Page 296 - Making PIC Microcontroller Instruments and Controllers

PROJECT ? 29]Z

The thermistor I used had the following properties:

Resistance at 707 1000 ohms

Detta 25 ohms per degree Fahrenheit in positive direction
Inw tempemture ofinterest 327
High tempeRture ofinterest 300"F

Under ideal conditions, the bridge we design would provide a signal close to 0 volts
at 32' and close to 5 volts at 300T. The network would have an overali (lower) resist-
ance of about 5k ohms so we don't overload the power supply. A simple network is not
going to be able to do this, but a usable netwo* is possible.
From the prece.dhg data we can calculate that the ihe.misior has about 50 ohms
resistance at 32"F and about 6750 ohms at 300T. If we put a 5000 ohms resistance in
seies with it, the thermistor can be connected to the PIC, as shown in Figure 21.3 The
analog signal value we will read on the A-to-D conversion will be ftom 146 to 252.

=
(50/5050)*2ss 2.s2 255-3=252 at32'F
=
and 6'75l/ll'15cD*255 146.49 255-146=109 at 30O'F

SIGNAL lo PIC
3K to 5K SO TOTAL
RESISTANCEWITH
IS
THER[4ISTEB
5K TO 1OK

ffUre Clrcultry for a thermislor connoc-
iion lo Plc. (Thermislor resistance can increass or
decrease with temoerature,)