bimetallic thermostats

JB Controls, in close collaboration with Sensata Technologies, offers the most complete range of bimetallic thermostats for industrial, railway, aerospace, military, and space applications. These thermostats, preset for a temperature range from -54°C to +284°C, can be supplied in isolated units or in bulk quantities, with options for clip attachment or printed circuit board mounting. Thus, we will find the ideal product to meet your needs depending on your environment.

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What is a Bimetallic Thermostat?

M1 thermostat

A bimetallic thermostat or bilame uses the difference in thermal expansion between two metals to regulate the temperature of a system by opening or closing an electrical circuit. There are several types of bimetallic thermostats, the most common of which is the bimetallic disc thermostat.

In the industrial sector, it is often used as a safety device that functions as a switch. It triggers at a defined temperature, most often set at the factory. This allows the temperature to be regulated, for example by cutting off the power to a heating element in a heating system once the desired temperature is reached.

Sensata Technologies has been developing and marketing thermostats under its Klixon® brand for years. These thermostats operate on the principle of a bimetallic disc, which is made up of two metals with different coefficients of thermal expansion. These discs are stamped to allow for a rapid and decisive change from a concave to a convex shape.

The two metals in the disc react differently to changes in temperature, causing a change in shape from concave to convex and vice versa, thereby either interrupting or ensuring the electrical continuity of a circuit.

logo klixon (JB Controls' partners) fournisseurs industriels

In summary, the bimetallic thermostat is a switch that turns on or off the power supply of a given circuit at a temperature defined by the manufacturer. In industrial settings, bimetallic disc thermostats are often referred to as Klixon® in reference to the American brand that belonged to Texas Instrument, which became Sensata Technologies since 2006. However, there are other manufacturers of bimetallic thermostats that use the same principle of operation with the disc.

How does a Bimetallic Thermostat work?

The operation of a bimetallic thermostat is based on the difference in thermal expansion of the two metals used, for example, brass and stainless steel. These metals have different coefficients of thermal expansion, meaning they react differently to changes in temperature. Additionally, when the temperature increases, one layer of the bimetal expands more than the other, triggering a deformation of the bimetal.

Operating principle, choice of 2 metals:

Operating principle, choice of 2 metals

Diagram illustrating the deformation of the bimetallic strip:

The two metals are joined together, and for the example, let’s consider the deformation caused by a temperature increase from 20 to 100°C.

Diagram of the change in state of the bimetallic disk

The metal 1 having a higher coefficient of thermal expansion than metal 2, the bimetallic strip is then deformed and gives a convex shape to the strip. This deformation is progressive as the temperature increases. The change of state is slow and not instantaneous. That type of bimetallic strip cannot be used as a safety device.

Here is where the bimetallic disc comes in, which is stamped to give it a concave/convex shape at room temperature. This operation allows for a quick and immediate change of state at a given temperature.

How does a Bimetal Disc Thermostat work?

The bimetallic disc thermostat is therefore a stamped bimetallic disc to obtain a bimetallic disc whose change of state from concave to convex is instantaneous.

Diagram of the quick-action change of state of a bimetallic disc :

Schematic representation of the change in state of the bimetallic disk

The bimetallic disc thermostat is thus a stamped bimetallic disc in which the change in state from concave to convex is instantaneous. This allows the thermostat to be used as a reliable safety element in a multitude of industrial applications. The change in state of the bimetallic disc that triggers the thermostat is called “instantaneous action”, meaning that the switch is quick and definite. Additionally, for specific applications, the majority of bimetallic thermostats are pre-set at the factory, with manufacturing being done based on the specific needs of the application.

The bimetallic disc can return to its initial state in one of two ways:

  • Automatically, at a defined temperature when it cools down, which is known as an “automatic reset” thermostat.
  • Manually, where an operator presses a button to reset the disc to its original state, which is called a “manual reset” thermostat.

General schematics of the Klixon® type bimetallic disc thermostat:

close scheme
open scheme

The tilting of the bimetallic disc causes the movement of the ceramic plunger which in turn causes electrical contact between the two terminals of the thermostat. It is now closed, and current flows between the two terminals.

A large number of parameters are taken into account in the definition of Klixon® thermostats, including :

  • Initial state of the thermostat: Normally Open (NO) or Normally Closed (NC)
  • Trigger temperature and its tolerance, for example: 49°C +/-3°C
  • Differential: the temperature at which the thermostat returns to its initial state in the case of an automatic reset Klixon®
  • Numerous other factors such as the electrical power to be cut off, vibrational environment, shock resistance, mechanical configuration (fixation), are taken into account.

What are the common applications of a bimetallic thermostat?

Due to their role as safety and regulation devices, Klixon® thermostats are ubiquitous in individual and collective households, businesses, and a large number of industries:

  • Home appliances : Thermostats are incorporated in refrigerators, freezers, ovens, microwaves, dryers, and coffee makers to ensure a stable temperature.
  • Heating and cooling : Thermostats are a key component in heating and cooling systems for homes and commercial buildings, as well as for refrigerated transport in the HVAC field.
  • Industrial equipment : Bimetallic thermostats are used in industrial equipment such as electric motors, transformers, pumps, electronic cards, etc.
  • Laboratory equipment : Many thermostats are used in laboratory equipment and in the medical field to protect costly analysis, diagnostic, and patient monitoring tools.
  • Vehicles : Bimetallic thermostats ensure proper operation of vehicle engines by detecting any abnormalities in engine oil and coolant temperature. They are also used to monitor the batteries of electric vehicles and prevent any risk of fire.

In conclusion : From a few Euro kettle to multi-million Euro satellites and launchers, bimetallic thermostats ensure the safety of systems ranging from the simplest to the most complex.

What are the advantages of a bimetallic thermostat?

The advantages of bimetallic thermostats are numerous, we will list below the main points:

  • Speed : The Klixon® disk changes state suddenly and sharply, ensuring its proper switching function. The thermostat is either open or closed, there is no in-between.
  • Cost : Bimetallic thermostats are cheaper than electronic thermostats and are much more reliable because they rely on mechanical operation.
  • Reliability and accuracy : Bimetallic thermostats are simple mechanical devices that do not require energy to operate, so they are very reliable and less prone to failures. Thus, each thermostat is configured to last a minimum number of cycles. This number of cycles also depends on the electrical power to be cut.
  • Pre-calibrated : There are adjustable bimetallic thermostats (often with a screw) or factory pre-calibrated. The latter often offers the advantage of being protected against the environment (dust, water, oil, etc.). In addition, pre-calibrated bimetallic thermostats are more accurate and reliable over time.
  • Adaptability : The Klixon® is available in a multitude of sizes and finishes, as well as with a wide range of fixings, whether to stick, fix, clip, etc. We find it on electronic cards as well as on heavy-duty pumps and motors installations.

What are the main applications of bimetallic thermostats?

As seen previously, bimetal thermostats are present in most electronic devices that can overheat or require temperature regulation. We will list, in a non-exhaustive way, the following markets:

      • Household appliances
      • Industrial
      • Military
      • Space
      • Railway
      • Aerospace
      • Energy
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