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Olin repiles:
> Bob Blick wrote:
>> OK, how about this one I posted long ago? Since the design you
>> posted had no freewheel diodes, I assume you are not doing high
>> speed PWM, so this design also has no diodes. Use jellybean parts.
>
> I like the basic topology.  Note however that the two inputs can't be
> raised
> all the way to the same supply driving the H bridge.  If they were, the
> top
> transistor wouldn't come on very far, if at all for practical purposes.
>
> The input transistor base is a the + rail.  Its emitter is one B-E drop
> lower.  However it's collector also needs to be one B-E drop lower for the
> top transistor to turn on.  There is at least one saturated C-E drop
> missing
> in the top transistor drive circuit.  Chances are the top transistor will
> need a little higher B-E drop due to its larger current.
>
> The solution is easy and only costs two extra resistors.  Put the existing
> resistor in series with the input transistor collector, not emitter.  Then
> add a base resistor for the input transistor.

Let me explain a better way, please bear with me. The circuit I posted had
a resistor in the emitter of the input transistor. This gives you a fairly
constant current drive to the output transistors, and that's pretty good.
The drawback is that if the logic input is as high as the motor supply,
the input transistor loses the use of it's collector. This isn't a problem
when the logic supply is the same as motor supply, because if you're
driving it with a PIC, the logic voltage only takes a very small amount of
loading to drop a quarter of a volt or more. But when the motor supply can
get lower, the solution is to add a resistor in series with the base of
the input transistor. That way if the motor power supply goes lower than
logic voltage, as input current increases, the input resistor drops more
voltage, keeping the input transistor's base voltage below the motor
supply voltage. Just make sure the input resistor value is greater than
the emitter resistor value. I would suggest 100 ohms for the emitter and
2200 for the input.

This makes the circuit almost perfect(you still get the nice constant
current drive) and you've only added one(well, two) resistor.

Attached is the schematic.

Cheerful regards,

Bob

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