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Power! I want more POWER! 36V4 isn't it

No, I’m not power-mad. But my stepper is. We want to use the 36V4 “High power” stepper driver to drive a NEMA 23 or maybe even a NEMA-34 class stepper. It may be high power relative to a NEMA-8 or NEMA-17, but it doesn’t have the oomph for larger motors. Yet we want that 1/256-step stepping. Any suggestions how we can add an outboard set of MOSFET drivers with more-than-adequate current rating?

We are building a small prototype with NEMA-17 as proof-of-concept, so we can use the driver “as is” for this small prototype. But as soon as we know it all works, we want to build a “full-sized” unit. We are all retired and budgets matter; the controllers for the high-powered motors are far, far outside our budget limits. Hence the desire to grab a soldering iron and build our own “controller amplifier” that we could hook up to the 36V4, and do it from our existing parts inventory.

Suggestions, Web links, etc. would be appreciated. We have an assortment of both PMOS and NMOS transistors in TO-220 packages, and a 36V OMG-A supply for the three motors. Our MOSFETs are rated 60V, 30A, with heat sinking.


I do not see a practical way to add MOSFETs to the outputs of our High-Power Stepper Motor Driver 36v4 to increase their current capabilities. You could consider using our schematics for the board as a starting point for designing your own. There are small-run PCB houses like OSH Park that make getting just 2 or 3 custom boards reasonable.

How much current do your NEMA 23 and NEMA 34 motors draw?


The NEMA numbers specify only the dimensions of the motor face plate, and there are hundreds of different step size, torque and current configurations for each type. For useful advice, post model numbers and/or links to the specifications of the motors you intend to use, as well as your system requirements for torque and power.

Yes, I know what NEMA is all about. However, a NEMA-23 or NEMA-34 motor is also a lot heftier than a NEMA-17, and requires more power. A small NEMA-34 and a very large NEMA-17 might be comparable, but let’s just use the convenience of the terminology and assume that bigger motors take more power.

I’m looking at 8 amps plus change per phase.

8A plus change per phase.

I spent last night reading the spec sheet for the chip, and it is designed to drive external MOSFETs, in particular, plain old NMOS MOSFETs. So I’m thinking that an Xacto knife to scrape solder mask off a trace and a few wires might solve the problem.

I would have to look at your schematics. That is my plan for this weekend.

The schematic is not forthcoming on the part numbers. It looks like you are using a part #1903 for the output drivers, but this is rated at a max of 30V, but has a current limit of close to 20A, presumably with heat sink cooling. I haven’t worked out the thermal equation yet. I see some options here:

Provide a lot of cooling for the MOSFET drivers. Heat sinks w/good thermal grease.

A bit of board rework to remove the existing MOSFET drivers and solder some bigger MOSFETs on. This involves getting MOSFETs that fall in the nC range of the 8711. More research.

Remove the existing MOSFET drivers and add wiring to go to an outboard set of eight MOSFETs

Leave the existing MOSFETs in place but ignore them. Run wires to an outboard set of eight MOSFETs. This may confuse the world because the AOUT/BOUT signals would not be correct.

Design a new board. Doable, but maybe not as simple as the above options. Most likely, more reliable.

With cooling (large heatsinks and forced airflow) you might be able to get a few more amps out of the 36v4 stepper motor driver, but I am very skeptical of getting to 8A. If you already have some drivers and MOSFETs on hand, and good rework skills, you could try changing out the FETs or rewiring with some external ones, but if you have a little experience with board design, I suspect that route will end up being easier and as you mentioned more reliable.