Ok, so the weekend is here and finally I’m getting closer to having my own heated bed upgrade .
All of the electronics are sorted, first of all I changed my Marlin config, slightly different pins than @shelxle used, I connected my MOSFET to PK6 so it’s next to the thermistor pin and I used a 10k NTC thermistor because that’s what I have already bonded to my bed with previous version controlled by W1209 thermostat module.
I used a 2x9 goldpin header and connector to connect the pins with the bed. This is how it looks on the board:
And no job is complete without a bodge resistor, this one is the 4.7k pullup for the NTC. I’ve put it here instead of near the bed because when the thermistor gets disconnected the printer would display noise.
To connect the printer side and the bed side (and be able to remove the heatbed completely when it’s not necessary) I used a 4 pin JST SM connector and printed a bracket for it that attaches to the back tower of the delta.
As for the bed I used a typical MK3 aluminum heatbed rated for 12V. Made a little detent in the center for the thermistor and soldered it to the pads originally meant for an indicator LED (after scraping the traces that would connect it across the heater of course). The white stuff around the NTC is thermal adhesive. I wanted all of the electronics to be as compact as possible so I chose to mount the MOSFET (IRL7833) directly to the bed, thermal glue would probably be enough but since the mounting pad of the package is the drain I just soldered it to the pad directly, why I didn’t choose a DPAK version of the transistor you ask? I dunno . The rest of the connections are made on a piece of perf board and it is glued to the bed as well as soldered to the FET which secures it just fine. The green connector is 12V power directly from a separate supply and the 4 pins are what goes to the printer. The resistor here is a 10k pulldown for the gate of the FET. The soldering is pristine I know… It looks much worse in the photo than in real life…
Only thing thats left now is to finish designing and print the risers so the bed can be mounted to the printer. I’m also thinking about adding an option to Marlin so that it doesn’t display the bed temperature at all when the NTC is disconnected (now it shows the lowest possible value which is -34°C in my case).