Use an LM317 (or LM780x) in constant current mode.
See the datasheet for specifics on how this is done,
but basically, connect a resistor from the common terminal
of the 317 (780x) to the output terminal. =20
And from the output terminal through your resistor string to gnd.
You now have a constant current through your resistor string.
LM317 or 780x regulators are less than a dollar almost anywhere.


Regards,

Jim

> -------- Original Message --------
> Subject: Re: [PIC] Reading DIP switch with A/D
> From: Dwayne Reid <dwayner@planet.eon.net>
> Date: Tue, September 28, 2010 11:28 am
> To: "Microcontroller discussion list - Public." <piclist@mit.edu>
>=20
>=20
> At 07:03 PM 9/27/2010, RussellMc wrote:
>=20
> >Using a constant current source gives even better results - not needed
> >with only 4 switches.
>=20
> The problems (several) with using constant-current source (or sink)=20
> is: extra component cost and accuracy.
>=20
> Its HARD to make a very low cost constant current source that is=20
> reasonably accurate.
>=20
> You can still purchase constant current diodes but they are expensive=20
> and need significant supply voltage (at least 3V higher than the=20
> highest A/D input voltage used).
>=20
> You can use the old 2-transistor constant current source but its=20
> current varies with temperature.
>=20
> You can use an op-amp and a transistor - works well but uses a TON of=20
> components.
>=20
> Unless you can suggest something I haven't mentioned.
>=20
> Don't get me wrong - I think that reading a string of resistors with=20
> a constant-current source is the ideal way to tackle this=20
> problem.  Its just that the cure seems to be way worse than the=20
> original problem.
>=20
> dwayne
>=20
> --=20
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
> www.trinity-electronics.com
> Custom Electronics Design and Manufacturing
>=20
> --=20
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist


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