Custom size mk3 heatbed


ATM i have 60 in stock and 51 beds is used out there. I hope everyone have good use of their bed :slight_smile:

Best Regards
Ted aka Marlark, Cinya


Hey, thank you very much for the hearbed - it looks good - tomorrow i install it.

i have one question - i use the same one but in large 215x215 - in the PDF file for “dummies” he only use pin 1+2 vor 12V and pin 3 is not in use.
but normaly you MUST have a bridge beteen 2+3 if you use pin 1 on 12 or 15 V (pin 1=plus pin 2+3=minus)
if you dont have a bridge between 2+3 the bed only run on half power and one side is not realy heated.


The black line extends into a half circle between 2 and 3. Might be a bit hard to see on some monitors, but it’s there. So the bridge between 2 and 3 is correct.


WOW -0,7 Ohm, 21 Ampere on 15 V (320W) - in 4 Minutes on 100 Celsius with 4mm Glas - i have inside only 380W - at my first try it kills the bridge between 2+3 (thin cable)

i test it on print but i think i take the 24V pins !


ok, on 24V Pin with 15V it don´t work - only 66 degrees with fan 100%
i think on 12 V the www.reprap power-expander do not live long because he goes up to 140 degrees after 4 minuites.
i try some more :wink:


Use a 40A SSR. It works. And use 2.5 mm wires.


Here’s how I went about installing the heat-bed sold by Marlark.

I was going to use a new 15 V supply and remove the stock one.
Using Kelvin measurement I got .645 Ohm between the +12 V terminal and minus on the heat-bed. So the 0.65 figure quoted is correct. 15 V and 0.65 ohm will draw 23 A so the power supply needs 345 W for the heat-bed alone. 15 Volt is quite low and voltage drops will be noticeable in the time required to reach the temperature set point. In order to minimize the drops I used 4 mm^2 cables.

For switching I decided to use a low Rdson MOSFET. You can go the DC SSR route but unless you want an alibay Fotek clone/fake/uprated one with unknown on resistance and no datasheet and a yeah-right max amp rating, prepare to spend more on it than the heat-bed.

After looking at some MOSFETs, I decided on IRF1324PbF (

Worst case calculations:
Rdsonmax = Pdmax / IDmax^2 = 300 / 195^2 = 7.89 mohm.
RthetaJ = 62 K/W.
Max power dissipation at roomtemp (25 degrees C): Pmax = (Tjmax-Troom)/RthetaJA = 2.42 W (no surprise there, it’s a TO-220).
Max current at roomtemp: I = sqrt(Pmax/Rdsonmax) = 17.5 A.

Now, since we need it to be able to carry 23 A continuously we have to heat-sink the sucker so it can dissipate 23^2*7.89m = 4.17 W.
Assume ambient temperature of 40 degrees C, max junction of 175 degrees C. According to datasheet RthetaJC + RthetaCS = .5+.5 = 1 K/W.

Rtheta junction to ambient = (175-40)/4.17 = 32.4 K/W. Total needed is 32.4 - 1 = 31.4 K/W.
A heat-sink with a rating of <31 K/W is needed for some margin. (Using the ASSMANN WSW V6560W 9 K/W, the MOSFET or sink never gets uncomfortable to hold heating from 25->100 degrees C)

The circuit I made was this. R2 and D1 are placed directly on the heat-bed. R2 was selected according to my preference of light intensity with the LED I’m using.

A PCB will be quite unpractical for this purpose, but certainly doable. Prototyping board? Forget it. The trace width required is… vast.

Using an opto-isolator like the RepRap power expander board is certainly possible, but the main reason for doing it, apart from isolation, is: the bed output pins on the main controller board is low side switched with a power N-mosfet already present. I wanted a very simple wire harness and I didn’t have an available opto-isolator.

Since this can’t be connected directly to the bed heater output, as noted above, SIGNAL is connected by piggy-backing on T5’s gate pin. Ground to power supply minus.

Note that there’s no flyback diode in the circuit. Even though the load is resistive, there’ll be some inductance. If it’s enough to kill the mosfet fast, I really can’t say. Yet.

D1 and R2 mounted on heat-bed. The solder pads are not exactly placed optimally for the 12 V power option.

