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Installing an AC bed heater

Posted by PRZ 
Installing an AC bed heater
January 21, 2018 12:38PM
With the advent of larger printers, we see more people willing to installed AC heat beds, notably for CoreXY and ‘Ultimaker type’ printers. This is fairly more critical than on Delta printers with fixed bed, and the question come regularly on the forums. I also intend to do so on a CoreXY with a bed 400x400. Most people don’t quite capture all the safety concerns such installation may cause and that is quite frightening.

I decided to create a page on the RepRap Wiki about AC bed installation and then I did the following drawing:
Before publication I would like to get comments on that, disagreements (if any), complementary required information and so on.
There are still stuff I am not sure how to be done:
  • Silicon heat beds are delivered with long high temperature wires with fiberglass sheath, however these wires are not that flexible and I am trending to cut them and install a screw terminal block on the bed support (for CoreXY and the like). What do you think about that ?
  • On my delta, I did not used the bed pad thermistor and installed the bed thermistor on the aluminium plate instead. This is more precise and have the advantage that as the aluminium plate is earthed, any problem with thermistor wiring will ground them, so it is quite safe electrically. However, as I already experienced, silicon heat pads may inflate and in that case the plate thermistor is of no use, heater no longer being in contact with the plate, the heating system being only heating the pad itself, using the pad thermistor will be safer. However, on these pads, the electrical separation between the thermistor wires and the power wire is not very good and this is a concern for me. On my delta, the bed being installed on a ceramic fiber mat and firmly maintained by aluminium clamps, this may not be a problem. On an open installation for a CoreXY, I am still undecided of what is the safest installation. Opinions or advices?
  • My schematic shows a direct power switch while it shall be normal to use a pushbutton and power relay maintained active by the board, which may decide to shutdown all power in case of trouble. The signal does exist on the Duet, with a MosFET sufficient to drive a mechanical relay, but most other boards did not propose this functionality. For me, this shall be installed on ALL printers, whatever bed heating type, if any. How shall I present that ?
  • Is a cable chain guide better than an ICTA sheath? It will drive to shorter radiuses, so more stress, but there will be no pinch risk. I have to add a warning about pinching cable. Also broken wire can go through a chain.
  • H07RNF rubber cables or more flexible silicon insulated fine stranded wires ?
    [edit] Someone suggested These wires with 400 strands which makes them very flexible and quite resistant to flex. But they have no rubber enveloppe, so abrasion might be a concern.

What I mean by a thermistor in the bed plate (in fact a block screwed to the plate):

Edited 3 time(s). Last edit at 01/21/2018 05:14PM by PRZ.


- Safety [reprap.org]
- Embedded help system for Duet and RepRap Firmware [forums.reprap.org]
- Enclosed delta printers Lily [rouzeau.net] and Lily Big [rouzeau.net]
- OpenScad delta printer simulator [github.com]
- 3D printing on my site [www.rouzeau.net]

Re: Installing an AC bed heater
January 26, 2018 09:46AM
So, no comments ?
Anyway, I updated my schematic :
- Added enclosure, its earthing and metal connectors earthing
- Added recommendation for a thermal fuse (could be a thermal switch either), though bad installation may cause more harm than good, but this is normal practice for heating equipment from decades.
- Recommend a screw terminal on bed support to change cable type (if needed) for moving beds only.
I am still undecided about recommendation between chains and plastic sheath, chains may create abrasion on wires and you need complementary textile sheath (usually in PET)

I tried to be exhaustive and there is a lot of text which reduce legibility. However, I think that in addition to a web page with links, a self-supporting document may be useful.
I published the source file (Libre office draw) on my GitHub account here :

It is an evidence that as this is unpaid work, I cannot take any responsibility whatsoever, but I really tried to make my best on that. [Edit] Living in EU, the context I know is EC regulation, however, as far as I know, the USA electrical code (NEC) may not differ much, but there may be not something quite equivalent to the CE machinery directive as a unified regulation.

Edited 1 time(s). Last edit at 01/26/2018 09:56AM by PRZ.


- Safety [reprap.org]
- Embedded help system for Duet and RepRap Firmware [forums.reprap.org]
- Enclosed delta printers Lily [rouzeau.net] and Lily Big [rouzeau.net]
- OpenScad delta printer simulator [github.com]
- 3D printing on my site [www.rouzeau.net]

Re: Installing an AC bed heater
January 29, 2018 07:02PM
Thanks for this. I'm in the process of building a coreXY and am just about ready to start wiring the AC heat bed.
Re: Installing an AC bed heater
February 08, 2018 11:50AM
I just wanted to pop this in for future readers of this thread.


Things often go wrong and when they do its catastrophic. You can get a fire with a 12v system when things go too hot, takes a while. With a mains system, takes seconds....

Smoke detector.....
Thermal fuse protection.....
Dont leave it unattended.....
Dont leave any flammable items near your printer.....

Better it takes out a separate building, shop, garage etc than your house with you in it.....

google '3d printer fire' for some idea of what can go wrong!!

Edited 1 time(s). Last edit at 02/08/2018 11:53AM by JustSumGuy.
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