16F870A, ULN2803, Stepper motor = very low torque, any problems with code?

[HenrikOlsson]

Hi,
A unipolar step motor with a 12V winding running on a 12V supply won't give you much torque.
Usually, with steppers, you want to use a voltage much much higher than the windings rated voltage and then actively limit the current to the windings rated current. Decades ago when unipolar motors was "common" it was usually done by inserting BIG power resistors in series with the winding. Today it's done by sensing and regulating - much more effective.

I think you'll see that the torque is much higher when the motor is stationary compared to when it's turning. A motor with a high inductance winding may have an impressive holding torque for a given current but the running torque falls off much more rapidly compared to motor with a lower inductance/higher current winding. The cure for it is more voltage, much more voltage,

Do you have any data on the motor?

/Henrik.