Ultimaker and Prusa MK2 heated bed mod

Ultimaker Prusa MK2 Heated bed solution

image of heated bed
After battling through the Ultimaker wiki and the reprap wiki to figure out how to get a heated bed working ont he Ultimaker, I decided to write up my solution.
The Documentation and information available the moment is pretty sketchy. After trying a couple of things and blowing up half my Ultimaker, I decided the wiki was a terrible resource, lacking in opertinent information at best, and leaving massive assumptions.
The biggest assumption on the ultimaker wiki is that you can connect the headed bed to the onboard FET driver directly. While this worked for me temporarily, the heat from the FET caused it to stop working. My advice is to NOT connect directly via the onboard FET and use a relay.

The instructions below are for how to connect a Prusa MK2 heated bed to the Ultimaker with board revision 1.5.6.

Required parts:
1x Prusa Mk2 heated bed
1x Glass/mirror/aluminium cut to size
1x 100k thermistor
1x 4.7k metal film resistor
1x 18v Relay (http://uk.farnell.com/jsp/search/productdetail.jsp?SKU=1861593)
2x Terminal Block
245mmx245mmx8mm MDF sheet
1x drill bit 3mm
1x drill bit 4mm
4x Bulldog clips
100cm 16AWG Red cable
100cm 16AWG Black cable
20cm 24AWG Red cable
20cm 24AWG Black cable
200cm 26AWG Yellow Cable
1x 3 pin molex female
3x molex terminal pins
2x self tapping 6mm woodscrew
1x 12v PSU at least 10A
4x 20mm screw
10cm heatshrink sleeving
some kapton tape

Required Tools:
Saw (unless MDF pre cut)
crimping tool or pliars
wire cutters
soldering iron
small flathead screwdriver

About 2 hours

Assemble Electronics Hardware

image of wiring
image of wiring

  1. Remove the bottom fan cover from the PCB. Unclip the fan from the connector and place the cover to one side. This will expose the PCB.
  2. Now unscrew the PCB from the bottom of the machine. This is so you can solder the resistor in place (see below).
    * You should ideally remove all connectors and place the PCB in a sensible position to put the resistor in. I cheated and did it in place witht the help of some "helping hands" holder.
  3. Solder in the 4.7k resistor in the third empty resistor hole as per the diagram.
  4. Screw the PCB back onto the Ultimaker.
  5. Strip 2 pieces of 24AWG 100cm yellow wire and crimp on the molex connectors.
  6. Clip the molex crimps into pin 1 and 3 of the 3 pin molex shell socket.
  7. Connect the molex connector to the PCB. It is marked on the PCB as "heated bed"
  8. Solder 20cm of wire to each of the coil legs on the relay. See diagram.
    image of wiring
  9. Using 30cm of red 16AWG cable, strip and tin the cable. Solder one end to one of the relay contacts.
  10. Strip and solder the remaining 70cm of red 16awg cable. Solder one end to the other relay contact.
  11. Connect the other end of the 30cm red cable to the terminal block
  12. Connect one end of the 100cm black cables to the terminal block. The other side of the terminal block goes to your PSU.
  13. Feed the red and black cables through the hole on the top right of the Ultimaker.
  14. Mount the relay on the bottom of the Ultimaker. I used hotglue.
  15. mount the terminal block in a suitable place. Put a 6mm woodscrew through the middle.Trim and tidy cables as required.
  16. Reconnect the PCB fan.
  17. screw the PCB cover back on.
  18. route the thermistor cables through the same hole you routed the heated bed cabled in step 14

Make a mounting plate
image of wood
The prusa mk1 heated bed is smaller than the Ultimaker. The mounting holes have no easy way to attach. We are going to make an adapter.

  1. Cut the MDF sheet to 245x245mm
  2. Get the existing plastic bed from the Ultimaker. Put this on top of the MDF sheet and tape down.
  3. Clearly mark with a pencil the holes onto the MDF sheet. it is important to get the slits as well as the larger hole.
  4. optionally drill thruogh the holes in the acrylic through the MDF. This is what I did.
  5. You should now have a piece of MDF with the same holes as the original bed. Try this by attaching it to the Ultimaker. Clean and file until it fits nicely.
  6. Get the heated bed and tape to the MDF sheet.
  7. using a pencil, clearly mark the holes.
  8. Remove the heated bed and drill the holes out.
  9. Put a 20mm screw in each of holes in the heated bed and a nut on the reverse.
  10. push the screws into the holes on the MDF sheet. Put a screw on the reverse of the MDF.

Final fitting
image of wood

  1. Get the yellow cables from the thermistor and the red and black cables for the heated bed and clip the cables to the MDF bed. Route them to the back of the machine but leave enough slace for full movement.
  2. Place 5cm of heatshrink sleeving on the thermistor cables and push away from the end.
  3. Solder the yellow thermistor cables to the thermistor.
  4. Push the heatshrink over the thermistor to the thermistor head. Heat with a lighter to shrink down.
  5. Put the thermistor in the hole in the prusa heat bed. Make sure the top of the thermistor is level with the top of the bed.
  6. Stick down with kapton tape.
  7. Solder the red and black cables to the heated bed.
  8. Attach your mirror/glass


Go to The new Marlin builder http://marlinbuilder.robotfuzz.com (disclaimer, this is mine)

  1. In the hardware section, choose "Heated bed tempterature sensor" option "100k Thermistor"
  2. Click the bottom button "build Marlin"
  3. Download the new hex file and save.
  4. using Cura, update the firmware on the Utimaker.


  1. without the new external PSU attached, load Cura and connect to the machine.
  2. Click the option to set the temperuate of the heated bed. You should hear the relay click. If it does you have the software working. Put a multimeter between the red on the heated bed and the red on your PSU. It should be a short.
  3. Click the option "Check Temp". It should return the temperature of the heated bed. if this is a sensible looking temperature, and not 0, then the thermistor is working.
  4. Turn on the PSU. if there is heat on the bed and no fire it is working.


Main Content: 


heated bed for ultimaker

I tried to buy the MDF sheet 1,8 mm, but I cannot find it.

The thinnest I can find is 3 mm. Can I use this to?

next question: What kind of glass is the best (harded ?) and how thick should it be ?



I don't really understand steps 5 until 11 in the Assemble Electronics Hardware section, mind if you put pictures of them?

Is it possible if I want to use the same electronics assembly but using a self-made heated bed using aluminum plate + nichrome wire?

Please advice. Many thanks!




I have followed your excellent guide however now have a problem. The firmware builder is now broken and Daid is unsure on when he will be able to rebuild it. Would it be possible for you to upload a copy of the firmware you used or even email me a copy? I have the same setup as you with the same temp sensor.



sucks balls

oh man that sucks.

I can honestly say no, I don't have the settings, hex or otherwise. I guess I could extract the firmware, but I know since this article was posted I have rebuild for an ultipanel.

Sorry dude