Thermostat- Different Types and their Operating Principle

The thermostat is a device that helps to regulate the temperature of the physical object.  Based on the types of thermostats, the application can be in numerous sectors such as houses, vehicles, machines, etc. We will be discussing these types in the upcoming paragraphs.

Types of Thermostat

1. Bimetallic Strip Thermostat

This is the simplest type of thermostat available in the market.

Bimetallic Strip Working Principle

The bimetallic strip consists of two different types of metals combined together. One end of the strip is fixed at a point. Similarly, the other end is set free. They have two different coefficients of thermal expansion. So, when the strip is heated, the metal with a higher coefficient bends towards the other metal. Similarly, when the strip is cooled down, the metal with a lower coefficient bends towards the other metal.

When the bimetallic strip expands to a certain level, the circuit opens. Thus, the heating element will not be able to get voltage to turn it on. So, the heating element begins to cool down. Similarly, when the temperature of the probe of the thermostat decreases, the strip comes back to its original state. Thus, the circuit becomes closed and the heating element gets turned on.

The temperature dial is connected to the circuit. When the dial is rotated the gap between the strip and the contact point varies. In this way, the temperature can be set in the machine.


  • Highly robust
  • Cheap
  • Simple
  • Easy to operate
  • No need for a power source
  • Applicable for a wider range of temperature (up to 500°C)


  • Less accuracy
  • Not suitable for very low temperature
  • Not much applicable in the industrial sector

2. Capillary Thermostat

The capillary (or capillary and bulb) thermostat is widely used in medical equipment such as water bath, incubator, oven, stability chamber, etc for safety purposes.

Capillary Thermostat Working Principle

Working Principle of Capilary Thermostat
Working Principle of Capillary Thermostat

It is based on the principle of thermal expansion. The internal space of the thermostat consists of liquid (eg, Mercury) or natural gas. When the temperature of the bulb filled with natural gas increases, the gas undergoes an expansion. Thus, the gas expands along the path of the capillary tube and finally reaches the bellow (diaphragm). The expansion of the fluid causes an expansion of the diaphragm on both sides. So during an expansion, it presses the switch which changes the external circuit from closed to open stage.

When the circuit becomes open, the current can not flow through the heater. Thus, the heating element becomes turned off.  Now the machine begins to cool down. The fluid within the thermostat comes back to its original state.

By rotating the temperature dial (knob), you can change its gap with the diaphragm. On rotating clockwise, the gap decreases and vice versa.


If there is any leakage in the bulb, capillary tube, or the bellow, the compressed gas present in it will escape out. Similarly, if the switch present in it becomes too rigid to move up and down, then there can be the chance of overheating the machines such as an oven. Similarly, if the switch becomes too flexible, then there can be a chance of underheating the machine. In any way, if the thermostat gets damaged, there is no way to repair it. Thus, you need to be very careful in handling it.

3. Wax Type Thermostat

Wax type is another type of thermostat which you can find in the engines of vehicles. For more details go through the video shown below.

4. Digital Thermostat

The digital thermostat is a type of thermostat that consists of a thermistor as a temperature sensor. Besides thermistors, the technology can also use other semiconductors such as resistance temperature detectors. No matter whichever technology you use, they are almost similar. They both are capable of converting the surrounding temperature to resistance.

Since the thermostat is digital, you can see the display of an actual temperature. It also allows setting the required temperature. You may need to operate it with one or more batteries. However, some of them can be operated with 24 VAC as a power source.

Digital Thermostat Working Principle

Thermistors can operate in one of two ways. The first way is the PTC (Positive Temperature Coefficient) and the other way is the NTC (Negative Temperature Coefficient). PTC implies that there is a direct relationship between temperature and resistance. Similarly, NTC implies that there is an inverse relationship between temperature and resistance. The main thing is that, whenever there is a temperature variation, there will also be a variation in the resistance of the thermistor.

The thermistor is connected to the bridge circuit. Thus, when the bridge circuit is balanced, there will not be any flow of voltage. Similarly, when the bridge circuit is unbalanced, there will be a change in the voltage signal. This voltage signal then goes to the controlling circuit. So, on sensing the value of the voltage signal, the controlling unit changes the state of the load from ON to OFF or vice versa.

There can be many more variations in the digital thermostats. A single digital thermostat can have multiple features such as wifi, humidity sensing, etc. Nowadays, digital thermostats have been improved to form smart thermostats.

5. Smart Thermostat

A smart thermostat is also a type of digital thermostat. It is the latest and the most advanced form of the thermostat. It is portable and easy to use. It is used for the purpose of home automation such as heating or cooling of the house, ventilation, etc.

Different companies have different models of smart thermostats in the market. They have different features and can slightly differ from one another. They are a type of programmable thermostat that helps to maintain a certain temperature at a certain time period. They may have inbuilt sensors such as humidity sensors. The humidity sensor helps to control the level of humidity in the room. They are also a type of wifi- thermostat. Thus they can be controlled remotely with the help of a laptop, smartphones, etc.

Some of the most popular smart thermostats in the market are as follows.

Once you purchase it, you need to have a general idea of the installation and the operation of the machine. The video is shown below, which talks about the general installation and the operation of the Honeywell thermostat.


  • It is easy to install and operate. You can remotely control the unit.
  • It automatically maintains a certain temperature at a certain time. So, it helps to reduce energy consumption.
  • You can use multiple settings for multiple rooms.


  • If you work from your house, you do not need to install the smart thermostat. This is because you will be able to control the home environment manually.
  • It is quite expensive to purchase.
  • It may not be compatible with the heating or cooling units, with which you want to connect. In that case, you will have to buy a new compatible heating/cooling unit.

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