Charles Jean wrote:
On Sat, 10 Mar 2007 17:11:29 -0800, Peter Bennett
wrote:

On Sun, 11 Mar 2007 00:24:31 GMT, Charles Jean
wrote:

I have a project that involves using a PIC to control 3 independent
arrays of LEDs. Each array requires a voltage of 12 volts and
consumes about 70 ma of current when operating. I found the enclosed
template schematic on the internet using a UNL2803A chip to interface
a PIC cotroller to various DC loads. It seems to fulfill my
requirements for the project at hand, but I thought it would be a good
idea to run it by the gurus out there first. Any comments,
criticisms, etc. would be appreciated.

I'm having a little trouble interpreting the data sheet on some
things. Pin 10 of chip is variously labled as common, Vcc, Vdd,
depending on whether I'm looking at the datasheet or the schematic. It
evidently supplies 12v to operate the chip. In the schematic(and my
project), the same 12v would also be used to power each of the loads.
But what if one or more of the loads happened to have a different
voltage requirement? Can the chip be operated in this mode? If so,
what is the operatinal voltage range of the chip itself and what kind
of load voltage ranges will it tolerate?

Pin 10 is the common cathode of a group of diodes which have their
anodes connected to each output. These can be used as the "catch"
diodes that are normally connected across a DC relay coil.

The ULN2803 and related parts are just a bunch of darlington
transistors with their cathodes all connected to pin 9, and collectors
to the respective output pins. Various versions of these parts have
different input circuits to adapt them for use with TTL, CMOS and
higher voltage logic sources.

Unlike normal logic chips, the ULN2803 family does not require any Vcc
supply to operate.
_____
Does that mean I can use different supply voltages for each load then?
If so, what voltage do I put on pin 10 of the ULN2803A?

Your schematic shows both LED and relay loads. The diodes
that all connect between the collectors and pin 10 really
have no practical use for the LED loads, since these produce
no voltage when their transistor switches off. They do have
a practical use for the relay loads, since these produce a
voltage that adds to their supply voltage, briefly, when
their transistor turns off. The diodes detour the inductive
current back to the positive supply, till the stored
magnetic field collapses. If you have several different
supply voltages feeding different loads, and some of the
loads are inductive, you could connect pin 10 to the most
positive voltage, so that none of the diodes can become
forward biased by any of the normal load swings, but can
still provide a path for the inductive current.