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
>
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>
>

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Thanks,

Scott Thomas

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