> I didn't respond to Russell's challenge because I thought he was 
> limiting
> the spec so much as to make thinking outside the box impossible.  To 
> get the
> right design, you have to ask questions about what the ultimate 
> purpose is,
> what the motor has to do, etc.  For example, is there maybe some 
> other
> higher level way to sense what the motor is doing?  Also, how often 
> is the
> motor direction reversed?  Where is the PWM coming from?  It sounds 
> like the
> motor is always full on or off each direction, but I thought there 
> was
> mention of PWM.

Just to clarify.

No PWM.
Motor is fully on or off.
It drives in either direction as required.
On for from ABOUT 1 second to 20 seconds.
Overlap between directions is processor controllable and usually long 
(30 seconds) so not a problem.

Any parts MIGHT be used but this is a desperately NIMBY situation.
I have already majorly changed a Taiwanese design and getting stuck 
into their H bridge will not make me friends.
Using identical components is a great plus. Whatever I use has to be 
testable in NZ and buildable easily in Taiwan. Parts that I am CERTAIN 
can be got in taiwan are not too bad (eg LM385). Specifying pats that 
SHOULD be easily available in Taiwan can lead to bad surprises. Using 
identical parts makes it certain.

Cost is a main priority but not the overwhelming one.

At present the motor speed is very very poor when battery is only half 
flat and rather fast when battery is new..
Using JUST an improved simple H bridge with no current limiting would 
make motor VERY fast and MAY lead to damage as this is an unknown 
condition.

Try in to get motor specs would be a lost cause.

If I was doing this from scratch I'd do it differently. But, as this 
is a real world existing design with a resistant implementer I want to 
try to keep it as apparently the same as possible - even though my cct 
so far is a radical redesign - even though it takes only a few extra 
resistors.

One point - with high supply voltages the lower driver dissipation 
needs to be checked during design as it can have several volts across 
it by design. On battery this is liable to be OK. If an AC adaptor is 
used (not mentioned in design challenge) the V+ at motor loads needs 
to be checked.





        RM



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