What is a Thermistor? Thermistor Types, Thermistor Circuits

(Last Updated On: August 21, 2020)

Thermistor

Thermistor Overview:

What is a Thermistor? Thermistor Types, Thermistor Circuits- A Thermistor is a semiconductor temperature dependent variable thermal resistor that changes its resistance with temperature. Thermistor is combination of two words thermal and resistor. Technically, all resistors are thermistors their resistance changes slightly with temperature but the change in these resistors are usually very small as compared to thermistor and therefore it is difficult to measure. Thermistors are made in such manner that its resistance is changing with temperature. Thermistors are used as temperature measuring or sensing devices in electrical circuits to compensate for temperature variations of other components.

In this article I am going to explain all the details including the Thermistor types, how to use a Thermistor in different types of circuits, how to use a Thermistor with Arduino board, etc.

Types of Thermistors:

There are two types of Thermistors:

Negative temperature Coefficient (NTC) thermistor:

NTC thermistor has inverse relation with temperature when the temperature increases its resistance decreases and when the temperature decreases the resistance of the Thermistor increases.  NTC types Thermistor are susceptible to very small changes make it ideal to be used as temperature sensor. An NTC is commonly used for temperature monitoring and inrush current limiter. It acts as inrush current limiter it partially blocks the inrush current by the high resistance of thermistors and is shade as heat. As the Thermistor heat up its internal resistance drop allowing more current to flow until reached the standard current flow and temperature level. It is used in ovens, air conditioner and fire detectors.


Positive temperature Coefficient (PTC) thermistor:

PTC thermistor has direct relation with temperature when the temperature will increase its resistance will increase and when the temperature will decrease so the  resistance of the Thermistor will decrease.

With PTC thermistors, resistance increases as temperature rises. PTC Thermistors protect against overcurrent condition and is used in series combination as resettable fuses.

PTC thermistors are typically placed in series with a circuit as inline and resettable fuses to protect against overcurrent situations. When over current situation occur its temperature repeatedly increases causing the thermistor resistor to increase as well. There are two varieties of PTC:

Silistor Temperature sensor:

One is silicon based Thermistor called Silistor which has temperature characteristics that follow a linear temperature curve with resistance gradually rising as the temperature rises. They are not very common but do exist.

Switching type PTC Thermistor:

Other is switching type thermistor they behave like NTC until they cross a temperature barrier called Curie point. The Curie temperature is a defined temperature trip point at which certain material loses their permanent magnetic properties which is short, affects the device by creating barriers when it reaches the Curie temperature to allow the resistance to increase rapidly.

PTC thermistors are used in thermostat, small heaters and motors.

Disk and Chip Thermistors

These are manufactured by using metal surface contacts. Disk and chip type thermistor has slow response then bead type thermistor because it is in large size. It consists of copper rod due to which its sensitivity is about ±1%.   The power dissipation by this thermistor is proportional to the square value of the current. So due to which it is better than bead thermistor these thermistors has maximum current handling capacity. The fabrication of chip thermistors is done by tape casting. The size of it is 0.25 to 25 mm.

Epoxy:

Epoxy dip coated and soldered between jacketed Teflon / PVC wires. Their small dimension allows for easy installation and they can be point or curve matched.



Bead Thermistors

These types of thermistors are made up of lead wires of platinum alloy and are directly interfaced in the ceramic body. Bead type thermistors have advantage over disk and chip thermistors by the following feature

  • Fast response times
  • Better stability
  • Ability to operate at higher temperatures

Because of its fragile nature of bead thermistor, while using in the circuits these are sealed in the glass body. Due to this construction there stability is not affected and safe from damage. The size of it is 0.075 to 5 mm.

Glass Encapsulated Thermistors

These types of thermistors are designed for high temperature above 150 °C. They are designed by encapsulating in the airtight glass. These are more stable and are protected from environmental changes. Glass Encapsulated Thermistors range is between 0.4 to 10 mm.

Probe Assemblies:

These types of thermistors are used in various types of house hold appliances. This type of thermistor is used in air conditioner, geyser.

Surface mount Thermistors:

These types of thermistors are used in mother board, laptops

Mathematical Equation for Thermistor:

R2=R1 e(β(1/T_1 -1/T_2 ))

The temperature will be measured in Kelvin.

