Provided feedback appreciated.

>> 2. Why did you make it current limiting? It also limits your lowest
>> supply voltage.

It does. I wanted some sort of control (see below) and I desparately 
wanted to use available parts if possible and something that "looked 
similar" to the original design if possible. As I have said, there is 
a VERY strong NIMBY factor present here. Also a very strong 
parts-availability factor. These aspects cannot, alas, be discounted. 
If I was doing this with only engineering constraints I would probably 
do it quite dofferently. I don't mind adding glue parts within reason 
if theya re cheap, relatively compact and certain to be available.


> I agree with Bob here.  I know you put current limiting in the spec, 
> but I
> would look for other ways to achieve the same thing, probably at a 
> higher
> level.

I do not have ready access to the "higher level" or any control of 
what others will do. It should ideally plug in in place of the 
existing circuit. This is commercial reality in this instance. There 
are language, geographical, cultural, pride and (now) time 
constraints.

> The schematic I posted was current limited as per spec, but I would
> rather just get rid of the emitter resistors and put base resistors 
> in
> instead.

Current limiting with base resistors is, as you noted, affectred by 
transistor beta and can vary over a wide range. A 2:1 variation would 
not be unusual.

> I have revised my schematic (attached) to loose the current 
> limiting.  This
> is about a simple as it gets, with only 4 transistors.

The circuit looks good in an ideal world, but wont work as it is in 
this application. I'd really like it to be suitable!
I looked at only 4 transistors but there are drive issues. (Life 
usually fails to be simple ;-) ).
The 360r input would take about 10+ mA drive which is not available. 
You need significant drive inles syou can GUARANTEE the Beta of the 
driven transistors to be high at required load.

Your circuit would be potentially OK with FETs as the bottom drivers.

Driving the top transistors off the collectors of the bottom drivers 
(which I also found attractive as it simplified the high side drive 
requirement) produced oscillation in this application when I tried it. 
Not insoluble I'm sure, but one more issue to address.

> Only one of the
> inputs at R5 and R6 may be driven high at a time.  This H bridge is 
> desinged
> to supply a minimum of 300mA to the load with a minimum supply of 
> 3V.  The
> transistors are assumed to have a minimum gain of 50, which is quite
> reasonable for this voltage and current range.  Power dissipation in 
> the
> pass elements is minimal since they are always run in saturation 
> when on.
> This also means minimal headroom is used by the H bridge and most of 
> the
> voltage goes to the load.  The control inputs are meant to be driven
> directly from PIC pins with the PIC supply being 3-5V.

I agree that the circuit is excellent for a certain range of tasks. In 
this case an extra issue is that the motor sped varies very very 
widely across the voltage range if no limiting is provided, which is a 
reason for the current limiting. At lowest battery the motor is 
extremely sluggish and may stall. At full battery or, worse, if an AC 
adaptor is used, the motor revs alarminly high. I have no specs for 
the motor, do not know what is safe, don't know the effect on the 
plastic gears etc. And getting that data is very very close to 
impossible. Being able to restrict motor speed in some way is at least 
prudent and possibly essential. Voltage control may be better.

Based on all of the above, how about this?
Diagram may follow.

Top drivers bipolar driven off opposite low driver collectors as per 
Olin etc.
Bottom drivers are FETs.
Two extra transistors connect across FETs gate/source to shunt drive.
These are driven by zeners off own side FET drain =~ motor voltage.
Bottom driver is effectively a shunt voltage regulator with the motor 
as the series pass element.
Top drivers are on/off

Less than 100% on my requirement wrt needing new parts (FETs) but 
otherwise very standard.



                RM




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