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Revision to Choosing a bed heater

Tony

* 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 and connecting stepper motors]].
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 and connecting stepper motors]].
 
=== Bed heater types ===
=== Low voltage bed heater driven directly from the Duet ===
 
Duet and Duet WiFi provide a terminal block for connecting a bed heater. The voltage supplied to the bed heater is the voltage you apply to the [[Glossary_of_Terms#VIN|VIN]]VIN terminals from your power supply. The maximum current you can safely draw depends on the Duet version.
Duet and Duet WiFi provide a terminal block for connecting a bed heater. The voltage supplied to the bed heater is the voltage you apply to the [[Glossary_of_Terms#VIN|VIN]]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.
==== Duet 0.8.5 ====
 
The safe bed heater current has not been measured, but should be somewhat above 10A because the PCB trace between [[Glossary_of_Terms#VIN|VIN]]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 +([[Glossary_of_Terms#VIN|VIN]] )+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.
The safe bed heater current has not been measured, but should be somewhat above 10A because the PCB trace between [[Glossary_of_Terms#VIN|VIN]]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 +([[Glossary_of_Terms#VIN|VIN]] )+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 WiFi and Duet Ethernet) ====
 
The Duet WiFi 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 [[Glossary_of_Terms#VIN|VIN]] +VIN+ and bed Heater + is duplicated on both sides of the PCB.
The Duet WiFi 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 [[Glossary_of_Terms#VIN|VIN]] +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 [[Glossary_of_Terms#VIN|VIN]]VIN terminal block. On the Duet 0.8.5 and Duet WiFi, the - bed heater terminal is the one nearest the [[Glossary_of_Terms#VIN|VIN]]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 [[Glossary_of_Terms#VIN|VIN]]VIN terminal block. On the Duet 0.8.5 and Duet WiFi, the - bed heater terminal is the one nearest the [[Glossary_of_Terms#VIN|VIN]]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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