Where

  • R1 is the resistance at temperature T1
  • R2 is the resistance at temperature T2
  • T1 is the first temperature point in Kelvin
  • T2 is the second temperature point in Kelvin

Example No1

A thermistor has a resistance of 10KΩ at 25°C and 30KΩ at 0°C

a)     What are the value of beta constant

b)    What will be the NTC resistance at 150°C

Solution:

R1=10KΩ
R2=30KΩ
T1=25°C
T2=0°C

Now to find bet value we will first convert temperature in kelvin by using the formula

°K=°C+273
T1=25°C+273=298°K
T2=0°C+273=273°K

The formula to find the beta value is:

Thermistor

By doing the calculation we will get

β=3575

What will be the NTC resistance at 150°C

Thermistor


Thermistor Measurement:

Now to calculate the voltage value for the thermistor we will use voltage divider circuit.

Thermistor

Vt=R/R+Rt  * Vdd

Now to solve this for  we will get

Rt=Vdd –Vt/Vt * R

Vt is our primary measurement while  and R are our auxiliary measurement. Measure the resistor R value before connecting to the voltage divider through ohmmeter. Select the R value to maximize measurement sensitivity and range.

Thermistor Specifications:

Following are the parameters that everyone need to know before using thermistor:

Resistance:

The resistance of the thermistor varies with the temperature. The temperature specified by the manufacturer company of thermistor is  usually 25°C. The table show the resistance with respect to temperature.

                     Temperature Resistance
0°C 30kΩ
25°C 10kΩ
50°C 4kΩ
35°C 1kΩ
35°C 1kΩ

Tolerance:
Tolerance gives us the information that how much the resistance can be changed from the specified value. Thermistors are usually specified with either a temperature tolerance or a resistance tolerance. It is expressed in percent. For example, if the specified resistance at 25°C for a thermistor with 10% tolerance is 10,000 ohms then the measured resistance at that temperature can range from 9,000 ohms to 11000 ohms. Temperature tolerance is need when measuring the temperature measurement over the temperature range instead of use of resistance tolerance.

Beta constant

A value that represents the relationship between the resistance and temperature over a specified temperature range. The manner in which the resistance of a thermistor increases or decreases is related to a constant known in the thermistor industry as beta (β). For example:

Thermistor

By doing the calculation we will get

β=3575

Beta constant value 3575 indicates a temperature range from 273°K to 298°K.

Operating Temperature Range:

Mostly operating temperature range of a thermistor is  from −55 °C to +150 °C, though some glass body thermistors have a maximal operating temperature of +300 °C.

Thermal Time Constant

When the temperature changes, the time it takes to reach 63% of the difference between the old temperature and new temperatures       (T1 –T2) is known as thermal time constant.


Thermal Dissipation Constant

In thermistor heat are produced when current is pass through it. This is the amount of power required to raise the thermistor temperature by 1°C above ambient temperature. It is specified in mill watts per degree centigrade (mW/°C). Normally, power dissipation should be kept low to prevent self-heating in insignificant temperature measurement. In some thermistor applications self-heating are required to raise the body temperature above the ambient temperature due to which thermistor detects even subtle changes in the thermal conductivity of the environment. Some of these applications include liquid flow measurement, liquid level detection, and air flow measurement.

Maximum Allowable Power

Maximum power dissipation is the range of powers that can thermistor with stand. It is specified in Watts (W). Exceeding form this power specification will cause damage to the thermistor.

 Resistance Temperature Table:

Thermistors operate at specific range on temperature. Its temperature range is typically -50 to 300°C depending on type of coating and construction. Glass Encapsulated Thermistors are designed for high temperature.

Thermistor structure & composition

Thermistors come in a variety of sizes and shapes like flat disk, and they are made from a variety of materials dependent mostly metallic oxide of manganese, cobalt, nickel, copper, iron  upon their intended application and the temperature range over which they need to operate. To make a thermistor first the metallic oxide are crushed in powder form. Then heat is given to this powder form and compact it in form of mass this process is also known as centring process. Now various shapes are given to it according to their application like flat discs where they need to be in contact with a flat surface. Temperature probes used beads or rods type of thermistors. The shape of a thermistor is depend on the requirement for the application. The temperature range of metallic oxide thermistors are from 200 – 700 K. Lower temperature thermistors are made up of semiconductors. Germanium thermistors are used for temperatures below 100 K. Silicon thermistors can be used at temperatures up to 250°K.

