Newbie- stepper driver, voltage vs duty cycle

[Spec1alFx]

Hi all!
I'm trying to build a unipolar stepper driver using a microcontroller (dsPIC with a feature rich, hardware PWM), 4 mosfets and other components... But first I need to understand some simple concepts that are too simple to find explanations about

The first question is: voltage. Stepper's datasheet tells me 4.8V / 2.0A per winding. But if I look at typical torque curves the reference voltages are 24 or 48 volts. As I understand the trick is the duty cycle: if I can excite a coil with 4.8V / 2.0A in a time "t" I can do the same thing with higher voltage / amperage in a shorter time. Is it correct?

If yes: for the Ohm's Law, if I use 48V I have 10x amperage too... So I should use 1% duty cycle? Possible?

Thank you very much,
Claudio

[vger]

Claudio,

Most drivers use feedback from a sense resistor to control the power going through the motor. A low ohm (0.25 or 0.1 ohms) resistor is connected between the common lead of the motor and ground. The end of the resistor connected to the motor will have a voltage on it, in relation to ground, that is proportional to the current through the resistor, and the motor. This voltage is compared to a reference voltage (often adjustable) that clears a latch or flip-flop which cuts the drive to the output transistors to stop the current through the motor. An oscillator then sets the latch turning the current through the motor on again, and the sense circuit shuts it off when it reaches the limit. The cycle repeats at the oscillator frequency.

The power supply must be able to deliver at least the "average" current through the motor(s). A little overhead doesn't hurt.

Steve