Heat Bed Sizing

My current printer has an acrylic build platform that is not heated. I am looking to upgrade this and have purchased a 12"x12" piece of stone to use as my new build platform.

The question is this: If I am going to spend the majority of my time printing in an 8"x8" area, do I need to get a 12"x12" heater? I understand that a 12"x12" heater would heat the platform quicker and more evenly, while using a lot more power. An 8"x8" heater would heat slightly slower using less power. The 8x8 would heat the center uniformly to the desired temperature, but not the outer areas as well.

Here are the two heaters:
[store.quintessentialuniversalbuildingdevice.com]
[store.quintessentialuniversalbuildingdevice.com]

Thanks for reading.

From the description, both of those heaters draw the same amount of power. The description for both says up to 15A, which may or may not mean they draw the same power. If the 12"x12" heater really does draw 15A or less, that means it is only using 180W of power, which is a bit low for that size of heater, and will heat quite slowly especially with a bed that has a large thermal capacity (such as a thick piece of stone). A good rule of thumb is around 2W/square inch of bed surface for reasonable heating capability. That means you would want ~300W of power for heating the full 12"x12" bed. This cannot be powered directly by most electronics, which are generally limited to less than 200W of power to the bed at 12V (~17A). Many cannot support more than 150W at 12V (~12.5A).

I use a 12"x12" AC heated pad for my print bed (specifically, this one), and use an SSR to drive it from the regular heated bed output of my RAMPS board. This pad uses 360W, which still doesn't heat the aluminum and glass of my heated bed terribly quickly.

My recommendation is that you use the 8"x8" heater pad if you won't be printing anything bigger than that (but then why do you have such a large printer?). If you want full use of your bed, and don't mind messing with AC voltage, an SSR and AC heater pad works well. Another option (if the electronics you are using supports it) is to run 24V, which reduces the current required by half from 12V for the same power output, and means that your electronics may be able to directly drive a 300W heated bed (which would now draw ~12.5A). Make sure the heater is rated for 300W at 24V though, if you put 24V through a heater rated for 300W at 12V, it will actually draw 1200W!

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I am using a RAMPS 1.4 and the stone is 1/4" thick. I would prefer to use 12V as the increase to 24V would require some additional changes. I like the AC idea, but I want everything to run off one power supply and configuring that to work with RAMPS seems like it could be tricky?

I think I will just go ahead and get the 8x8 to see how it works.

Also, the 12x12 says up to 20A.

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ARosch
Also, the 12x12 says up to 20A.

You are correct, it does say 20A for the 12"x12" heated bed. I misread that. This is still a bit under-powered, and would draw too much power to be fed directly by RAMPS.

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ARosch
I like the AC idea, but I want everything to run off one power supply and configuring that to work with RAMPS seems like it could be tricky?

The advantage of AC power is that you can tap it directly from the wall, no additional power supply required. I just tie wires to the AC input of my power supply. There is no configuration to be done. Simply connect the live side of your AC power to the SSR, the AC output of the SSR to your heater, and the other heater lead to AC neutral. Connect the RAMPS heated bed output to + and - of the SSR, observing correct polarity. Then you are done. Now RAMPS will control the heated bed the same as if it was directly connected. You can even keep PWM on for the heated bed with many SSR's, if so desired (I do).



In this image, the ~ is the AC power, with the live side connected to pin 2 of the SSR, and the neutral side connected through the heater to pin 1. Connect + and - of RAMPS' heated bed output to pins 3 and 4, respectively. The SSR I use is similar to this: [www.ebay.com] (they are also available on Amazon).

Edit: If you do decide to play with an AC heated bed, be aware of the electrocution hazard. Do not leave power connected while you are working on your printer, and make sure you properly insulate all wires and joints. Common sense should apply here.

Edited 1 time(s). Last edit at 03/05/2014 04:56PM by NewPerfection.

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Wow that is a lot easier than I expected. I will definitely consider this idea, thanks!

Very interesting. I have a 12v 300mm x 300mm silicone heater which draws around 21 amps on my dc supply. So I have a SSR which I was going to turn the switch mode supply on and off with but a PWM on 230VAC seems a more elegant solution.

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RodF
Very interesting. I have a 12v 300mm x 300mm silicone heater which draws around 21 amps on my dc supply. So I have a SSR which I was going to turn the switch mode supply on and off with but a PWM on 230VAC seems a more elegant solution.

Or just get a DC-DC SSR, and switch the output of the 12V supply rather than the AC input. This would probably be a lot cheaper than getting a new silicone heater rated for 230VAC. You should still be able to use PWM if the SSR supports it.

Edited 1 time(s). Last edit at 03/10/2014 02:40PM by NewPerfection.

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Or just get a DC-DC SSR, and switch the output of the 12V supply rather than the AC input. This would probably be a lot cheaper than getting a new silicone heater rated for 230VAC. You should still be able to use PWM if the SSR supports it.[/quote]

Thanks for the input Cameron. I have ordered a DC-DC SSR. Hopefully it will be OK with PWM, the specs say it will switch on and off in 10ms. I have a Rumba board with HB mosfet which I suppose can have the frequency set in the firmware (My machine is unfinished so I haven't started commissioning it yet).

Edited 2 time(s). Last edit at 03/10/2014 10:04PM by RodF.

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NewPerfection
I use a 12"x12" AC heated pad for my print bed (specifically, this one), and use an SSR to drive it from the regular heated bed output of my RAMPS board. This pad uses 360W, which still doesn't heat the aluminum and glass of my heated bed terribly quickly.

Sorry to drudge up this post, but I'm curious why you chose the 360watt sheet, rather than one of the higher wattage sheets? Since you are running off an SSR the power supply is basically not being used. The medium sheet pulls less than 7 amps and I would think would heat up much quicker. Any thoughts on this?

So I ordered the medium watt sheet, and only after doing so did I realize that it is very likely that it will pull too many amps for my home wall circuit.. I'll let you guys know how it works though.