Iíve looked everywhere but canít find a practical chopper circuit to limit the current in straightforward unipolar drive circuit.
Iím using 8 discrete transistors (BD912) to drive a rated 4v 2.2A stepper on the x-axis and rated 4v 1.1A stepper on the y-axis of my first CNC machine (Iím new to this hobby!). Iím going to run these off a 24v supply, but obviously need to limit the current to the rated values, and rather than a wasteful current limiting resistor, a simple chopper circuit built entirely in hardware is the way to go. I donít want to have to buy a stepper driver system from someone else, nor do I want to use an interpolator IC and driver combination with Vsense lines for chopping.
My present driver (assembled onto veroboard) uses four lines from a PCís parallel port for each motor, individually controlling each coil of the motor (software can full and half-step). Iíve tested this with low voltages across the steppers and it works happily, so I just need to add the current limiting chopper circuit. Thereís already a NOT gate on each of the lines from the PC (active low) to the drive transistors (active high) so I figured Iíd be changing this for a NOR gate to allow the transistor to be turned on and off using the presence of a logic signal coming from a comparator (I had the LM358/LM324 in mind configured off a single+5v supply) which simply monitors the voltage across the motor coil and compares with a reference voltage.
Iím not sure where to go from here though! Attached is a schematic Iíve drawn up in Ďcrocodile clipsí showing the conceptual circuit I have in mind for each channel of the unipolar drive (since Iíll be half stepping, thereíll need to be a current sensing resistor on each channel). Has anyone got details of a successful practical circuit along these lines? (Iím sure thereís lots more needed than just this, particularly in terms of the comparator to TTL interfacing).
Use of oscillators or PWM seems a little overkill, but maybe a simple time-decaying RC timing circuit or op-amp feedback configuration to give some hysteresis would improve performance?
Any ideas and comments would be much appreciated!
Last edited by fairorgan; 01-20-2007 at 10:38 AM.
Look at this thread http://www.cnczone.com/forums/showthread.php?t=25361 (unipolar motor microstepping)
There are two ways to implement a chopper: fixed frequency, and fixed Off time. We implemented fixed fequency.