Page 461 - The Mechatronics Handbook

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Thermistors
Thermistors are bulk semiconductors made from an oxide of nickel, cobalt, manganese, or other metal.
The oxide is ground to a fine powder and then sintered to produce the actual thermistor material that
is then incorporated into a sensor. Thermistors are resistance temperature sensing devices with several
notable differences from RTDs such as their large negative temperature coefficients, and extreme nonlinear
response. The resistance of a thermistor is usually so large (several thousand ohms) that lead wire
resistance is rarely a concern. Thus, they are inevitably two-wire devices unless multiple thermistors or
components are included in the probe. There must be some means of electrical bonding between the
wire leads and the thermistor semiconductor. This bonding and the typical epoxy encapsulation places
a limit on the maximum usable temperature, even though the thermistor itself is a refractory material.
There are several schemes for dealing with the nonlinearity of thermistors, ranging from applying a
correction curve with a computer to having multiple thermistors with differing characteristics complete
with nonthermal resistors as a bridge within a single encapsulated probe. For moderate temperatures
spanning 200 K, a simple external bridge can be used to linearize the signal.
Although not normally called thermistors, germanium, silicon, and carbon are semiconductors that
can also be used to monitor temperatures by measuring their resistance. Germanium is used for very
precise measurements at cryogenic temperatures down to less than 1 K. The change in resistance can
be very large and very nonlinear, but still very repeatable with a typical unit going from 7000 Ω at
2 K to 6 Ω at 60 K (Doebelin, 1990). Silicon can be used at room temperature and, depending on its
doping, can have a very steep temperature curve. It is rarely used as a temperature sensor since other
methods work better over its useful range of –200°C to 200°C. Carbon resistors out of a parts drawer
found in any laboratory can be used for cryogenic measurements from 1 to 20 K, but they must be
individually calibrated.

Integrated Circuit Temperature Sensors
The base-to-emitter voltage drop of a transistor operating at a constant current is a simple function of
absolute temperature. Thus, any transistor can be used as a temperature sensor. In reality, this is much
more of a problem with building thermally stable electronics than a convenient means of measuring
temperature. Integrated circuits are available that monitor the collector current, amplify, and linearize
the base-to-emitter voltage to yield an output that is proportional to absolute temperature. Common
integrated circuit temperature sensors are available with outputs of 10 mV/K, or 1 µA/K. The temperature
range over which they may be used is limited to −50°C to 150°C by the construction techniques of
integrated circuits. This makes them very useful for referencing one junction of the thermocouple and
most ambient temperature measurements. Although not intrinsically water proof, the ICs are small metal
cans or plastic cases resembling signal transistors and can be potted or used in thermowells.
The IC sensors with a voltage output are commonly two terminal devices, with a possible optional lead
for trimming the response. When a small current of about 1 mA is allowed to pass through it, it will have
a voltage drop directly proportional to the absolute temperature (National, 2000). Even simpler IC trans-
ducers are available with separate excitation and signal leads. These are usually calibrated to 10 mV/°F or
°C. These have an inherent limitation of not being able to measure below a few degrees above 0°F or
0°C unless both positive and negative power supplies are available.
Voltage output ICs are very convenient where the temperature being monitored is local to the readout
and the voltage drop across the lead wires is not a concern, but for remote sensors, which require long
lines, current sensors are preferred. Current sensors are also two terminal devices that behave as high
impedance current sources so whatever lead resistance present may increase the voltage, but will not affect
the current through the sensor (Analog, 1997). Both types can be individually adjusted by trimming resis-
tances on the chip with a laser during manufacture to provide the rated output or they can have an external
adjustment lead. Even with trimming and calibration, the accuracy over the entire span from −50°C to
+150°C is rarely better than two or three degrees. Several individual ICs may be hooked up to give
minimum or average temperature. Voltage ICs are placed in parallel for minimum temperature and in

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