PWM control of unipolar stepper motor?

mike56

I want to control a stepper motor that's rated at 3.6V @ 200mA using a ULN2803A.· However, I only have a 24V supply voltage.· Can I just drive each coil at 15% duty cycle PWM instead of using a different power supply or a power wasting resistor?· The parallax propeller makes this much easier than other chips.

If I have 4 pins then I would:
-PWM 15% pin 1, all other pins 0% pwm
-delay 1ms
-PWM 15% pin 2, all other pins 0% pwm
-delay 1ms
-PWM 15% pin 3, all other pins 0% pwm
-delay 1ms
-PWM 15% pin 4, all other pins 0% pwm
-delay 1ms
repeat

Correct?

T Chap

Steppers are usually rated low compared to what you actually drive them with (voltage) in practice. The thing that must be done is to limit the current to the motors rating, using some method of chopping. There are many examples on line, as well as many IC's that will manage the current regulation for you with only a trip pot or fixed set of resistors to set the current. You would want to stay away from resistors attempting to limit current.

Toby Seckshund

To maintain a good speed of current rise, and therefore keep up the stepping rates, a higher voltage through a "resistance" will be required. A constant current should be even better. The current supplied to each winding should be individually regulated, but usually a common feed is used which·affect the pull in torque.

The old teleprinters coils would pull over at 12v but to get operating speed 80+ volts through a resistor was used.

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