Joe,

Oops, I forgot to mention this in my post.  Yes, I used a 15v zener
diode "pointing towards" the common pin (pin 10) of the darlington
to spike-protect from back EMF.  Is that sufficient and/or appropriate?

By the way, "pointing towards" is what a mechanical guy like me says
because I can't remember which is anode and cathode (ugh!).  I'll beat
that into my brain before I'm done. :-)

My new design separates the power supplies, but see my
earlier post today about how/where to separate them, as I'm not sure
where to do that.

Thanks,

Gustaf

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Joe McCauley
> Sent: Friday, February 27, 2004 4:00 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] Release of magic smoke
>
>
> Have you tied pin 10 (the output diode common ) to the supply voltage
> for the motors? If not then maybe back emf from the motor coils have
> killed the PIC.
>
> Joe
>
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Gustaf J. Barkstrom
> Sent: 27 February 2004 07:45
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE:] Release of magic smoke
>
>
> One of my 16F84A's just died after some high duty-cycle running of
> stepper motors.  The circuit I built is basically the one found at the
> bottom of this page: http://www.dakeng.com/u2.html Source code is also
> available on that page.
>
> However, I buffered the inputs of the PIC with a 74LS244 (octal line
> driver), and used a Darlington array (ULN2803) on the outputs to drive
> the steppers instead of the discretes shown in the schematic.  Note also
> that I am not using a resistor between the PIC output pins and the
> Darlington array input pins.
>
> An ASCII block diagram of my circuit:
>
> [parallel port data pin] -> [74LS244] -> [PIC] -> [ULN2803] -> [Stepper]
>
> The entire circuit, including motors, is using the same 5+/GND regulated
> power supply. PIC is RC clocked, and the power/gnd pins are not bypassed
> on either the PIC or the line buffer/drivers.  Pin 3 of the PIC floats,
> as the page linked above says "leave this pin floating if you like."
>
> I ran two steppers (Vexta 2-phase, 5V/1A nominal) on this circuit for
> several minutes, but no more than 30 seconds at a shot.  So, I got brave
> and told my software to run the motors over a long distance, and they
> ran continuously for about 4 minutes. After that, the chip froze (motors
> stopped); held MCLR low and released it high again, still nothing.  I
> touched the top of the Darlington arrays and the buffer/drivers, barely
> warm.  I touched the PIC and about got a 2nd degree burn.
>
> I turned everything off, went to bed, and turned it back on the next
> day. Normally, the motors lock into an initial position at power-up, but
> now no power is going to either of them.  Before I troubleshoot further,
> I would like to get some input on the flaws of my circuit.  PCB art and
> schematics available at your request, if the flaws of the circuit are
> not already blatently obvious...
>
> I think (in a sophomoric way) that my question is: why is so much
> current going through the PIC?
>
> By the way, the circuit worked very well until I turned up the
> duty-cycle.
>
> Thanks,
>
> Gustaf
>
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