Transcript Transducers/Sensors: Sample Device Thermistor

Sample Device
Thermistor
Transducers/Sensors:
• A thermistor is a temperature-sensing element
composed of semiconductor material (typically a
mix of metal oxides) that exhibits a large change
in resistance in response to a small change in
temperature
• Thermistors usually have negative temperature
coefficients, the resistance of the thermistor
decreases as the temperature increases
(Positive temperature coefficient thermistors also exist,
they make good current limiting devices)
Sample Device
Thermistor
Transducers/Sensors:
• Accuracy: Thermistors are one of the
most accurate types of temperature
sensors. Typical accuracy of ±0.2°C.
• Range: Thermistors are fairly limited in
their temperature range, working only over
a nominal range of 0°C to 100°C .
• Stability: Thermistors are chemically
stable and not significantly affected by
aging.
Sample Device
Thermistor
Transducers/Sensors:
Sample Device
Thermistor
Transducers/Sensors:
• Linearity: Thermistors are highly non-linear,
following a law called the Steinhart-Hart
equation
A, B and C are constants
found experimentally by
measuring R at three
different temps and
substituting back in the
equation
Graphs or tables of RvsT
are usually used
Sample Device
Thermistor
Transducers/Sensors:
Typical Thermistors
Sample Device
Thermistor
Transducers/Sensors:
• Using a Thermistor to
sense/measure
temperature 1:
If the thermistor is part of a
potential divider then the
output signal will vary
with temperature.
This simple circuit lacks
precision, it’s often used
in a set-point switch
Sample Device
Thermistor
Transducers/Sensors:
Using a Thermistor to
sense/measure temperature 2:
The thermistor is one of the
resistors in a Wheatstone Bridge
– say R4.
The output voltage is now
related to temperature and will
vary as the temperature varies.
Calibration will enable
temperature to be measured.