Temperature Switches
A temperature switch is an electrical switch which changes its state when it reaches a preset temperature.
In this lesson we will look at a few common temperature switch designs.
We'll examine how these switches function internally, and the principles they work on.
Different types of switches have different advantages and disadvantages, so we'll look at those too.
How It Works
Temperature switches come in many different shapes and sizes but the vast majority fall into one of two basic categories.
They can be either a fluid switch or a bimetallic switch.
Whether fluid or bimetallic, every temperature switch contains some basic components.
A sensing probe, which is what is actually placed in the heated enviroment.
A mechanical switch. This can be normally-open or normally-closed.
A way to set the desired temperature that will trigger the switch. Usually, though not always, this is done using a spring or adjustable screw.
Let's look at fluid-filled temperature switches.
These switches operate on the principle that fluid expands when it is heated.
They can be built using a liquid, gas, or in some cases, both.
Different types of fluids are used depending on variables such as the required temperature range, the response speed, and the switch sensitivity needed.
As the temperature increases, the fluid expands, causing it to inflate a flexable bellows.
The bellows works in opposition to a spring. When the pressure in the bellows rises enough to overcome the resistance of the spring, the switch will be forced closed.
When the temperature decreases, the spring overcomes the pressure in the bellows and the temperature switch resets.
Note that due to hysteresis (the tendency of an object to maintain its current state), the reset temperature will be cooler than the trigger temperature.
A risk with fluid-filled switches is that, if the temperature continues to rise even after the switch has been actuated, there is a point at which the increasing pressure will cause permanent damage.
Something, somewhere in there, will burst!
Then there are the bimetallic temperature switches.
Bimetallic temperature switches work on the principle that metals expand when heated, and different metals will expand different amounts when exposed to the same temperature.
Bonding the two pieces of metal together will cause them to flex when they are heated.
The metal that expands more is forced to curve by the metal that has not expanded as much.
Bimetallic temperature switches are commonly made using either a bimetallic strip or a bimetallic disk.
A bimetallic strip can be used to open or close a circuit path itself while a bimetallic disk is used to move a contact.
The temperature at which a bimetallic switch is triggered is determined by the types of metals used to make it.
Both fluid-filled and bimetallic switches are passive devices.
This means that these switches do not need any sort of external power source in order to function, making either switch a good choice as a safety device.
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