New power supply installed. The supply screw holes were slightly larger than the previous supply, so I printed some new standoffs. 4 mm^2 wires for the head-bed. I also replaced the 0.75 mm^2 wires for the printer board power with a 1.5 mm^2 one. T5 piggy-backing is underneath the flatcable slightly above the bed heater output pins.

MOSFET, heat-sink, bullet connectors, thermistor wires, control wire and bed supply wires.

Next modification will be the whole build platform; this is just too thick.

Heating the bed 25->100 degrees C just lying on the kitchen table was 2 minutes 13 seconds. In the sandwich above, it’s 4 minutes 2 seconds.
Current draw starts at 23 A at 21 degrees C and goes down as temperature goes up. At 100 degrees C it’s 18 A.


Hi Danf,
I ripped off the alu plate and did ist this way:

And some info here…



(2016-02-21) Current Stock Status : 55, Sold : 56

Passed the halfway point :wink:

// Marlark


Hi, I was wondering if nobody is using any (white) silicon thermal grease between the aluminum and the glassplate, to enhance the heatflow? Or even a thermal conducting rubber mat? Those techniques are used between semiconductors and their heatsink for that purpose.


I guess,
you have a mess, when cleaning the glass plate.
And need always new paste to keep it running.

I am fine with the heating at the moment…


Do agree with frank that it will be messy and to be honest i think it is a solution that is not needed.

// Marlark


and if I am honest,
(stolen from MavRaven): If we are printing 3-8 or more hrs, waiting a few minutes longer to heat up, does not make any difference :slight_smile:
But I do understand the technical point of view


Using a thermally conductive material between glass and aluminium plate would be a good idea if:

  • your glass is cracking due to uneven heating
  • don’t want to replace glass or heated aluminium plate
  • you need a the initial glass surface temperature to correspond an undefined “somewhat better” to your temp set point
  • don’t mind reapplying thermal paste or have a large thermal pad

As the temperature sensor is located on the opposite side of the aluminium plate the thermal paste (or other gap filler) will increase the apparent heating time, i.e. there’s less insulating air trapped. Since I don’t use anything between glass and aluminium (mine appears quite plane parallel and conduct even enough) I really don’t mind the fact that print head heating is done after bed heating - it’ll help heat distribution before print.

In comparison to a transistor or other small component - you generally need to heat sink it with thermal compound when you run the risk of altering characteristics or destroying it if you don’t. At only 10 W it needs to dissipate orders of magnitude more (W/mm^2) than my heat-bed at 345 W.


I did the modification for a leaner build platform.

Print with ABS plastic on kapton sheet. Owl at 50% scale.

Bed setpoint: 115 degrees C.
Heatblock setpoint (Velleman temperature): 215 degrees C.
Heatblock actual temperature at start: 241 degrees C.

Data after 2.5 h of printing:
Printer ambient temperature, at same height as MOSFET: 30 degrees C.
MOSFET Heatsink: 41 degrees C.


All frequently mentioned sites for buying a replacement PSU in this thread seem to be wholesale only displaying prices excluding VAT or unable to buy as an individual.

Cheapest for a Meanwell RSP 500-15 I found was 118 Euros including VAT and shipping.


[quote=“Alfred”]All frequently mentioned sites for buying a replacement PSU in this thread seem to be wholesale only displaying prices excluding VAT or unable to buy as an individual.

Cheapest for a Meanwell RSP 500-15 I found was 118 Euros including VAT and shipping.[/quote]

I got one of those, and the fan is a little noisy.
I don’t mind since my printer was banned to the cellar :-))

If heating up the bed and both nozzles at the same time, the fan runs most of the time.
It won’t fit into the old position, mine is on the very left site parallel to the left site of the printer


(2016-03-10) Current Stock Status : 53, Sold : 58

I have some left if someone want a custom sized heatbed for their printer. All the details to get one is in the first post in the thread.

Best Regards


I just ordered one, Can’t wait till I got all the parts to mount it! :slight_smile:
Thanks a lot!


Finally had some time to start working on my Marlark heatbed.
Opted for a powerful replacement PSU, a 500W 15A Meanwell unit. Way to big to fit in the old place so it’s now along the left side of the box.
I decided to install it with it’s fan pointed to the back in contrast to Danf’s solution.