Homebrew PCBs

Robert Hedan wrote:
> Problem is now I have to reconsider a totally new driver circuit 'cause
> these are bipolar steppers.  Can I modify my schematic without TOO much
> trouble?  Or am I better using a new design?

The discrete driver contained in "stepper_driver_mod.JPG"
can be modified for bipolar drive by creating a totem
pole structure of an upper PNP and lower NPN device.
E of PNP to V+, C of PNP + C of NPN becomes the drive
output, E of NPN to GND.  You will need a means to
drive B of the PNP devices opposite polarity w/r/t
the NPN devices.

One of these structures will be needed for each of the
four leads of the bipolar stepper resulting in a
total of 8 driver devices.  Do you actually intend to
drive 28V rated steppers off this design or are you
intending the higher voltage to enable faster stepping
rates?  If the latter you will need some means of
current control to avoid overheating the windings.

Concerning the transistor base drive you might want
to check the gain (hfe) available and verify if the
optocouplers and added series resistance results in
sufficient base drive current.

Though the circuitry is minimal from a gate count
perspective, I'd prefer to see a single uC package
displace the 2 SSI packages and buy the design
flexibility of control software.  IMHO I'd toss
the steppers along with half of the required driver
transistors, constant current source, add the
positional feedback and DC motor, and create if you
will a "digital motor".  You are going to need the
the control software to run somewhere and pushing
it to a dedicated uC for each drive motor can
greatly simplify the implementation.

-- 
uhmgawa@...        www.gnu.org