Thermistors

PowerPoint Presentation on Thermistors Transducer

1. THERMISTORS BY SHIWANI RAJ

2. CONTENTS INTRODUCTION OF THERMISTORS TYPES OF THERMISTORS CONSTRUCTION OF THERMISTORS CHARACTERISTICS OF THERMISTORS RESISTANCE-TEMPERATURE CHARACTERISTICS VOLTAGE-CURRENT CHARACTERISTICS CURRENT-TIME CHARACTERISTICS ADVANTAGES, DISADVANTAGES AND APPLICATION OF THERMISTORS REFERENCE

3. Thermistors Thermistors is a contraction of term “Thermal Resistors”. Thermistors are essentially semiconductors which behave as resistors with a high negative temperature coefficient of resistance. In some cases the resistance of a thermistor at room temperature may decrease as much as 5 per cent for each 1 ° C rise in temperature. This high sensitivity to temperature changes make the thermistors extremely useful for precision temperature measurements, control and compensation. Thermistors are widely used in the temperature range of -60°C to +15°C. The resistance of thermistors ranges from 0.5 Ω to 0.75 Ω.

4. TYPES OF THERMISTORS Thermistors are of basically two types: NTC Type: Where the resistance decreases with an increase in temperature. PTC Type: Where the resistance increases with an increase in temperature. NOTE: NTC stands for Negative Temperature Coefficient and PTC stands for Positive Temperature Coefficient.

5. Construction of Thermistors: - Thermistors are composed of sintered mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron and uranium. They are available in variety of sizes and shapes. The thermistors may be in the form of beads, rods or discs. Fig 1: Commercial forms of Thermistors BEAD Smallest in size Diameter: 0.015mm to1.25mm Sealed in tips of solid glass rods to form probes. GLASS PROBES Diameter: 2.5mm Length: 6mm to 50mm DISC Made by pressing material under high pressure into cylindrical flat shapes. Diameter: 2.5mm to 25mm

6. CHARACTERISTICS OF THERMISTORS There are three important characteristics of thermistors given below: Resistance-Temperature Characteristics Voltage-Current Characteristics Current-Time Characteristics

7. Resistance-Temperature Characteristics The resistance-temperature characteristics of thermistor is non- linear. The resistance-temperature characteristics show that a thermistor has a very high negative temperature co-efficient of resistance, making it an ideal temperature transducer. The resistance-temperature characteristics show that even for a small change in temperature the change in resistance of a thermistor is very large. Between -100°C and 400°C, the thermistor changes its resistivity from 10 -7 to 10 Ω cm, a factor of 10 7. FIG 2: Resistance-temperature characteristics of a typical thermistor and platinum

8. The mathematical expression for the relationship between the resistance of a thermistor and absolute temperature of thermistor is R T1 = R T2 exp. [ β ( - ) ] where R T1 = resistance of the thermistor at absolute temperature T 1 K, R T2 = resistance of the thermistor at absolute temperature T 2 K, and β = a constant depending upon the material of thermistor, typically 3500 to 4500 K A linear approximation of the resistance-temperature curve can be obtained over a small range of temperatures. For a limited range of temperature, the resistance of a thermistor is given as R T1 = R T2 [1 + α T1 ∆T] where α = the temperature coefficient of the resistance which is typically 0.05 Ω / Ω - ° C ∆T = Change in temperature = T 2 – T 1 An approximate logarithmic relationship used for resistance-temperature relationship for a thermistor is given as: R T = aR e b/T where R T = resistance at temperature T K, R = resistance at temperature T K, a and b are constants   NOTE: A linear approximation means that we may develop an equation for a straight light which approximates the resistance versus temperature curve over a specified span.

9. VOLTAGE-CURRENT CHARACTERISTICS If the applied voltage to a thermistor is small, the current is small and the thermistor obeys Ohm’s law. At low voltage current is proportional to voltage. When the current in the thermistor is large enough to raise the temperature of the thermistor appreciably above the ambient temperature, the resistance of the thermistor will be decreased and more current will flow, further increasing the temperature and decreasing the resistance of the thermistor. This current increases until the heat dissipation of the thermistor equals the electrical power supplied to the thermistor. The current increases with increase in voltage until a maximum voltage is reached. Beyond this the current increases with decrease in voltage and the thermistor has a negative resistance. . FIG3: Voltage-Current Characteristics of a Thermistor

10. Under any fixed ambient conditions the resistance of a thermistor is largely a function of power being dissipated within itself. Under such operating conditions, the temperature above thermistor may rise 100°C to 200°C and its resistance may drop to 1000 th of its value at low current The characteristics of self heat of a thermistor can be used to measure flow, pressure, liquid level, composition of gases etc. If the rate of heat removal is fixed, then the thermistor is sensitive to power input and can be used for power level or voltage control. NOTE: In the self heat, the thermistor is sensitive to anything that changes rate at which heat is conducted away from it.

11. Current-Time Characteristics The current-time characteristics indicates the time delay to reach maximum current as a function of applied voltage. When the self heating effect occurs in a thermistor network, a certain finite time is required for the thermistor to heat and the current to build up to a maximum steady value. This time may be changed by changing the applied voltage or the series resistance of the circuit. This time-current relationship provides a simple and accurate means o providing time delays ranging from micro seconds to minutes. Thus a thermistor may be used for providing time delays. FIG 4: Current-Time Characteristics of a Thermistor

12. ADVANTAGES Thermistors are compact, rugged and inexpensive. Thermistors have good stability. The response time of thermistors are fast. DISADVANTAGES Non-linear High sensitivity allows the thermistor to work at low temperature range Not suitable for wide temperature change Shielded cable have to be used Applications of Thermistors: - 1). The major application of thermistors is in the field of measurement of temperature. 2). Temperature compensation in complex electronic equipment, magnetic amplifier and instrumentation equipment. 3). Measurement of power at high frequencies. 4). Measurement of thermal conductivity. 5). Vacuum Measurements.

13. REFERENCE A COURSE IN ELECTRICAL AND ELECTRONICS AND INSTRUMENTATION BY A.K. SAWHNEY

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