PROTOTYPE: Heated bed mosfet relay V3

This is the new and improved version of my "Ultimaker heated bed MosFET relay hack".

3D printer parts and enhancements

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Based on original design: Ultimaker heated bed MosFET relay hack - V2 by Jonathan Bischof


/Important note: There is a (fatal) error in the design! It doesn't work the way it's published here! Please stand by until I find the time to get it sorted out. Thanks

NOTE: This is an untested prototype design. I currently have very limited time available and won't be able to make and test one myself in the next few weeks. However I wanted to get this out because I know some people have been waiting for it for a long time now.

Disclaimer: UNTESTED DESIGN! Please try out responsibly and be prepared to face problems with this. I believe it will work fine. It's a simple design and I am confident that there are no errors. If you don't think you're the right person to test this out, then please wait for someone to confirm that it's working fine.

Some important documentation is still missing, but you should be able to make and use this using the information from the old design.

About the improvements:
Version 3 uses an optocoupler to isolate the input from the output voltages. This makes it universally usable with any 3D printer electronics platform and enables you to use one power supply unit for everything, or separate units. Performance is identical to V2 - this thing shouldn't get warm during operation, at least not in "bang bang mode". PWM mode is not recommended for the heatbed. You could try, but keep track of the mosfet's temperature.

Materials and methods

Printed (!) circuit board can be made from PLA or a higher temperature material as you wish. With careful soldering, PLA works fine.
The electronics components should be ordered from using the BOM provided. Please do not use any other parts unless you're knowing exactly what you're doing.



Issues are used to track todos, bugs or requests. To get started, you could create an issue.


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3422c8458623792e6e28b1fb4013ede3?default=blank&size=40Szabolcs Számel commented ago

Hi thanks for this project. I fixed the circuit for myself and its working perfectly. Just look there:

You need the +12v or +24v from the psu, take out the R2 resistor and put the 5v regulator fed by +12/24v connected to collector of optoisolator. Thats all. But please check the datasheet of MOSFET what you are using, look at the Transfer characteristics, I used 75344G and for me the 5volts on gate is enought for 60A drain current cca.

Mini sheepJonathan Bischof commented ago

I'm swamped at the moment. Plus I won't be needing to build any of them myself in the near future (v2 works fine for me) so it could take some time, sorry :(

You could look into version 2, which should work fine as long as you read the whole description and make sure your setup matches with the version 2 requirements.

De90ad97a065677ec0ea94b8dccd73f0?default=blank&size=40Edvardas commented ago

When you think you can post updated design? I really love the idea of isolating printer from heated bed.

Mini sheepJonathan Bischof commented ago

The problem with the design as it is, is that the "positive" wire coming from the heated bed isn't actually positive anymore as soon as the heated bed is switched on. I didn't see that, but it renders the whole thing completely useless because there is no more gate voltage for the mosfet to work with. I suppose that, what actually happens, is that the mosfet gets only partially switched on, up to a point where it's resistance is around the same as the heated bed (instead of almost zero) which leaves half of the heated bed voltage on the bed, and half on the mosfet. The problem is, it also leaves half of the power on the mosfet which blows it pretty much instantly...

I know, it's not "half" but somewhere in that region. In any case, it won't work.

De90ad97a065677ec0ea94b8dccd73f0?default=blank&size=40Edvardas commented ago

Yay... I've tested this few weeks ago and got some smoke instantly, now there's a short in first diode and after reading this i'm no longer sure that it was my fault. Haven't noticed a flaw in design...

Mini sheepJonathan Bischof commented ago

Hi Peter,

Dang it, you're right. I didn't see that... I need to think this over. Your workaround is fine, but needing another wire isn't what I wanted. Sure that can be done better...

I thought I used the same terminal blocks, maybe I accidentally picked the wrong 3D model o.O

Thanks for pointing that out, and sorry for the inconvenience!

9e29e604b461c46376f068999a2f60d0?default=blank&size=40Peter1956 commented ago

There is a mistake in the schematics. The Gate voltage is schorten by the MosFet. The MosFet burns out in a few seconds. You need V+ from the Heatbed Power on the Voltage divider (R2/OC/R3). Then it works fine.

By the way, the X2-terminal is to small. Yon need a bigger one like from the V2.


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Mini sheepJonathan Bischof commented ago

Hi koyashi,

I don't know about the ANET A8 (or any Prusa i3) in detail, but it should work without any problems as long as you get the right components for the right voltages.

You can also use two of these in parallel with the same external power supply because they are isolated and should not interfere with each other.

Best regards,


471ecf3ea368ce64e75cca3195b2ca95?default=blank&size=40koyashi commented ago

Hello Jonny.

I have a ANET A8 and was wondering if this is suitable for both the heated bed and the extruder?

Just wondering because I wanted to order enough parts from mouser to build 2 of them.


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