What is thermistor

Theory of thermistor:

Michael Faraday, the English scientist first discovered the concept of thermistor in 1833 while reporting on the semiconductor behaviour of silver sulfide. Through his research, he noticed that the silver sulfides resistance decreased as the temperature increased. This discovery would later lead to the commercial production of the thermistors in 1930 when Samuel Ruben invented the first commercial thermistor. Since then technology has improved paving the road to improved manufacturing processes along with the availability of higher quality material.

Definition:

A thermistor is defined as a kind of resistor whose electrical resistance depends on varies with the changes in temperature. A thermistor is a temperature sensitive device. Basically, the thermistor is made of semiconductor materials that mean their resistance lies between the conductor and the insulator.

Thermistor symbol:

The variation in the thermistor resistance shows that either conduction or maybe power dissipation occurs in the thermistor. The circuit diagram of thermistor uses the rectangular block which has a diagonal line on it.

Thermistor symbol

Types of  Thermistor:

The thermistor is classified into two types of based on how they behave with the change in temperature

• Negative temperature coefficient temperature thermistor (NTC)
• Positive temperature coefficient  temperature thermistor (PTC)

So now let us discuss these two types of the thermistor to better understand this topic:

Negative temperature coefficient thermistor (NTC): 

In this type of thermistor the temperature increases with the decrease of the resistance. The resistance of the negative temperature coefficient thermistor is very large due to which it detects the small variation in the temperature.

Positive temperature coefficient thermistor (PTC): 

The resistance of the thermistor increases with the increases in the temperature. 

Application for NTC and PTC thermistor include:

• Temperature compensation 
• Temperature control
• Temperature measurement
• Inrush current limiting

Benefits of NTC and PTC thermistors:

NTC thermistor is basically rugged, stable, reliable and equipped more than other types of temperature sensors to handle extreme environmental conditions and noise immunity.

• Compact size
• Fast response time
• Cost efficient
• This thermistor was the ability to obtain a specific resistance at a particular temperature

Advantages and Disadvantages of thermistor:

Advantages of thermistor:

• It is a small size
• Easy to use
• They are fast in operation
• It has good sensitivity in NTC region
• Fast response over the narrow temperature range
• Cost is low
• Very responsive to changes in temperature
• High accurate
• Repeatable
• Options for customization
• Easily interfaced to electronics instrumentation

Disadvantages of thermistor:

• Thermistor need for shielding power lines
• Extremely non-linear
• Passive
• More fragile as they are semiconductor devices
• Susceptible to self-heating errors
• The excitation for large temperature range

For detailed information:
Read  more >> Advantages and disadvantages of a thermistor

You have also read:

1. Thermocouple - Advantages and Disadvantages
2. RTD - Advantages and Disadvantages 

Application of Thermistor:

• It is used in automatic temperature controllers
• It is widely used in temperature measurement
• The thermistor measures the thermal conductivity
• It is also used for biasing and compensating circuit of a transistor
• In electronics circuit used for the temperature compensation
• A thermistor is used in the control devices actuated by temperature
• It is used the measurement of flow
• It is used for the measurement of high-frequency power
• It measures the composition of gases
• It is used to measures the vacuum and provides the time delays
• The thermistor measures the pressure of a liquid
• Used for the thermal relay circuits and time delay circuit
• It is used in the control devices actuated by temperature

Read more >> Application of Thermistor

You have also read:

1. Thermocouples Applications
2. RTD applications

What is thermocouple

History of thermocouple:

Thomas Johann Seebeck accidentally discovered the thermocouple in the year of 1821. He was experimentally determined that a voltage exists between the two ends of a conductor when the conductor's end is at different temperatures

Meaning of thermocouple:

The thermocouple is basically a temperature measuring device.  The thermocouple is used for measuring the temperature at one particular point. It issued to measure the temperature at one specific point or in the form of the EMF or in an electric current. The temperature can be measured at this junction and the change in temperature of the metal wire stimulates the voltage.

So the thermocouple is comprised of at least two metals joined together to form two junctions. One is connected to the body whose temperature is to be measured this is the hot or measuring junction and the other junction is connected to a body known as temperature. This is the cold or reference junction.  So, therefore, the thermocouple measures the unknown temperature of the body with reference to the known temperature of the other body.

Working principle of thermocouple:

• See back effect: This type of effect occurs among two different metals. When the heat offers to any one of the wire, the electrons start flowing from hot metal to cold metal wire. Therefore the direct current induces in the circuit. In short this phenomenon in which the temperature difference between the two different metal induces the potential difference between them. The see back effect produces small voltage for per kelvin of temperature.
• Peltier effect: This effect totally opposite to see back effect. This effect state that the difference of the temperature can be created between any two dissimilar conductors by applying the potential difference or variation between them. 
• Thompson effect: This effect states that when two disparate metal fixed and join together and if they create two joints then the voltage induces the entire length of the conductor because of the temperature gradient. The temperature gradient is a physical term which shows the direction and rate of change of temperature at a particular location.

Construction of thermocouple:

• Ungrounded junction: In this type of junction, the conductor is entirely isolated from the protecting cover. So this one can be used in high-pressure application work. The major benefits of using such type of junction are that it decreases the stray magnetic field effect.
• Grounded junction: In this type of junction the metal wires and protective cover are welded together. The grounded junction use for measuring the temperature in the acidic atmosphere and provides resistance to the noise.
• Exposed junction: The exposed junction is applicable in the area where fast response requires. This type of junction is used for measuring the temperature of the gas. The metal used to make the thermocouple basically depends on the calculating range of temperature.

