Roland and all,

Okay, I don't feel too stupid now.  I was going to post
a question about separate power supplies, but then this
happened.  So, here's my question about power supply
separation:  Where is the separation?  What I mean is,
I understand using two areas of the board, but do I need
two wall-warts, two regulators, etc?  Do I have to separate
them back up to mains AC?

Yes, I chose the ULN2803A because it has the 2.7k resistors
on input; the datasheets say this is specifically for
5v TTL/CMOS connection, so I thought I was good on inputs.

Again, I'm using a 15V Zener to pin 10 (common) on the ULN2803
to prevent back EMF.  Is that sufficient?

Thanks again,

Gustaf

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Roland
> Sent: Friday, February 27, 2004 7:00 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE:] Release of magic smoke
>
>
> The 2803 has a 2K7 series resistor on the input, so that should be OK.
> Whats more likely happening is that flyback from the stepper coils is
> causing the 5V to spike, and the pic is doing the clamping!
>
> try and use separate supplies, as shown in the diagram, or adequately
> isolate the two regions, logic and power.
>
> Regards
> Roland
>
>
> At 02:45 AM 27/02/04 -0500, you 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
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> >
> Regards
> Roland Jollivet
>
>
> JeM Electric cc
> PO Box 1460
> Kloof
> 3640
> Kwazulu Natal
> South Africa
> Tel: +27 31 7024412
> Fax: +27 31 7011674
> Cell: +27 83 255 6017
> Email: enquiries@caon.co.za
>
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