PSI Automation – Formerly Baccara Australia

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ph | +61 3 9729 3250   e |

All About Mechanical Temperature Switches!

Low cost, compact, and accurate

All About Mechanical Temperature Switches!

Low cost, compact, and accurate

What are Temperature Switches?

A temperature switch is a mechanical or electrical device that reacts when a certain set temperature is reached. It is utilised in manufacturing and industrial processes by turning on or off when the temperature rises or falls to the set degree. Examples of their uses include turning off a machine before it overheats, or to turn on the cooling cycle in your fridge to get it to the right temperature.

What are the different types of temperature switches?

There are a number of different types of temperature switches that use different operating principals. They categorised into either mechanical or electrical. Here we will be talking about mechanical temperature switches.

Mechanical temperature switches operate without a power source, making them a cost effective and reliable solution. There are two main types of mechanical temperature switches.

Bimetallic Temperature Switches

Bimetallic switches utilise the bimetallic strip, which converts a change of temperature into a mechanical displacement. A bimetallic strip is a strip consisting of two different types of metal, often steel and copper (sometimes brass) fused together.

They are then fixed in position at one end with the other end free to move.

As temperature increases, the metal with a larger thermal coefficient will try to expand and get longer. However, as it is fused with another piece of metal, the only way it can do that is by bending.

This physical movement is then used to connect or disconnect a circuit.

This demonstration features a normally closed switch, as the circuit remains closed at rest, they are also made as normally open.

Fluid Expansion Temperature Switches

Fluid expansion temperature switches come in a variety of constructions, however they all rely on a chamber or bulb that is filled with a gas or liquid. Upon heating, the liquid inside expands, causing the physical movement of a part such as a bellow, that opens or closes a circuit.

Our range of PDTF temperature switches use the fluid expansion principal, utilising an internal bellow that pushes the switch contacts when the liquid expands.  Let’s look at how them in detail.

Parts of our fluid expansion temperature switches:

  • A fluid filled chamber within the temperature probe
  • A bellow that extends as the liquid expands inside
  • An adjustment screw to adjust the distance between switching contacts, in order to change the set point
  • A spring to provide resistance against the bellow
  • Switch contacts that allow current from an external power source to pass through them when they touch.
  • Terminals to connect the external power source to the contacts.
  • Process connection, a threaded port to the switch to the process unit

In the demonstration below we have a normally open temperature switch, the process connection would be screwed into the system with the probe within the media.

  1. Here the system is at resting temperature, therefore the liquid is settled, and the electrical contacts are not touching.
  2. As we increase temperature in the system, it causes the liquid to expand, and the bellow starts extending
  3. Once the bellow extends enough to reach the set point, the electrical contacts touch, and a circuit is created.

This demonstration features a normally open switch, as the circuit remains open at rest, they are also made as normally closed.
However, for ease of functionality across multiple applications, they are commonly manufactured to have both NO and NC components, we call this “changeover“.

PDTF temperature switch

Temperature Ranges: 5°C – 150°C
Circuit: SPDT
Port Size: ½”
Protection: DIN Plug IP65
Max Working Pressure: 52 bar
Tolerance: ± 3°C
Max Operating Temp: 163°C
Max Overload: 28°C above set point

Ready to buy?

We stock an extensive range of temperature switches ranging from 45°C to 120°C, with access to ranges from 5°C to 150°C

At PSI Automation our specialised team can help you choose the best switch for your system.