On Mon, 7 Feb 2000 10:50:56 -0200 "Cassiano J. Pereira"
<cassiano@EXATAS.UNISINOS.BR> writes:
> Hi all!
>
> How can I to build an ohmimeter using a Pic16c711?
> If somebody to know of something, answers me.
> Thanks!

Andrew T Kelley [k_andrew@JUNO.COM] replied:

That is easy.

5V
|                   |
|___[res]__| ADC Pin
            |

Read the adc pin.
then calculate the voltage drop.
Like this:
    drop = 5 - adcvalue;
Then figure out what value resistor drops x amount of voltage.




IT may NOT WORK...




Hope this helps,
Andrew

I can be a little more definate.  It won't work!  The input impedance of the adc is pretty high and worse, is not precisley defined, so unless the the resistor you are measuring has a very high value, you will only drop a very small amount of voltage.  Remember a resistor will only drop voltage when a current is flowing through it.

The normal way to measure a resistor is to put a known current through it and measure the voltage drop.  You extend the range of an instrument, you would have several ranges that would put different amounts of current through the resistor.  You simply apply ohms law R=V/I.

The tricky(er) bit is making a precision current source which isn't my speciality, but I'm sure someone on the list will oblige.

Mike