DC motor drivers running very hot with stepper motor

Hi,
I try to use a SY42STH47-1684B stepper motor with a standard MakerBot extruder board. As far as I can tell, the SY42STH47-1684B is a tested stepper motor for building a Mendel extruder.
My problem is, that the both DC motor driver chips on the extruder board run very(!) hot after a few seconds. At least 70-100°C after test driving the motor (without filament or something) for 10 or 20 seconds. The motor turns as expected.

The extruder board currently still runs Makerbot Firmware (compiled for stepper motor drive).

Is this normal? Do I something wrong?

The rated current for the motor is 1.7A IIRC but I only run them at 1A or less to keep the motor and the chips cool.

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[www.hydraraptor.blogspot.com]

Is it possible to limit the motor current on the Gen 3 extruder controller (v2.2)? Or do I need a separate stepper driver board for the extruder?

In the Mendel build instructions, it looks like I could use the Gen 3 extruder controller board v2.2 directly!?

It uses PWM control which I think you can set with an M code or set it zero and use the pot on the board. It is a bit of a bodge because if you don't set it correctly you can fry the chips. Open loop PWM is not good for steppers because of the BEMF but the extruder only moves slowly so it should be OK.

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[www.hydraraptor.blogspot.com]