Advantages of Thermistors:

The following are the advantages of the thermistor.

  1. Incubator use thermistor for monitoring temperature.
  2. Thermistor is cheaper than other temperature sensor like LM35.
  3. The thermistor is shape is simple which make it long durable, compact, and less expensive.
  4. Thermistor is waterproof.
  5. The self-heating of the thermistor is avoided by minimizing the current passes through it.
  6. It is used in consumer appliances like dryers, toasters, coffee makers, freezers
  7. The properly aged thermistor like bead thermistor has good stability.
  8. The response time of the thermistor changes from seconds to minutes depending of the thermal capacity of the thermistor.

Thermistor Working and Testing:

The working principle of a thermistor is that it depend upon temperature. Its resistance varies with temperature. We can check the thermistor with the help of Multimeter or ohmmeter. Now consider we are checking PTC type thermistor we will connect the PTC terminals with the probes of Multimeter and set the Multimeter on resistance mode. Then we will give heat to the sensor we will see that the resistance of the thermistor is increasing which shows us that it is PTC thermistor because it has direct relation with the heat. Similarly when we check NTC Thermistor with Multimeter its resistance will be decrease with the increase of heat.


Thermistor based projects:

Temperature controlled Automatic fan controller using a Thermistor:

In this project the fan will be automatically turned on when the temperature increases.

Components used:

  • Thermistor 10K
  • Resistor 10k
  • Fan
  • BC547 transistor
  • 9 volt battery

Thermistor

Thermistor Circuit Working

The circuit consists of a 10kΩ thermistor connected with 10kΩ Resistor to form a voltage divider circuit. The Thermistor used in this project is NTC thermistor, whose resistance increases when the temperature decreases and its resistance decreases when the temperature increases. At room temperature  the thermistor has 10kohms resistance. Fan is connected with the transistor. Now when we heat the thermistor by bringing a match stick near it its temperature will be increase and resistance will be decreases which will cause the fan to switch on. When the temperature will be decrease the fan will be turn off.

Fire alarm system using Thermistor:

In this project the buzzer will be automatically on when the temperature increases. This project can be helpful in homes. The working of this project is similar with the automatic fan controller the only difference is that we use buzzer in place of fan.

Components used:

  • Thermistor 10K
  • Resistor 10k
  • Speaker
  • BC547 transistor
  • 9 volt battery

Thermistor

Thermistor with Arduino:

In this project we have connected Thermistor with Arduino board. This project can be used for multi purposes such as temperature monitoring, fire alarm etc.

Thermistor

We have connected one terminal of the 100K thermistor with 5V and other terminal with the analogue pin of the Arduino the Thermistor is then connected with a 100K resistor to form a voltage divider circuit. Now when the resistance of the thermistor vary the output voltage at the junction will also vary. As the Thermistor and resistor forms a voltage divider, so on the Arduino’s Analog  Pin A0 we will get different voltage values as the temperature changes.



Thermistor Arduino Programming:

Thermistor Arduino Code Explanation:

I started off by defining a pin for the Thermistor. A wire from the voltage divider is connected with the A0 pin of the Arduino board.

int thermistor= A0;

Next, I defined a variable “thermistorval” for storing the value.

int thermistorval;

Inside, the setup() function, I activated the Serial communication using Serial.begin() and selected 9600 as the baud rate. Using the pinMode() function I set the Thermistor as the input.

void setup(){

Serial.begin(9600);

pinMode(thermistor,INPUT);

}

Inside the loop() function, we read the Thermistor value using the analogRead() function and store the value in variable thermistorval. Next we display the value on the Serial Monitor.

void loop(){

thermistorval=analogRead(thermistor);

Serial.println(thermistorval);

delay(1000);

}

You can easily modify this code, add an if condition if you want to control an output device, or LED.

