I'm driving a stepper rated at 5V/1.7A using an A4988-based driver circuit with a 12V power supply. An Arduino controls the stepping sequence.
According to the datasheet for my A4988, I should be calculating Vref as:
Imax = Vref / (8 * Rs) so,
Vref = I * (8 x Rs) (where Rs is 0.1 ohm on this particular board and confirmed with meter).
So, Vref should be (1.7A x .8) or 1.36V to drive 1.7amps.
The problem is, I cannot adjust the A4988 to get Vref up that high.
With the adjustment pot full swing, the best I can get is .9 to 1V. When I energize and hold the motor and use an amp meter, the coil is drawing about .9 amps -- which is still less than the 70% of 1.7 Amps I would like (using full-step mode).
Is Vref potential limited by the input voltage of 12 volts? Is there something else I'm missing?
Thanks in advance.