I'm currently playing with small uni-polar steppers. 12v taken from 5.25 floppy drives and other small steppers from printers and hard drives. Really just trying to learn about it, not build the ultimate machine. At this time.
I have a 27v transformer that I wish to power the 12v stepper with. Therefore I'm learning choppers and PWM with my pics and stand-alone comparators and such.
Here are my questions/reasonings:
1. I measure about 68ohms across my unipolar, that is double coils for more torque. Therefore I could calculate the desired voltage between the ground side of the motor and a current sense resistor using the voltage divider formula?
2. I can also just run 12v across two coils of the stepper and measure the voltage at the bottom side before a current sense resistor.
3. Using the voltage measured at #1 or #2 I can set my comparator to turn off when it exceeds that voltage, i.e. 27v is applied to the stepper and it rises beyond the 12v value.
4. What is a good value for a current sensing resistor? Can I get away with only 1 ohm or is that too small?
5. Can I take my voltage measurement after the coil selecting transistors and still get an ok result?
Thanks in advance.