Hi All.

I'm making a stepper motor controller to hopefully use on a set of automatic
gates (will also have a RF remote, beam to detect when i've driven past,
etc - sick of getting out in those rainy days!). I've got a 3V 6.7A unipolar
stepper which i'm driving with a PWM'd 50V to increase torque. The 50V is
turned on via the PIC (16F877) sending a set signal (pulse) to a flipflop
(4043) which turns on a BDW47 PNP BJT to the common of each winding
(seperate ckt for each winding). The current is measured through a 0R1 sense
resistor and when a comparator (LM358) senses the current above 6A it resets
the flipflop. This is repeated every 125uSec. The other terminals of the
windings are switched in regular fashion with BDW42 NPN BJT's and the rate
at which these signals change adjusts the stepper speed.

It all works well when i set the current limit for the resetting of the PWM
at a low level but as soon as i go above an amp or two i start getting large
oscillations which eventually destroy the transistors. I have flybacks from
the NPNs to ground. Placing them also from the PNP's to +50V seems to kill
the whole thing. Is there some fundamental flaw i'm making here? Does anyone
have any experience with this kind of stuff?

Any help would be greatly appreciated.

Regards, Kresho.
Sydney, Aust.

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