In addition to what Olin said, sometimes it is easier to use a relay to
act as the switches to set the direction. When this is done a transistor
is put in series with the motor and relay circuit to do the PWM. This
transistor is easier to drive when the emitter or drain is grounded.


-- Rich


Olin Lathrop wrote:
>
> > When using PWM to control the speed of a motor that is controlled by an
> > H-bridge, what gets turned no-and-off?  I mean, do you send the PWM to the
> > transistors that control the direction, or do you send the PWM to a
> > transistor that is in series with the power supply?  Am I making any
> sense?
>
> For any one motor direction, you need one top and one bottom transistor on.
> Either one can be pulsed because they are in series.  However, the bottom
> transistors are usually a little easier to drive and may be faster.
> Typically, the top transistor is left on as long as the direction is not
> changed and the bottom transistor is pulsed by PWM.
>
> ********************************************************************
> Olin Lathrop, embedded systems consultant in Littleton Massachusetts
> (978) 742-9014, olin@embedinc.com, http://www.embedinc.com
>
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