In this lecture, we will learn about the thermistors, the resistance temperature characteristics of thermistors, the construction of thermistors, the application of thermistors, and some features of thermistors. So let’s start with the basic definition of thermistors.
Thermistors
- The word thermistors are called THERM al + res ISTORS=> THERMISTORS. It means thermistor is a contraction of the term “thermal resistor”.
- Thermistors are generally composed of semiconductor materials.
- Thermistors are temperature sensing devices that have a negative temperature coefficient. I.e. their resistance decreased with the increase in temperature.
- Thermistors are widely used in applications that involve measurements in the range of -60oC to 15oC.
- The resistance of thermistors ranges from 0.5\Omega \; to\; 0.75 M\Omega
Resistance temperature characteristics of Thermistors
- The mathematical expression for the relationship between the resistance of a thermistor and absolute temperature of thermistor is:
\boxed{R_{T1}=R_{T2} \exp \left [ \beta \left ( \frac{1}{T_1}-\frac{1}{T_2} \right )\right ]}
Where,
RT1= resistance of the thermistor at absolute temperature T1; (oK).
RT2= resistance of the thermistor at absolute temperature T2; (oK).
\beta= a constant depending upon the material of the thermistor, typically 3500 to 4500 oK
Construction of Thermistors
- Thermistors are composed of a sintered mixture of metallic oxides such as manganese, nickel, cobalt, copper, iron, and uranium. They are available in a variety of sizes and shapes. The thermistors may be in form of beads, rods, and discs.
Features of Thermistors
- Thermistors are compact, rugged, and inexpensive. Thermistors when properly aged, have good stability. response time of the thermistor can vary from a fraction of a second to a minute, depending upon the size of detecting mass and thermal capacity of the thermistor.
- The upper unit of temperature for thermistors is dependent on physical changes in the material.
- The measuring current should be maintained to as low as possible so that soft heating of the thermistor is avoided otherwise errors are introduced.
Applications of Thermistors
- Measurement of temperature.
- Contro of temperature.
- Temperature compensation.
- Measurement of power at high frequencies.
- Measurement of thermal conductivity.
- Measurement of level, flow, and pressure of liquids.
- Measurement of vacuum and composition of gasses.
- Providing time delay.
Frequently Asked Questions on Thermistors
-
What is a thermistor used for?
Answer: Thermistors, derived from the term thermally sensitive resistors, are a very accurate and cost-effective sensors for measuring temperature. Available in 2 types, NTC (negative temperature coefficient) and PTC (positive temperature coefficient), is the NTC thermistor that is commonly used to measure temperature.
-
Is the thermistor a resistor?
Answer: A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of thermal and resistor.
-
What is the difference between a thermistor and a thermostat?
Answer: The difference between a thermistor and a thermostat is that A thermostat is a regulating device component that senses the temperature of a physical system and performs actions so that the system’s temperature is maintained near a desired setpoint, but a thermistor is a resistor whose resistance fluctuates fast and reliably with temperature and can thus be used to detect temperature
-
Which material is used in thermistors?
Answer: The most common materials to be used for these thermistors are Manganese oxide, nickel oxide, cobalt oxide, copper oxide, and ferric oxide. Semiconductor thermistors are used for much lower temperatures.
-
What is a thermistor example?
Answer: Toasters, coffee makers, refrigerators, freezers, hair dryers, etc. all rely on thermistors for proper temperature control. NTC thermistors come in bare and lugged forms, the former is for point sensing to achieve high accuracy for specific points, such as laser diode die, etc.
Also Read:
