A bimetallic strip is a simple device which converts thermal energy into mechanical motion. It is used as a thermally activated switch or heat indicator and works on the principle of differential expansion of heated dissimilar metals. The bimetallic strip is made up of two different metals which are bonded together to form a straight, flat strip or a concentric coil. When the strip is heated, one of the metals heats up and expands faster than the other, causing the strip to bend. This mechanical deflection is then harnessed in various ways to switch electrical circuits or move a dial to a give heat value indication.
When any solid, fluid, or gas is heated, its molecules start to move away from each other leading to expansion. As long as a heated material is not contained at some point, it will expand in a balanced fashion. If, for some reason, the material is contained or prevented from expanding along one of its surfaces, the unrestrained expansion in the rest of the material will cause it to deflect or bend. Obviously some materials will heat up quicker or expand more than others depending on their molecular structure. If two such dissimilar materials bond together, the one that heats slower will restrict expansion along one face of the other and cause this deflection phenomenon to occur.
This principle of thermal deflection of dissimilar materials is utilized to supply heat-related mechanical motion in a wide range of electrical and measurement equipment. Bimetallic strips are generally constructed of two dissimilar metals with brass and steel and copper and steel being typical combinations. The two materials are typically welded, brazed, or riveted together to produce an even, secure union between the two contact faces. Bimetallic strips may be formed as straight or concentric coil spring type elements. Their physical deflection when heated is then harnessed in a variety of ways to achieve different functions.
One of the most common methods of utilizing the deflection of a bimetallic strip is employing the strip as an electrical contact to make or break circuits. Heater thermostats are a good example of this type of application and use the straight, unheated strip to maintain contact with a switch point and keep the heater circuit active. When the temperature rises to a predetermined point, the strip heats and bends, thereby breaking the circuit and switching the heater off. When the strip cools down again, it straightens and reactivates the heater circuit. This allows a constant temperature range to be maintained.
The coil type bimetallic strip tends to unwind as it heats and turns the needle of a dial type thermometer to indicate a temperature value. Bimetallic strips are also used as thermal overload devices to cut the power supply to electrical equipment when they draw excessive current or physically overheat. The known expansion characteristics of the different metals in the bimetallic strip allows for very accurate tailoring of these devices to operate at exact temperatures. This makes the strips a reliable and affordable method of using heat as a control or measurement input.