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. The Duet 2 (Wifi and Ethernet) is 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 and connecting stepper motors.

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 2 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.

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.

Duets 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! Do not "tin" the ends of the wires. 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 Duet 3 Mainboard 6HC is safe up to 18A heater current on OUT0 in a normally ventilated environment.

The Duet 3 Mini 5+ is safe up to 15A heater current on OUT0 in a normally ventilated environment.

The Duet 2 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/d....The trace between VIN+ and bed Heater + is duplicated on both sides of the PCB.

Duet 0.6 The board is comfortable with 10A bed heater current, but not much more. The weak link is the PCB trace on the top side of the board that connects the Power In V+ and Bed Heater V+ terminals. If you want to use a bed heater that draws more than 10A, solder a thick wire between those terminals on the back of the board.

Duet 0.8.5

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.

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 2, the - bed heater terminal is the one nearest the VIN terminal block.

Caution: when using a high-powered bed heater, in the event that temperature control fails and the bed heater is turned on at full power for an extended period of time, you should either make sure that the bed heater and bed will not exceed a safe temperature, or else install a thermal cutout to disconnect the bed heater or its power supply before excessive temperatures are reached.

When using PID to control the bed heater, RepRapFirmware 2.x uses a low PWM frequency (10Hz) so as to be compatible with all standard types of SSR.

RepRapFirmware 3.x does not set different PWM frequencies for different outputs. If using an SSR then set the "Q" parameter in the M950 command to a low frequency like 10 Hz.

Use a zero-crossing DC-AC SSR such as Crydom D2425, Kudom KSI240D25-L or Fotek SSR-25DA (note: many Fotek SSRs on sale are fakes, using triacs rated at lower current than the marked rating of the SSR). For 230V bed heaters, SSR-10DA may be sufficient. If your bed heater draws more than about 1/4 of its rated current then the SSR may need a heatsink.

Take appropriate safety precautions when using a mains voltage bed heater. In particular:

• Connect metal parts of the printer to mains ground. This includes the printer frame, the bed plate if it is conductive (e.g. aluminium), and any other metal parts that the bed heater or SSR wiring might come into contact with if wires break.
• Ensure that it is not possible for the user to touch the SSR terminals or any other exposed mains wiring, or for you to touch the mains wiring when you are working on the printer with power applied. If your SSR is not supplied with a clear plastic safety cover, buy one (for the Crydom SSR listed above, the part number is KS101).
• If the bed is moving, use highly-flexible wire or cable with a sufficient voltage rating to connect the moving bed heater to the stationary wiring. Cable intended for use in multimeter tests leads is one possibility.
• If the bed is moving, you must use strain relief at both ends of that cable, to reduce the risk of the cable fracturing with repeated movement.
• If the bed is moving, use a cable chain or similar to make sure that the cable can't get chafed or trapped.
• Provide a fuse for the bed heater circuit or the whole printer appropriate to the current draw and the current rating of the mains lead. One option is to use a panel mount IEC mains inlet connector with a switch, neon indicator and fuse built in. See http://www.thingiverse.com/thing:965396 for an example setup on a delta printer.
• It is highly recommended that you power the printer via a Ground Fault Current Interruptor (GFCI) - more commonly called a RCD (Residual Current Device) in the UK - to protect against electric shock in the event of a fault.

If in doubt, consult a qualified electrician.

Use a low voltage drop DC/DC SSR such as the Auber Instruments MGR-1DD80D100 or Crydom DC100D40. The SSR may need a heatsink, depending on the current. Do not be tempted to use a cheap DC-DC SSR such as the SSR-40DD, which is basically useless for this application because of its high voltage drop.

add M305 command to config.g [M305 command specs]

if you want to remove heatbed from the system add

M140 H-1

There are three ways of using two thermistors to control one heater:

1. Connect them in series. The temperature readings will be averaged with a bias in favor of the colder one. If any wire breaks, you will get the usual -273C reading and the heater will fault.
2. Connect them in parallel. The temperature readings will be averaged with a bias in favor of the hotter one. If one of the thermistor wires breaks, the temperature will under-read, so you will end up heating too much.
3. Use one to control the temperature, and the other as an over-temperature cutout, configured using M143.

Note: if connecting 2 in series, the resistance of the thermistors needs to be combined in config.g. So if each thermistor is 100k at room temperature, they need to be set as 200k when declaring the sensor with M308. And if connecting in parallel, the resistances need to be half their combined average (or just half of one if both are the same).

However, when connecting thermistors in series or in parallel, although the B value remains the same, the effective C value changes. So it's easier to adjust the R parameter instead. For example, on a Duet 3 the normal R value is 2200. If you connect two identical thermistors in parallel, you can specify the usual T, B and C parameters for the thermistors, but double R to 4400. Likewise, if connecting two thermistors in series, halve the R value.