Re: Question re: about current control schematic

Jon- thanks for the additional information and clarification. It's
nice to have a comment from someone authoritative like yourself.

Grant

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:
>
>
> grantfair2001 wrote:
>
> >An article about current control in steppers by Suded Emanuel at:
> >
> >http://www.eio.com/crntctrl.htm
> >
> >has a schematic. When I asked on the Nuts and Volts Forum about using
> >a mosfet instead of the Darlington, I got this reply:
> >
> >"Wow, that schematic appears to have multiple errors. At least two at
> >first glance. It looks like they are trying to do a classic buck
> >regulator. The base of the BDW84 is grounded, which means the PWM chip
> >can't do anything to affect it.
> >
> Yup, the line should be broken. The diode should be grounded, and not
> connected to the base. But, the way they have it drawn, the
transistor is
> STILL hooked up wrong. It looks like they have the emitter and
collector
> interchanged!
>
> > D1 should be grounded, but not the
> >transistor base. Also, looks like the BDW84 is in backwards (collector
> >and emitter reversed). So, be aware of that in any case."
> >
> >I don't know enough about electronics to know if this is true. Any
> >comment from the electronically unimpaired appreciated.
> >
> >
> But, finally, this is a HORRIBLE circuit! It runs the motors as
> unipolar, and makes no provisions for individual control of the
> current in each phase! It will have HORRIBLE performance at
> higher speeds, and will likely blow T1-T4 with any motor larger
> than the ones in floppy drives, since no provision is made at all
> for the inductive energy in the windings. I'd drop it and look for
> a better circuit right away.
>
> Jon