Thermistors are a type of resistor whose resistance changes significantly with temperature. They are made of semiconducting materials like metal oxides and their resistance decreases with rising temperature (NTC thermistors) or increases with rising temperature (PTC thermistors). NTC thermistors are used in applications like temperature sensors and overcurrent protection, while PTC thermistors are used in self-regulating heaters and current-limiting devices. Thermistors have a fast response time, are compact and inexpensive but have non-linear resistance-temperature characteristics and may self-heat.

1. 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.
Thermistor
NTC
Thermistor
PTC
Thermistor
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. Beads
•Smallest in size
•Diameter of 0.015 to
1.25 mm
Rods
•Length varies from 6
to 50 mm
•Diameter of 2.5mm
Discs
•Made by pressing
material under high
pressure into cylindrical
shapes of diameter 2.5
to 25mm
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,
RT1 - Resistance of the thermistor at temperature T1 in Kelvin.
RT2 - Resistance of the thermistor at temperature T2 in Kelvin.
β -temperature depending on the material of thermistor.
The resistance of the thermistor changes
from 105 to 10-2 at the temperature
between -100°C to 400°C.

9. Applications:
ɠ Digital thermometers
ɠ Used in Automobiles to determine and record the temperature of
the oil and coolant found in the engine.
ɠ Rechargeable battery has a Thermistor built in it, to regulate the
temperature and helps the battery not to get burnt.
ɠ 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
x Non-linear output is seen
x Not suitable for high temperature
measurement
x Shielded cables must be used to minimize
interference
x 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