>     Actually, this circuit doesn't work well with logic levels. But maybe
I
> could use some pullup resistor and use a PIC output to drain current to
> ground the CONTROL-X inputs.
>
>     What are your thoughts about this circuit?

Note that your low side drivers are in the common emitter configuration.
This makes sense because ideally you want them to act as switches.  However,
the high side drivers are emitter followers which I don't like for this
purpose.  The high drive voltage is then limited to the PIC logic high
voltage minus the B-E drop.  This will cause the high side drivers to
dissapate about 3 times the power of the low side drivers.

I would use PNP high side drivers symmetrical to the NPN low side drivers.
This does mean that opposite corner transistors can't be driven directly by
the same signal because the base signals are inverted.  One way to deal with
this is to have an NPN emitter follower drive the base of each PNP high side
driver.  This also allows a higher voltage for VCC* than the PIC power
supply.  It will be limited by the voltage the 6 transistors can withstand,
not the by the 5V logic high level coming from the PIC.

By the way, make sure that the VCC* is not the same power rail feeding the
PIC because this motor driver will feed back noise onto its power input.  If
you configure the H drive as I suggested above, then VCC* could be the
unregulated supply, depending on what the rest of the circuit looks like.
The grounds need to be connected, but make sure there is only one conducting
path between the noisy and "clean" parts of the circuit.  Maybe that's what
you already meant with the "*" in VCC* and GND*.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

--
http://www.piclist.com#nomail Going offline? Don't AutoReply us!
email listserv@mitvma.mit.edu with SET PICList DIGEST in the body