Thermistor and Its Applications Report

Introduction

Electrical resistance is a measure of opposition of flow of electric current within a conductor. Devices which have been designed to specifically offer this ability with electrical importance are called resistors.

There are many types of resistors which are classified as either fixed resistors or variable resistors. The values of fixed resistors do not change, thus are fixed. However, variable resistors change based on their control specifications. They are also called potential meters and they include Thermistors, rheostats and varistors.

This paper examines the working mechanism and applications of Thermistors

Thermistor: Principle of Operation

Thermistors are special kind of variable resistors whose resistance changes with changing temperature i.e. a change in thermal temperature results in changes in its resistance value (Fraden 532). This change can be interpreted or converted to electrical signals. This is the basic principle of operation of all Thermistor

Thermistors are classified as electronic temperature sensors because they detect temperature changes within it application range or environment, and sends required signals within a designate circuitry. Their typical temperature values ranges from -40 to 200 ⁰C, and operational resistance in kW range (Fraden 532).

The major specifications to be considered when using a Thermistor include its resistance temperature curve, nominal resistance value, resistance tolerance and the better tolerance. Assumption to any of these factors may result in damaging a Thermistor.

The Thermistor equations below show some of the relations that may influence Thermistor’s principle of operation:

R = R₀ exp β (1/T-1/T₀) and P=C (T₂-T₁),

Where T, T₀ are ambient temperatures; R, R₀ are corresponding resistances and β is a constant, and for powered dissipation P, C is the thermal dissipation constant (Padmanabhan 426).

Types of Thermistor

Thermistors can either be negative temperature coefficient, NTC or positive temperature coefficient, PTC. For NTC Thermistor, resistance decreases proportionally with temperature while PTC Thermistor, resistance increase proportionally with temperature.

Most NTC Thermistors are made from metal oxides. The oxides are made to powder and compressed in the presence of heat. However, some NTC Thermistors are simply crystallized from semiconductors. PTC Thermistors are manufactured by introduction of minute amounts of semiconductors into a polycrystalline ceramic (Fraden 539).

Applications of Thermistor

Relay circuits

The PTC Thermistors are used regulating temperatures of digitally controlled oven. The principle of operation is relies on the ability of a Thermistor to convert thermal signals to electrical signals, and comparator circuitry to compare compared voltages (Padmanabhan 219).

The resistance of a Thermistor will increase with an increase in the Ovens temperature. At a certain critical temperature, the comparator will be unable to produce a signal that to maintain the on state of a circuit. Thus the secondary circuit for heating goes automatically off.

Battery packs

Some batteries can charge under all universal natural temperatures while others cannot. This ought not to be a problem of the user final consumer. Further, Rechargeable batteries must be controlled and protected during the charging period. Therefore manufacturers have installed smart designing circuits that operate under NTC Thermistor.

NTC Thermistor are employed in used to maintain an ambient temperature range for optimum working of charging systems or for quick charging of a battery. During the charging cycle , 5 to 10 seconds, the NTC Thermistor measures the battery’s can detect changes in the rise of battery’s temperature and can be protected from abnormal charging. To detect the remaining charge during discharge, the NTC performs temperature compensation for voltage arithmetic.

Thermal fans

In this type of setup, the Thermistor detects when the temperature goes above a certain preset limit. Above that critical temperature the fan goes on and runs until the threshold is reached, then it goes off.

The circuit involves usage of TRIAC as a switch. The TRIAC turns on only when the gate voltage is greater than the internal trigger current. The TRIAC will continue to conduct as long as the internal trigger current remains. The mains voltage fans are run on AC

Other application

Other applications include flow meter, TV deflection circuits, time delays circuits and vacuum gages.

Works Cited

Fraden, Jacob. Handbook of Modern Sensors: Physics, Designs, and Applications. London: Springer, 2010. print.

Padmanabhan, Tattamangalam. Industrial Instrumentation: Principles and Design. London: Springer, 2000. print.