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Thermistor Applications

Thermistor Thermometers: A basic circuit of a thermistor for temperature thermometer measurement can be shown in Fig. 4. In this diagram a thermistor is connected in series with a voltage supply, meter, and variac. At room temperature a current will flow through the thermistor following ohms law. As the temperature is increased, the current through the meter will increase and by calibrating the current through the meter in terms of temperature, a direct reading can be made. The variac can be useful in establishing the protection and the calibration of the circuit.

The circuit in Fig. 4 can be modified as shown in Fig. 5 where the current passes through a resistor and the voltage drop across the resistor is monitored. This voltage drop can now be calibrated in terms of temperature.

For improving temperature measurement accuracy, a wheatstone bridge circuit can be adapted as shown in Fig. 6.

If R₂ and R₄ are chosen equal, then R₃ = R₁ or R₃ will read direct resistance of the thermistor when balancing of the bridge occurs. By substituting a decade resistance box in place of R₃ to establish greater accuracy, one can establish a sensitive thermometer circuit. Various other combinations or changes to the circuit in Fig. 6 can improve accuracy and sensitivity.

Liquid Level Indicators: Thermistors can work well as liquid level indicators or controls. Fig. 7 shows a thermistor intended to sense the liquid level in a tank. When the thermistor is positioned in air, the steady state current will keep the relay coil (C1) energized and the relay contact closed. When the liquid level rises and makes contact with the thermistor, it becomes cooler resulting in a rise in resistance. This increased resistance reduces the current through the coil, opening the relay contacts. This in turn activates an alarm or other relays in the valve circuit which will shut off the flow of liquid into the tank. As the liquid level drops in the tank, the current through the thermistor increases, again closing the relay and starting the cycle over again

Temperature Compensation: NTC thermistors are being used successfully to compensate temperature in a circuit where a positive temperature coefficient prevails. A typical example is the copper wire in the field coil of a meter. A natural error is introduced because the copper has a positive temperature coefficient. This will create an error as temperature rises by the increase in the resistance of the wire therefore reducing the meter deflection over the full scale meter movement. A simple correction for this kind of error is to match the positive temperature coefficient of the copper wire with the negative temperature coefficient of a properly selected thermistor. In this correction application, the thermistor would be shunted with a resistor whose combined temperature coefficient would produce a negative slope of the same magnitude as the positive slope of the coil. The curves in Fig. 8 show the slopes to be considered and the resultant combination.

Surge Suppession: There are situations that exist where the surge current generated by the initial applied voltage in a circuit will result in damage to the filaments of tubes or incandescent light bulbs. By the selection and insertion of a thermistor in series, a current delay will occur. As the voltage is applied, the high value of the thermistor will limit current flow. As the thermistor warms up, the current flow will increase to the point where the circuit will operate in a normal manner preserving the life of the filaments.

 Flow Measurement: The dissipation constant of thermistors is a valuable characteristic when the measurement requires the accurate monitoring of liquid or gas flow. The small size of the thermistor and its fast time response all contribute to its application when they are used as two separate legs of a wheatstone bridge. The thermistors are generally inserted into two separate sections to be monitored and are operated in a self heat mode. The bridge is balanced with the same environment in each section. As the environment changes in one section, the bridge will become unbalanced indicating a flow change which can be calibrated in terms of resistance

Other Thermistor Applications: Flowmeters,  Vacuum Gages, Motor Thermal Protection, Fire Alarms, TV deflection circuits, Voltage regulation, Amplifier gain control, Time delay circuits, Thermal switches,  and many others..

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