Peter,

I am not sure I completely understand, but I think that separating
the power supplies is simpler.  After that, I will connect the ULN common
pin to V+ for the motors without the zener.  I will rely on the internal
kickback protection diodes.  I will also put a 1k resistor between
the output pins of the PIC and the inputs of the ULN chip.

Gustaf

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Peter L. Peres
> Sent: Saturday, February 28, 2004 12:01 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE:] Release of magic smoke
>
>
> You likely rised the Vdd supply by the kickback pulses from the stepper
> phases, through the kickback protection diodes in the ULN2003. Then the
> PIC latched up and started boiling water. You have to provide a path for
> the rectified kickback voltage/current to ground. In your case a simple
> zener across Vdd and GND would not have been enough. You would have used
> something like a dump resistor between the ULN2003 diode common and ground
> (and not connect the diode common to Vdd).
>
> Peter
>
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