I have a 3.15v stepper motor, and a ULN2003A Darlington Array. The motor is rated as 1 amp per phase at 3.15 Ohms per phase (Sanyo Denki 103H546-0440).
1st question: Am I right in thinking that although it's a 3.15v stepper motor and the arduino gives 5 volts (which could in theory be dropped down to 3.15v) I have to power the stepper motor separately due to the 1 amp rating (I don't pretend to understand why) and therefore I have go through the Darlington - rather than just use the OUTPUT pins direct?
2nd question: Am I right in thinking that using the arduino to give 5 volt high and lows to 4 of the input pins on the Darlington, I can then use those values on the equivalent outputs of the Darlington to excite the 4 coils in order?
3rd question: Can I then just put 3 volts from ordinary batteries down the two power cords of the stepper motor - without any resistor etc?
The problem is that the 2003 Darlington array can only handle 500mA pr. channel. But if you check the datasheet i think you will find that you can parallel two cahnnels to get higher amp rating. But even then you are running the 2003 array at it's absolute maximum which might not be good for extended perios of time. You should consider finding a array with higher current handling capabillity, or to use 4 darlington transistors from the TIP series that can handle 1 amp each. I cant remember the curent rating , but maybe teh TIP 120 can do it.
If you use ordinary batteries to run the motor they will run out of power rather quickly.
Even if it is rated at 3.15V, stepper are usually powered with much higher voltage, a few times their rated voltage. My 6V stepper is powered at 18V. The commercial driver I am using has a current chopper. I think the best for you would be to use L293 in bipolar (has usually steppers have better torque with bipolar). If it doesn't requires torque then the ULN2003 with paralleled outputs (so 4*2 = 8 you'll need 2 ULN2003 or 1 ULN2803) and unipolar will work, just be sure to check all the operating conditions of your motor (to be sure you will never miss a step).
Anyhow I found that using homemade solutions with steppers instead of a commercial driver, was always disappointing. It requires a lot of science to have these kind of motors running smooth. Fortunately it doesn't if it is just for making them turn with "very" light loads.
Mikmo and Jerome - a very big thank you. Got it working with two ULN2003As in parallel. What a joy to see that blighter step! Thanks, Ben I shall modify/improve in time.
I shall modify/improve in time.