The most common appl icat ion for the NTC thermistor
is temperature measurement. Accurate temperature
measurement can easily be accomplished by interfacing a
Wheatstone Bridge, 6K/30K ohm thermistor network and a
digital voltmeter integrated circuit as illustrated in Figure 5.
The IC consist of an analog to digital converter with built-in
3-1/2 digit LCD driver providing resolution of 0.1°C. Using
the 6K/30K ohm thermistor network makes it possible to
achieve an overall system accuracy of ±0.4°C from 0°C to
100°C. This digital thermometer can easily be interfaced
with additional circuitry to provide a temperature control
circuit with a digital display.
Click here to download the full size of the above Circuit.
The advent of low cost micro controllers used with precision
interchangeable NTC thermistors, provides the design
engineer with unlimited design possibilities for temperature
measurement and cont rol systems. These systems
are relatively inexpensive to produce yet offer very high
temperature accuracy and var ious software cont rol led
For example, a micro controller system utilizing remote
thermistor sensors can monitor and control the temperature
in several locations in an office building. For this case, the
micro controller is comprised of a built-in microprocessor,
analog to digi tal conver ter, RAM and several digi tal
inputs/outputs. The complete system Figure 6 utilizes the
micro controller, multiplexer, EPROM, digital display, keypad
and display driver.
The micro controller is programmed in assembler language.
The temperature measurement is calculated within the micro
controller using the resistance versus temperature algorithm
and the a, b and c, constants for the specific thermistor
resistance and curve material. Refer to the Steinhart
Equation on page 5. An alternative method to convert the
thermistor resistance to temperature is to program a lookup” table in EPROM. After programming, the micro controller
tells the multiplexer to send back temperature data from a
particular zone (room in the office building) and converts the
resistance of the thermistor into a...
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