Thermocouple junction

Working of thermocouple:

The circuit of the thermocouple is shown in the figure given below. The circuit consists of two dissimilar metals. These metals are joined together in such a manner that they are creating two junctions. The metal is bonded to the junction through welding

Let shown that in figure P and Q are two junctions of the thermocouple. The T1 and T2  are the temperatures at the junctions. As the temperature of the junction is different from each other, the EMF generated in the circuit 

If the temperature at the junction becomes equal, the equal and opposite EMF  generate in the circuit, and the zero current flows through it. If the temperature of the junction becomes unequal, the potential difference induces the circuit. The total current flowing through the circuit is measured through the measuring devices

Iron constant thermocouple

The electromagnetic force induced in the circuit is calculated by the following equation

E = a (Δ𝛉) + b (Δ𝛉 ) 2

Where  Δ𝛉  is the temperature difference among the hot thermocouple junction end as well as the reference thermocouple junction end and here a and b are constant.  

Advantages  and disadvantages of thermocouple:

Advantages or benefits of thermocouple:

• Fast response time
• They are a simple construction
• Low initial cost
• Durable
• Not required bridge circuit
• Good accuracy
• Does not required bridge circuit
• Good reproducibility
• High-speed response
• They are rugged
• They are a self-power active device

Disadvantages  or drawback of thermocouple:

• Not as stable as RTD
• Recalibration is difficult
• More susceptible to RFI/EMI
• Less sensitivity
• They require a reference for operation
  The stray voltage pick up is possible

For detailed information :

Read more >> Advantages  and disadvantages of thermocouple

You have also read:
1. RTD - Advantages and Disadvantages
2. Thermistor - Advantages and Disadvantages

Thermocouple applications:

• These are used in gas machines for detecting the pilot flame
• These are used to test temperature in the chemical plants, petroleum plants
• These are used in the food industry for cryogenic and low-temperature application
• These are used for a heat pump for performing thermoelectric cooling
• These are used for medical equipment
• It can be used for packing equipment
• It can be used as Diesel engine
• These are also used for plastic industry modeling machinery, steel industry

For detailed information

Read more >> Application of thermocouple

you have also read:

• RTD applications
• Thermistor applications

Thermocouple application

There is some application of thermocouple listed below:

• These are used as the temperature sensors in offices, homes, offices, and businesses
• These are in industries for monitoring temperatures of metals in iron, aluminium, and metal
• These are used in gas machines for detecting the pilot flame
• These are used to test temperature in the chemical plants, petroleum plants
• These are used in the food industry for cryogenic and low-temperature application
• These are used for a heat pump for performing thermoelectric cooling
• These are used for medical equipment
• It can be used for packing equipment
• It can be used as Diesel engine
• These are also used for plastic industry modelling machinery, steel industry

RTD application

Here this some application of RTD listed below:

Automotive:

• Automatic climate control
• Audio amplifier
• Emission control
• Home weather stations
• Coolant sensors
• Electric coolant fan temperature control
• Engine block temperature sensors
• Engine oil temperature sensors
• Oil level sensors
• Transmission oil temperature sensors
• Outside air temperature sensor
• Water level sensors
• Intake air temperature sensors

Computer:

• Power supplies 
• UPS system

Communication and Instrumentation:

• Cellular telephones
• Amplifier over temperature sensing
• Copper coil winding temperature compensation
• Oscillator temperature compensation
• Rechargeable battery packs
• Transistor gain stabilization
• Transistor temperature compensation

Consumer electronics:

• Toasters
• Pool and spa  control
• Oven temperature control
• Rechargeable battery packs
• Small appliance control
• Thermostats
• Fire detectors
• Clothes dryers
• Cellular telephones
• Air conditioners
• Washing machines
• Refrigeable battery packs
• Electric water heaters
• Electric thermometers
• Electric blanket controls
• Computer power supplies
• Dishwasher

Food handling and processing:

• Coffee makers
• Deep fryers
• Fast food processing
• Perishable shipping
• Thermometers for  use in food preparation
• Temperature controlled food storage systems 

Industrial electronics:

• Crystal ovens
• Commercial vending machines
• Gas flow indicators
• Fluid flow measurement 
• Industrial process controls
• HVAC equipment
• Solar energy equipment
• Microwave power measurement
• Photographic processing 
• Industrial process controls
• Liquid level indicators
• Welding equipment
• Water purification equipment
• Thermostats
• Thermoplastic molding equipment
• Thermocouple conductivity measurement
• Thermocouple compensation

Medical electronics:

• Blood analysis equipment
• Blood dialysis equipment
• Blood oxygenator equipment
• Infant incubators
• Esophageal tubes
• Skin temperature monitors
• Myocardial probes
• Intravenous injection temperature regulator
• Respiration rate measurement equipment
• Internal temperature sensors

Military and Aerospace:

• Aircraft temperature
• Baththermography
• Fire control equipment
• Satellites
• Missiles and spacecraft temperature 
• Physiological monitoring

Explore more information:

1. RTD advantages ad disadvantages