ToM Extruder Relay Fix

by rwensley, published

ToM Extruder Relay Fix by rwensley Jan 8, 2011



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ToM Extruder Relay Fix by rwensley is licensed under the Public Domain license.

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First this not my best work by far. I am staying in a hotel and have limited resources to be able to build this properly.

Some of the new owners of ToMs have been experiencing problems with the extruder motor failing after a short time. I was able to see that the problem is related to glitches on the extruder controller board possibly due to the power supply.

As a work around until MBI develops a proper fix, I have added this relay board to the output of the EC board. As a result I have been able to print several objects with out issue.


All of the parts are available at Radio Shack. Follow the schematic attached. Be careful of the diode and capacitor polarities. See cautionary notes on the schematic.

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@rwensley - thanks for this. My cupcake extruder motor (Plastruder MK5 on one of the last Ultimate cupcakes) died and I came across this.

Was only using one of the two relays on the relay board, so I wired the PWM direct to the input B and put a 220uF 25V cap (chosen for being there, not calculated) across it, then wired the output from the relay to the motor. Seems to do pretty well for a few prints so far.

Is this also useful for a cupcake extruder? I'm having similar problems as you described and the motor is no working anymore. But I'm using plastruder mk4 with a cupcake. What do you think?

I see no reason for this not to work with the cupcake. The DC will be 1 speed all the time and have full power available for operation.


I used this for quite a while before I upgraded to a stepper and it worked exactly as advertised.

I am an electronics moron so I have a question for you.

What would be the easiest way -- or is it even possible -- to change this circuit so that instead of keeping the motor on for 600ms it k
ept it on for more like 5 seconds?

I'm basically looking for the simplest way to get a simple switch to power the DC motor for about 5 seconds, if you have any ideas, I'll be very thankful.


Don't be so hard on your self electronics is a fun hobby but it does take time to learn.

You can determine the time delay by measuring the relay coil resistanceThe time will be approximately 2.5 C R. (For the record the formula for a time constant is t = C * R and it takes 5 time constants to complete) I use 2.5 because of the relay coils pick up aand drop out voltages.

If you
measure 400 ohms and you want 5 seconds then

5 = C * 400 or C = 5 / 400 so

C = 0.0125 F or 12,500 uF

I would go with the LM555 timer IC also! ;)

If I can help with any of your electronics questions just let me know.


Thanks, Ron. :) When I get around to the idea I'm planning, I'm definitely using the 555.

Just for the record, if I did the math right, I'd need a 4000uf cap, which is awfully big. I've decided I'll probably just go with a 555.

Here is another fix using MOSFETs to replace relays, and hence it supports PWM. Check this out:


I've come up with the true fix.


No relays required and actually fixes the problem.

Thanks I used your design, and as I had a spare relay, I added a reversible version.

Not that the ToM reverses very often :)

Same problem; this works for me as a temporary fix.

It definitely turned on my dead motor. Now I'm working on Skeinforge settings to improve print quality.