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Thermistors are used in a variety of temperature-sensing applications that are similar to the RTD because they change the amount of resistance as the temperature changes. This means that thermistors will mainly use Wheatstone bridge circuits to provide the signal conditioning necessary for use as a sensor signal. Notice from the examples in ill. 1 that the symbol for the thermistor is similar to the RTD in that a variable resistor symbol is used and the word thermistor or R, is used to indicate that the resistor is temperature sensitive. The first circuit example (ill. 1a) shows the thermistor used as a very accurate temperature sensor that is connected as one leg of the bridge. ill. 1b shows thermistors mounted in several locations that are all connected to one bridge circuit through a switch. The switch will make a connection to one thermistor at a time. ill. 1c shows the thermistor providing temperature compensation for a precision meter movement. The precision meter movement will not be accurate if the ambient temperature where the meter is located increases or decreases and the internal resistance is changed. If the meter movement is subjected to higher currents, it will also tend to heat up. The thermistor will change its resistance and automatically compensate for the temperature differences.
ill. 1d and ill. 1e show examples of two or more thermistors connected to the same bridge circuit. In this configuration, two thermistors can be located some distance apart to provide two separate temperature readings that can be used as a differential reading. e.g., one of the thermistors can be located at the inlet of a heat exchanger, and the other can be located at the outlet. The bridge circuit will show the temperature difference between the inlet and outlet. This type of configuration is also useful in air-conditioning system troubleshooting where the temperature differential of parts of the system such as expansion valves. evaporator coils, and condenser coils can be used to provide information for troubleshooting. ill. 1f shows a thermistor used as a temperature sensor for an alarm circuit, or it can provide on/off control for a heating element. The output signal from the bridge is sent to an amplifier that is connected to the coil of a relay. If the temperature causes the resistance of the thermistor to increase or decrease and make the bridge unbalance, current will flow to the amplifier and it will provide current to the relay. The contacts of the relay can be used to switch an alarm horn or light on or off if the circuit is used as an alarm. Or the contacts can switch the heating element on if the temperature is low, and off if the temperature is high. In this type of application the thermistor will act like a sensor and the variable resistor is used to adjust the alarm setpoint temperature. ill. 1g shows two thermistors used in a master/slave application. This type of circuit is used where two separate heating chambers or two separate liquid baths must be maintained at the same temperature. If the temperature of either chamber or bath becomes too hot or too cold, the bridge will become unbalanced and energize the output. The amplifier and relay part of this circuit are similar to the previous example.
Above: ill. 1 Seven interchangeable application
circuits for thermistors. (Click image above or here to
enlarge schematic).
