At 02:45 AM 2/27/2004 -0500, you wrote:

>I think (in a sophomoric way) that my question is: why is so much current
>going through the PIC?

It's called latch-up and results from current going through the on-chip
protection diodes (too much current) which triggers the GIANT PARASITIC
SCR which lives inside every PIC, right across the power supply. This
is essentially a self-destruct if your power supply has enough juice. The
SCR triggering itself is non-destructive, but the resulting current causes
enough heating to damage or destroy the chip.

>By the way, the circuit worked very well until I turned up the duty-cycle.

It was probably marginal.

I suspect your problem is one of circuit layout. You must not allow
motor current to flow between the ground pin on the darlington array
and the Vss pin on the PIC. IOW if you think of a "star" connection
at the ground of the power supply, one point goes off to the PIC
and one goes off to the ULNxxxx ground. You do NOT ground the PIC
to the supply and run the connection from there to the ULNxxx (although
the opposite would be okay). There should be a BFC (electrolytic)
paralleled with a 0.1 ~ 1uF ceramic close to the PIC and the motor
from ground to Vdd/5V. 1000uF 6.3V low impedance (like you
find on a PC motherboard) works fine.

BTW, you might want to put a 1K on each PIC output. Yes, there's a
resistor inside the ULNxxx, but trust me. There's also an isolation
junction diode to the substrate...

The zener on the catch diodes is just fine, provided it doesn't get too warm.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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