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unipolar steppers

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Rather than a step up converter, you're probably better off just building a new simple supply. The motors you have don't require much current, you could just make a bridged dc supply from a Radio Shack 12.6-0-12.6 center tapped transformer, that would give you ~25V. You should use a filter cap to smooth out the voltage, but as you will be controlling current and not voltage you don't need a voltage regulator. (edit) You could also use a simple voltage doubler circuit with this xformer to provide ~50v.

The L297 Vref (pin 15) is what the chip uses to determine the pwm chopper value and thus the motor current. It compares the sense resistor voltage to whatever voltage you feed in Vref, and if the sense voltage is greater it cuts power to the motor. The voltaage you feed Vref is calculated by what value sense resistors you are using and how much current you want the motor to have. I think the datasheet gives the formula for determining this value. For experimental purposes you could use a voltage divider network and a pot to vary the Vref. Because low value resistor are used for sensing current (to maximize power to the motor), the Vref voltage is usually quite low, maybe around a volt or less depending on your setup.

I would think this link has already been suggested, but you should Google a few pages on stepper motor control with the L297 to get a basic understanding of how to use it.
 
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