MP Select Mini – Extended Bed (Part 2: Bed Wiring)

If you missed Part 1 : Y Axis, you can read it here.

This surprisingly ended up being the easiest, most trouble-free part of this whole mod. I spent more time debating how to arrange wires than it took to actually do it.

I’m using an external MOSFET because I got a pretty beefy bed heater (12v 200W ~ 16A) and I wasn’t sure how much power I could pull through the original MOSFET. I printed a standoff for the MOSFET that I could use to mount it over top of the external power supply without anything touching. You can see from the side view how it is mounted above the power supply:

You can follow the wiring instructions for whatever you end up using, but basically the wall outlet wires to the the power supply input, and the power supply output goes to the input of the MOSFET. The main output of the MOSFET goes to the new hot bed heater. The original hotbed wires that come from the printer main board were extended to run to the control terminals of the MOSFET.

I wanted to be able to revert things to stock if needed, so spent a bunch of time thinking about installing some plugs / connectors to the outside of the printer, but in the end just used some bullet connectors and drilled a hole w/ rubber grommet in the back of the printer.

The whole thing sits behind my printer. At this point I have both the original printer power block and the new power supply both plugged in separately, but I think this power supply (12v 30A) should be able to power both the bed and the entire printer. At some point I’ll probably power the printer off the power supply (it does have 3 sets of output terminals).

Once things are back together, remember to do a PID tuning, since the new bed will heat at a different rate!

Thermistor Woes

At this point I got everything back together and found that the temperature of the new hotbed thermistor reads wrong on the printer. I tried both thermistor modes but they give the same incorrect reading. Actual 20C reads 40C and actual 50C reads 80C. The bed was supposed to come with a 100ohm thermistor, but the seller said that they had a few that went out with a 50ohm. I’m wondering if I was one of the unlucky few… Haven’t tested it yet though. I could replace it with the right thermistor or even the original one off the old bed, but I just got it back together. I don’t want to take it right back apart yet.

For now I just made myself a conversion table, so if I am printing PLA and want a 50C bed, I just set the temperature to 80C. You only really need to know a couple temperature points anyway.

Next steps:

Posted in 3D Printing

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CodingWithSpike is Jeff Valore. A professional software engineer, focused on JavaScript, Web Development, C# and the Microsoft stack. Jeff is currently a Software Engineer at Virtual Hold Technologies.

I am also a Pluralsight author. Check out my courses!

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