Howto Connect your Heated Bed to a Relay/Mosfet

The idea is pretty simple: The high current of the heated bed brings multiple problems to your mainboard. All the current for the bed has to go through the one connector of the bed in addition to all the other current for the hotend, the steppers and the logic units. The default connectors are not rated for the current that the heated bed can require.

←– connector and corresponding plug —>

 (EDIT: the latest version of the controlboard has newer/different type connectors fitted they are safer but if not connected properly even they CAN and WILL melt,  *remember transition resistance equals heatdevelopement))

Removing heated bed current from the mainboard significantly reduces stress on your mainboard, especially the connectors, and allows you to use a second PSU for bed power.

  • a MOSFET unit like this, and some wire(s)

General Add-on Heated Bed Power Expansion Module on aliexpress.com or this on banggood.com or on ebay.de

    Question: Could  i use a mechanical relays too?
    Answer: Yes you can, but the on/off clicking will drive you crazy very soon.
 NOTE: More than one version of the ANET controlboard is being shipped from China, always check for + and - (polarity) on the boards. 
       RED   =  +  (on psu marked as +V ) 
       BLACK =  -  (on psu marked as -V or COM ) 

 NOTE: I used an easy way to wire this using the stock 12 volts wire and without cutting it, just removing about 1 cm of the insulation 
       (to connect the mosfetboard), ran it from psu to mosfetboard to controlboard.(applies if you use 1 psu only)
       Other people might want to use 2 sets of wires, 1 for controlboard and the extra set of wires from PSU to mosfetboard.
  • unplug your printer's power cord
  • remove the power-cables of the heated bed from the mainboard and attach them to the power output of the MOSFET unit
  • Connect 2 wires from the bed output of the mainboard to the control input of the MOSFET (polarity matters! + to +, - to -)
  • Connect 2 wires from the PSU (either the same as the printer, or a second PSU) to the power input if the MOSFET unit
  • check twice against the graphic
  • plug your printer back in and test

Apply a second PSU ( 12 or 24 volts)

If you want your HOTBED to heatup faster you can add a second power supply unit to your printer.

Disconnect the wires from the PSU to the Mosfet Unit and apply them to the second PSU.

 The heating element that came with the Anet A8 will perform ok with an increase of the Voltage to 14 Volts. 
 This will help the bed heat up significantly faster.

Increasing Voltage on the (stock) PSU

 Note ** This does *not* apply to an ATX PSU! You cannot change the voltage output on these, don't try.

Near the power terminals, either on the left or the right of the terminals, is a small dial marked “ADJ +V”. Rotate it clockwise to increase voltage, and counter-clockwise to decrease it.

Always disconnect your power supply before making adjustments!

Check the voltage between one of the + and one of the - terminals, using a multimeter set for voltage DC.

PSU combinations

There are a lot of combinations possible

1 x 12 volts PSU

   NOTE: This is the standard setup

2 x 12 volts PSU, of the one used for the heatbed you could raise the voltage to circa 14 volts

   NOTE: If you have an ATX-Power supply laying around, take the original one for the bed and the ATX for the controlboard.

1x 12 volts + 1 x 24 volts, The 12 volts one is used to power the mainboard extruder and steppermotors, the 24 volts powers the heat bed (through the mosfet)

   NOTE: Make sure you use the 24 volts connections on the heater

12 volts PSU + 24 volts PSU combination

 The heating element in your HOTBED (as seen in the picture below) can be wired to support 12 or 24 volts connection.

The schematic printed on the surface shows pin 1 2 and 3 representation of the element →

If wired for 12 volts, the element is doubled up, resulting in less resistance/higher current/slower heating and less poweroutput.

If wired for 24 volts, the element is connected from both ends only resulting in higher resistance/lower current/faster heating higher poweroutput.

NOTE: There is more than one model of heating element available in the market, before you start any work check which one you have and if you can use the 24 volts option.

  V=R*I (Voltage equals current multiplied by resistance), P=V*I (Power output equals Voltage multiplied by current)

HOTBED 12 Volt con. resistance betw 1.0-1.2 Ω (work with 1.1 Ω)

• I=V/R the current drawn at 12 Volts is 12/1.1= 10.9 Ampere

• P=V*I Power output at 12 Volts is 12*10.9= 130.9 Watts

HOTBED 14 Volts con. resistance betw 1.0-1.2 Ω (work with 1.1 Ω)

• I=V/R the current drawn at 14 Volts is 14/1.1= 12.7 Ampere

• P=V*I Power output at 14 Volts is 14*12.7= 178.2 Watts

HOTBED 24 volts con. resistance betw 3-3.4 Ω (work with 3.2 Ω)

• I=V/R the current drawn at 24 Volts is 24/3.2= 7.5 Ampere

• P=V*I Power output at 24 Volts is 24*7.5= 180.0 Watts