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Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
I made a circuit on a breadboard for a project:
A PIC controller with a specific program that does blink the LEDs right, is supposed to step the unipolar stepper motor through a ULN2003A.
So I wired a microcontroller with a 5V supply, connected it to the array and powered the array with the 12V and the connected the motor in sink mode.
The ULN is an inversive array so I wrote my program to hold pins up for the motors windings to be off.
When I connect the stepper motor, the indicator LED, connected to one of the output pins, stops blinking. The motor doesn't turn or it vibrates, depending on the step frequency I program in to my PIC.
What is going wrong with this circuit? The input resistance of the ULN2003A is 470K or something similar. There can't be an overcurrent. Is it the 12 volts comming into the PIC vs PIC sourcing 5V.
I tried stepping the motor by rubbing a wire with 5V against the pins of the ULN in a sequence and it did move.
Not sure why PIC can't do the same thing. ULN doesn't want to work with 5V.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
you have probably exceeded the maximum step rate of your stepper motor. Try running it at half your current step rate. also, try having a 5-10step ramp up stage. if you just pulse a stationary rotor at full speed without ramp up, sometimes it will just slip.
to get fast response and rpm, real stepper drivers feed the motor with PWM at some 10s of volts, not 12V square waves.
Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
in the era of parallel ports, the steppers were stepped with square waves
Why an LED connected to the same pin of the microcontroller as the input for the array stops blinking as I connect the stepper to the output pins of the array?
The stepper may be skipping. But I changed the program so it steps really slowly.
I think the problem is in how the pic sinks/sources the ULN2003A. Not sure, where.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
The ULN is inverting logically, but since the stepper is wired with positive common, you need to pull the ULN inputs high to power the respective stepper winding. Only one ULN input should be high at a time, in other words (unless you're doing microstepping).
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The inductive kick from the stepper windings on turn-off may be crashing your PIC. Do you have some sort of snubber network to absorb it? I don't know what is usually used for this, but there must be something.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Do you have some sort of snubber network to absorb it? I don't know what is usually used for this, but there must be something.
The ULN device has a diode from each output, all paralleled to to a clamp pin, for diverting inductive kickback. IIRC, the darlington outputs are rated to 55v, so the clamp rail can go to anything below this. A sufficnetly large electrolytic and a 50v zener in parallel to ground for instance. With its input resistors, inductive clamps, and assuming the PIC and load share ground only, with sensible avoidance of ground loops (!!), the ULN device makes quite a good interface between sensitive stuff and coils.
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PIC16F628+ULN2003A+unipolar stepper motor with a 400 ohm winding.
