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Revision to Choosing a bed heater

Tony

[title|Choosing a bed heater]
 
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[summary]The bed heater is often uses the highest proportion of the power consumed by a 3d printer and many plastics print better on a heated bed. The Duet 2 supports up to 18A current on the bed heater channel, if more current is required then an external SSR can be used.[/summary]
 
 
 
== Heating power ==
 
If you want reasonably fast heating to around 100C for printing ABS, then a good figure to use is 0.4W per square cm of bed area. Here are some example bed sizes and the corresponding suggested heater power:
 
* 200x200mm (e.g. typical Prusa-style Cartesian printer): 160W. (Typical Mk2-style PCB bed heaters are around 120W at low temperatures, but the actual power varies quite widely depending on the copper thickness, and the heater power drops off as they get hotter).
* 300 * 300mm (e.g. large Cartesian or CoreXY printer): 360W
* 170mm diameter (e.g. Mini Kossel): 90W
* 330mm diameter (e.g larger Kossel): 342W
 
The usual options for powering a bed heater are:
 
* Direct from the Duet using 12V power. Duet 2s (Duet Wifi, Duet Ethernet) are rated for up to 18A bed heater current, so this is suitable up to 12 * 18 = 216W power.
* Direct from the Duet using 24V power. With the Duet 2s this is suitable up to 24 * 18 = 432W heating power.
* DC-DC SSR to control the bed heater, usually with a 24V supply.
* DC-AC SSR to control an AC mains voltage bed heater.
 
Before you decide to use a 12V bed heater, check whether you need to supply 24V power to the Duet in order to achieve your target movement speed - see [[Choosing stepper motors]].
 
== Bed heater types ==
 
These types pf bed heater are in common use:
 
* PCB bed heater. These typically provide less than the recommended heating power for their size (see above). For example, typical Mk2-style 200x200mm PCB heaters have a heating power of around 120W so as to be within the rated current of RAMPS electronics. The actual heating power often varies between different samples, probably because of poor quality control on the thickness of the copper. The heating power drops off as the temperature rises because of the increasing resistivity of copper with temperature. You can get more power out of a 12V heater by turning up the power supply voltage; for example at 14V the heating power is increased by 36%. Don't plan on doing this with a 24V heater because the maximum recommended input voltage of the Duet WiFi is 25V.
* Stick-on Kapton bed heater.
* Stick-on silicone bed heater. The heating elements are usually nichrome, which has a very low temperature coefficient of resistance, so the heating power doesn't drop off appreciably as they get hotter. You can get silicone heaters custom made to your own size, voltage and power specification inexpensively from suppliers on Alibaba/AliExpress such as Shenzen Ali Brother Technology.
 
Tips:
 
* When choosing a Kapton or Silicone heater, get one that is a little smaller than your bed plate so that there is room at the edges for bed mounting screws, and also for bed clips if you intend to clip glass print beds on to your bed plate.
* With a silicone heater you usually have the option of having a thermistor embedded within the heater. It is quite possible to use these thermistors (I do), however a thermistor that is directly in contact with an aluminium bed plate will give a more accurate reading.