We can covert this value in voltage by using the formula:

(Resolution of the ADC)/(system voltage)=(ADC Reading)/(Analog Voltage measured)

Where the resolution of the ADC is 1023 and system voltage is 5V and analog voltage is the reading of the thermistor.

ADC Reading =(Resolution of the ADC×Analog Voltage measured)/(system voltage)


Thermistor with a Voltage Comparator, Fixed Temperature:

This circuit given below consists of abThermistor and an operational amplifier (op-amp), this operational amplifier is used as the voltage comparator”because every Op-amp can be used as the voltage comparator, but the reverse may not apply” which drives Led 1 and Led 2, depending upon the value of the comparator. Thermistor will measure the resistance depending upon the temperature when the temperature will increase, its resistance will decrease and when the temperature will decrease its resistance will increase.

Components used:                      

  • Thermistor
  • Resistor
  • Led
  • Lm393(op-amp)
  • battery

Thermistor

There are two potential dividers in this circuit, in which one potential divider is fixed because there resistance is fixed which will act as reference voltage while other is potential divider is not fixed because of thermistor which resistance is changing these potential divider provide inputs to the operational amplifier. The comparator continuously compares the changing value of both potential divider by  subtracting one input value from the other input value (X) because we are using the inverting and non-inverting inputs of the op-amp.

Since there is no feedback loop, the operational amplifier will operate at saturation, and therefore the output voltages will swing just below +9 V or just above 0 V. When the output is close to ~9 V, the LED 2 will light, however, when the output is close to ~0 V, then the LED 1 will alight. In practice, op amps do not operate under ideal conditions and zero volts is not exactly zero, as a result, the red LED lights very dimly.

Thermistor with Relay, Variable Temperature:

In this project we will connect relay with thermistor. With the help of this project we can control ac bulb, fan etc. when the temperature of the room will be increase the relay will automatically on the ac.

Thermistor

Components used:

  • Resistor
  • Led
  • Transistor C945
  • Potentiometer
  • Relay

We will supply 12V supply to the circuit 1K resistance is connected with the 10K thermistor which will further connected with the potentiometer to form voltage divider  circuit. Two C945 transistors are used in this project. These transistors will switch on the relay. We will connect three terminal device with the relay on pin will be connect with normally open, one with common of the relay and other with the normally closed. We will connect one wire with common of the relay and the other with normally open passing through a switch.  If you think this circuit is hard to follow, then you can also use the following circuit. Which I have practically used in so many projects and is fully tested.


Thermistor with LM741 Op-Amp, Variable Temperature:

Thermistor

The heart of the circuit is LM741 Op-Amp which is an Operational Amplifier. In this circuit the LM741 is used as the voltage comparator. The two voltages available on the inverting pin 2 and non-inverting pin 3 are compared and the output is generated in the form of high or low available on pin 6 of the LM741.

A 10K ohm Thermistor is connected in series with a 10K ohm fixed value resistor. The Thermistor and Resistor R4 forms the voltage divider as the two components are connected in series. This way when the temperature varies we get different voltage values which are fed to the inverting input of the LM741 IC. So the voltage available on the inverting input varies as the temperature increases or decreases.

Another voltage divider is formed on the non-inverting input of the LM741, this voltage is used as the reference voltage. A potentiometer R1 is connected in series with R3 and R7. Using the potentiometer we can set different values. We can set any voltage which represent the temperature, this voltage is compared with the  voltage which is available on the inverting input of the LM741 IC.

If the voltage available on the non-inverting input is greater than the voltage available on the inverting input a High signal is generated at the output which controls the relay. So, anything connected with the Relay will be turned ON.

If the voltage available on the non-inverting input is less than the voltage available on the inverting input a Low signal is generated at the output which turns OFF the relay and this way the load connected is turned OFF.

At the output of the relay, you can connect AC or DC loads. The relay used in this circuit is of the type SPDT Single and Double Throw.

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About the Author: Engr Fahad

My name is Shahzada Fahad and I am an Electrical Engineer. I have been doing Job in UAE as a site engineer in an Electrical Construction Company. Currently, I am running my own YouTube channel "Electronic Clinic", and managing this Website. My Hobbies are * Watching Movies * Music * Martial Arts * Photography * Travelling * Make Sketches and so on...

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