Yes, I did read that, but here's where I got the Darlington idea,
as opposed to using the discretes:

http://www.doc.ic.ac.uk/~ih/doc/stepper/control2/connect.html

This Ian Harris fellow didn't use the resistors, and granted his circuit
was different, vastly different; he wasn't using a PIC for his
example.  I tried to "meld" the two circuits together.  I'm learning.

I have taken your advice for my next attempt at this thing. :)

Thanks,

Gustaf

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Scott Thomas
> Sent: Saturday, February 28, 2004 2:47 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] Release of magic smoke
>
>
> I would definitely put resistors in series with the Darlington inputs!
> Note the comment just below the schematic that Dakeng makes apologizing
> for any "magic smoke" because the resistors had been omitted on earlier
> schematics.
>
> -Scott
>
> Gustaf J. Barkstrom wrote:
>
> >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
> >
> >--
> >http://www.piclist.com hint: The list server can filter out subtopics
> >(like ads or off topics) for you. See http://www.piclist.com/#topics
> >
> >
> >
>
> --
>
> Thanks,
>
> Scott Thomas
>
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