AVR-Chat

Not sure if this original query was answered but here is a
stab at it.

For micro-stepped stepper motors, the windings are usually
driven directly with a PWM switch (ofen, using an
H-bridge). The PWM is not open-loop as would be the case
with PWM from most micros. Instead, it is feedback
controlled, varying the PWM on-time according to the
current actually flowing vs the desired current. The
current sense resistor is used to measure the actual
winding current. 

If you are not microstepping, then its a simple matter of
windings all on or all off. You don't need to measure the
current. But, you do need to be careful about the operating
voltage and current of the windings because the current
will rise to the value set by the simple winding resistance
(and the power supply voltage).

If you ARE microstepping, in my opinion, you are far better
off using one of the integrated microstepping drivers
(Allegro and JRC come to mind). These take ANALOG voltages
in to set the current in each winding and those analog
voltages can be made by smoothing PWM outputs from the
micro.

Hope this isn't too confusing!

Jim Wagner

>  In AVR-Chat@yahoogroups.com, "dv7839"
> <danvernon@e...> wrote:
> > I hope this isn't too far off topic.  I was
> doing some research for 
> an 
> > AVR powered stepper motor driver design when I came
> across a 
> reference 
> > to a current sensing resistor.  Could someone
> please explain what 
> this 
> > is, and why one might use it?
> > 
> > thanks,
> > dan
> 
> 
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