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ELECTRONIC CONTROL FUNDAMENTALS
Another material used in RTD sensors is platinum. It is
linear in response and stable over time. In some applications
a short length of wire is used to provide a nominal resistance
of 100 ohms. However, with a low resistance value, the
temperature indication can be effected by element self heating
and sensor leadwire resistance. Additionally, due to the small
amount of resistance change of the element, additional
amplification must be used to increase the signal level.
To use the desirable characteristics of platinum and minimize
any offset, one manufacturing technique deposits a film of
platinum in a ladder pattern on an insulating base. A laser
trimming method (Fig. 4) then burns away a portion of the metal
to calibrate the sensor, providing a resistance of 1000 ohms at
23 C. This platinum film sensor provides a high resistance-to-
temperature relationship. With its high resistance, the sensor is
relatively immune to self-heating and sensor leadwire resistance
offsets. In addition, the sensor is an extremely low-mass device
and responds quickly to changes in temperature. RTD elements
of this type are common. Early thin film platinum RTDs drifted
due to their high surface-to-volume ratio which made them
sensitive to contamination. Improved packaging and film
isolation have eliminated these problems resulting in increased
use of platinum RTDs over wire wound and NTC thermistors.
LADDER NETWORK OF
METALLIC FILM RESISTOR
LASER TRIM (INDICATEDBY
GAPS IN LADDER NETWORK)
CONNECTION PADS
Fig. 4. Platinum Element RTD Sensor.
Solid-State Resistance Temperature Devices
Figure 5 shows examples of solid-state resistance temperature
sensors having negative and positive temperature coefficients.
Thermistors are negative temperature coefficient sensors
typically enclosed in very small cases (similar to a glass diode
or small transistor) and provide quick response. As the
temperature increases, the resistance of a thermistor decreases
(Fig. 6). Selection of a thermistor sensor must consider the
highly nonlinear temperature/resistance characteristic.
C3098
Positive temperature coefficient solid-state temperature
sensors may have relatively high resistance values at room
temperature. As the temperature increases, the resistance of the
sensor increases (Fig. 6). Some solid-state sensors have near
perfect linear characteristics over their usable temperature range.
124
POSITIVE RTD
THERMISTORS
Fig. 5. Solid-State Temperature Sensors.
80K
70K
60K
50K
40K
30K
20K
10K
30
40
50
60
70
80
20
0
10
TEMPERATURE (DEGREES)
20K OHM NTC THERMISTOR
-20
10
40
TEMPERATURE ( C)
POSITIVE RTD
Fig. 6. Resistance vs Temperature
Relationship for Solid-State Sensors.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
C3077
20K OHM AT
o
o
77
F (25
C)
o
90
100
110
F
o
30
40
C
70
100
M15131
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