2.4 Thermistors ENSORS AND INSTRUMENTATION (UNIT V - SIGNAL CONDITIONING AND DAQ SYSTEMS
- 1. Department of Electronics and Communication Engineering UNIT – 2 Variable Resistance and Inductance Sensors 2.4 Thermistors AU3008 Sensors and Actuators (Open Elective – IV) ROHINI College of Engineering and Technology AUTONOMOUS INSTITUTION Approved by AICTE and Affiliated to Anna University NBA Accredited for BE (ECE, EEE, Mech) I Accredited by NAAC with A+ Grade Anjugramam – Kanyakumari Main Road, Palkulam, Variyoor Post -629 401, Kanyakumari Dist. R.Padma Kumar, Professor, Department of EEE ROHINI College of Engineering & Technology
- 2. Thermistors THERMal resISTORS Assume a simple linear relationship between resistance and temperature for the following discussion: ΔR = k ΔT where ΔR = change in resistance ΔT = change in temperature k = first-order temperature coefficient of resistance Source: http://en.wikipedia.org/wiki/Thermistor
- 3. Thermal resistors". Thermistors are generally composed of semi-conductor materials. negative coefficient of temperature resistance .. Detect very small changes in temperature precision temperature measurements 0.5 Ω to 0.75 MΩ. highly sensitive nonlinear characteristic Construction of Thermistors metal tube, leads, and temperature sensing element. insulating material Two leads connected to one of the arms of the bridge beads, probes, discs, and rods in a variety of sizes -100°C to +300°C.
- 4. Different forms of thermistor Characteristics of Thermistor: Three important characteristics of thermistor make them extremely useful in measurement and control applications. These are : the resistance-temperature characteristics. the voltage-current characteristics. A the current-time characteristics.
- 5. Resistance-Temperature Characteristics of Thermistors The mathematical expression for the relationship between the resistance of a thermistor and absolute temperature of thermistor is : where = resistance of the thermistor at absolute temperature;° K = resistance of the thermistor at absolute temperature ° K and = a constant depending upon the material of thermistor, typically, 3500 to 4500 °K
- 6. Voltage-Current Characteristics of Thermistors These characteristics are shown in Figure shows that the voltage drop across a thermistor increases with increasing current until it reaches a peak value beyond which the voltage drop decreases as the current increases. In this portion of the curve, the thermistor exhibits a negative resistance characteristic. If a very small voltage is applied to the thermistor, the resulting small current does not produce sufficient heat to raise the temperature of the thermistor above ambient.
- 7. Current Time Characteristics of Thermistor When we increase the applied voltage then the time delay also increases to reach the maximum current, and when decreasing the voltage level then the time delay to reach the maximum current is also decreasing, this happens because of the heating effect. At the low voltage, the thermistor produces low heat and at lower heat the resistance is high.
- 8. Advantages of Thermistors: 1. Temperature Sensitivity 2. Rapid Response 3. Small Size 4. Wide Temperature Range 5. Cost-Effective compared to other temperature sensor 6. Simple Circuitry 7. Good Long-term Stability: Disadvantages of Thermistors: 1. Non-Linearity 2. Limited Operating Range 3. Self-Heating Effect 4. Fragility if subjected to excessive mechanical stress 5. To achieve accurate temperature measurements, thermistors often require calibration 6. Sensitivity to Environmental Factors
- 9. 1. Temperature Measurement Thermistors are used as temperature sensors in various devices and systems because of their accuracy and wide temperature range. •Digital thermometers •Medical devices (e.g., body temperature monitors) •HVAC systems for room temperature control •Weather monitoring instruments Applications of Thermistors 2. Temperature Control Thermistors are used in systems requiring precise temperature control to regulate heating or cooling. •Refrigerators and freezers •Air conditioners •Incubators for medical and laboratory use •Ovens and industrial furnaces
- 10. Applications of Thermistors……… 3. Overcurrent ProtectionThermistors (especially PTC thermistors) protect circuits by limiting current during overload conditions. Power supplies Battery packs, Motors and transformers 4. Temperature Compensation Thermistors stabilize circuit performance by compensating for temperature variations. •Precision electronics •Semiconductor circuits
- 11. Applications of Thermistors……… 5. Automotive Applications Thermistors are extensively used in automotive systems for temperature monitoring and control. •Engine temperature sensors •Oil and coolant temperature monitoring •Cabin climate control systems 6. Medical Applications High-precision thermistors are used in medical devices for critical temperature measurements. Respirators and ventilators Catheters for internal body temperature measurement
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