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Revision to Choosing a bed heater

Tony

The usual options for powering a bed heater are:
 
* Direct from the Duet using 12V power. The Duet 2s (Duet Wifi, Duet2 (Wifi and Ethernet) areis rated for up to 18A bed heater current, so this is suitable up to 12 * 18 = 216W power.
* Direct from the Duet using 12V power. The Duet 2s (Duet Wifi, Duet2 (Wifi and Ethernet) areis 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.
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 WiFi2 Wifi or Ethernet is 25V.
* 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 WiFi2 Wifi or Ethernet 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 online.
=== Low voltage bed heater driven directly from the Duet ===
 
Duet and Duet WiFiDuets provide a terminal block for connecting a bed heater. The voltage supplied to the bed heater is the voltage you apply to the VIN terminals from your power supply. The maximum current you can safely draw depends on the Duet version.
Duet and Duet WiFiDuets provide a terminal block for connecting a bed heater. The voltage supplied to the bed heater is the voltage you apply to the VIN terminals from your power supply. The maximum current you can safely draw depends on the Duet version.
 
'''Important!''' the heat generated by high bed currents can cause the wires to creep, which in turn makes them loose in the terminal block. So you should re-check the bed heater terminal block screws for tightness regularly, especially during the first few days of use. This is especially important if you use stranded-core wire without crimping bootlace ferrules on the ends.
The safe bed heater current has not been measured, but should be somewhat above 10A because the PCB trace between VIN + and Bed Heater + is wider than on the Duet 0.6. As with the Duet 0.6, you can solder a thick wire between the Power In +VIN and Bed Heater + terminals on the back of the board to increase the current capacity. The bed heater terminal block is larger than on the Duet 0.6.
 
==== Duet 2 (Duet WiFiWiFi and Duet Ethernet) ====
==== Duet 2 (Duet WiFiWiFi and Duet Ethernet) ====
 
The Duet WiFi2 is safe at up to 18A bed heater current. This rating has been increased from 15A due to new [http://blog.think3dprint3d.com/2017/04/duetwifi-updated-thermal-testing.html].The trace between VIN+ and bed Heater + is duplicated on both sides of the PCB.
The Duet WiFi2 is safe at up to 18A bed heater current. This rating has been increased from 15A due to new [http://blog.think3dprint3d.com/2017/04/duetwifi-updated-thermal-testing.html].The trace between VIN+ and bed Heater + is duplicated on both sides of the PCB.
 
=== Bed heater driven using a Solid State Relay ===
 
You can use a solid state relay (SSR) to switch the bed heater by connecting its control terminals to the Duet bed heater terminals. Make sure that you get the wires to the + and - control terminals of the SSR the right way round. On the Duet 0.6, the + bed heater terminal is closest to the VIN terminal block. On the Duet 0.8.5 and Duet WiFi2, the - bed heater terminal is the one nearest the VIN terminal block.
You can use a solid state relay (SSR) to switch the bed heater by connecting its control terminals to the Duet bed heater terminals. Make sure that you get the wires to the + and - control terminals of the SSR the right way round. On the Duet 0.6, the + bed heater terminal is closest to the VIN terminal block. On the Duet 0.8.5 and Duet WiFi2, the - bed heater terminal is the one nearest the VIN terminal block.
 
When using PID to control the bed heater, RepRapFirmware uses a low PWM frequency (10Hz) so as to be compatible with all standard types of SSR.

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