of situation.
With mains AC power you absolutely must ground any metal frame elements that an AC wire could come into contact with. This means metal frame, aluminum bed parts, etc. It sounds like you've done this already. For additional safety you can connect the machine to your mains through a GFI device which would provide additional protection against electric shock.
Well good call on the stuck on condition, ill install a 120c thermal cutout fuse to be safe . I currently have a 400w mains heater installed that needs to be replaced due to leads coming out at the wrong location ( leads hit and jam on chamber heater when going down the z axis) and it takes about 10 or 15 mins (cant remember the exact time ) to get to a 100c, is that normal. Im in the process of ordered a custom pad and hoped 600w would get me closer to a 5 min warm up time.
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This calculator will give you a pretty good idea of heat-up time based on the bed construction: https://jscalc.io/calc/uS8JYjYISgIvzJ1x
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@mrehorstdmd sweet thanks, that helps
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I couldn't see you mention what you're attaching the heater to for the bed surface. I have a 600w AC heater attached to a 6mm thick 300x300 alu plate and heat up to 60c takes about 1 minute. 100c takes about 5 to stabilize.
If you have a thinner bed, 600w is probably overkill to the point of danger. But for a thick bed the speed sure is nice.
Doing a PID tune is a good idea, as is adding a thermal cut out in case of failed on situation.
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@Phaedrux
Good point its 6mm thick aluminum tool plate. -
@davidvh86 If you use the 468MP adhesive to hold the heater on the bed, you can expect to get about 2 years daily use from it before the adhesive lets go. As it lets go, portions of the heater will char. If there is no mechanical means of securing the heater to the plate, it will be possible for the heater to drop off the plate if the adhesive fails. You should therefore mount the thermal cutoff on the heater and not on the plate. If you secure the heater to the plate mechanically, then you can mount the TCO on the plate.
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@mrehorstdmd said in Mains heated bed suggestion How many watts is appropriate:
mount the TCO on
theres a tray below that carries the heated bed, but i thing attaching the cutout to the silicone pad is a good idea, any thought on a way to secure it to the silicone?
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@davidvh86 said in Mains heated bed suggestion How many watts is appropriate:
any thought on a way to secure it to the silicone?
Why, more silicone, of course. RTV silicone. (Room Temperature Vulcanization.) That's also the best way to attach the heater to the plate.
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@Phaedrux
lmao, thanks for the help this project is never ending, It went from a planned 2 week conversion to 4 months lol.
