Thermistor

Thermistor - Construction - Working prinicple - Applications - Pros and cons

1. Thermistor UFC C055 Semiconductors & Devices Slides by: 39120008 Syed Ibrahim J

2. CONTENTS

3. THERMISTOR (Thermal + Resistor) It’s a special type of resistor whose resistance changes with change in temperature consists of a semiconductor that acts as a resistor; invented by Samuel Ruben in 1930. Because, semiconductor have Negative temperature coefficient of resistance , per degree rise is temperature can make the resistance fall upto 5%. Due to this property of high sensitivity the thermistor is mainly applicable in precision temperature measurement, temperature control, and temperature compensation, especially in a lower temperature range of -100 °C to +300 °C.

4. PTC stands for   Positive Temperature Coefficient . As temperature rises, resistance increases . This type of thermistor is used in Current limiting devices Self regulating heaters Timer in degaussing coil Motors NTC stands for   Negative Temperature Coefficient . As temperature rises, resistance decreases . These are employed in Very low temperature thermometer Digital Thermostats Battery pack monitors In-rush protection devices

5. Available in Various size and shapes Construction : Thermistors are composed of sintered mixture of metallic oxides like manganese, nickel, cobalt, iron and uranium.

6. To make a thermistor , two or more semiconductor powders of metallic oxides are mixed with a binder to form a slurry. Small drops of this slurry are formed over the lead wires & dry it on into a sintering furnace. During this process, the slurry will shrink onto the lead wires to make an electrical connection. This processed metallic oxide is sealed by putting a glass coating on it. This glass coating gives a waterproof property to the thermistors – helping to improve their stability.

7. Working Principle : It works by its dependency on-resistance values on the change in temperature which is depending upon the material chosen in the construction of the thermistor The value of resistance can be measured by using an ohmmeter. These are connected in series with the battery and the meter.

8. The relation between the temperature and the resistance of the thermistor is mathematically expressed by the equation : Where, R T1 - Resistance of the thermistor at temperature T 1  in Kelvin. R T2 - Resistance of the thermistor at temperature T 2  in Kelvin. β -temperature depending on the material of thermistor . The resistance of the thermistor changes from 10 5  to 10 -2  at the temperature between - 100°C to 400°C .

9. Applications: Digital thermometers Used in Automobiles to determine and record the temperature of the oil and coolant found in the engine. Rechargeable battery has a Thermistor built in it, to regulate the temperature and helps the battery not to get burnt. Wield as circuit protection element. Pros Cons Compact, rugged and inexpensive Good stability and high sensitivity Response time is fast Not affected by stray magnetic and electric field Non-linear output is seen Not suitable for high temperature measurement Shielded cables must be used to minimize interference Self heating

10. Reference: https:// www.teamwavelength.com/info/thermistors.php https:// en.wikipedia.org/wiki/Thermistor https://www.electrical4u.com/thermistor / https:// www.omega.com/en-us/resources/thermistor