Temperature switches are designed for controlling and monitoring temperatures in various systems and applications. They switch an electrical contact when the temperature exceeds or falls below a certain level, thereby preventing overheating or ensuring sufficient heating.
This article deals with mechanical temperature switches that utilize the expansion of metals, liquids or gases. As the temperature rises, the substances in a temperature switch continue to expand until a switching contact is actuated when a defined temperature is reached. Switching currents of 10 A or more are no problem for most temperature switches. Small heaters can be controlled directly, for larger systems an actuator such as a power contactor must be used. Mechanical temperature switches are also passive switches and therefore do not require a supply voltage.
Electronic temperature switches are also available on the market. These measure the temperature with a resistance thermometer and evaluate its signal electronically. The switching point can be set using the keypad, for example, and they can be equipped with a display. The temperature can often be output via a standard signal (e.g. 4 to 20 mA).
The switching point (or switching temperature) of a temperature switch is the temperature at which a contact is opened as the temperature rises, thus switching off a heater. The switching point is fixed for some switches, but can also be freely set for other versions. In addition, the temperature switches have a so-called lower switching point (also known as reset point/switch back point), i.e. the temperature at which the contact is closed again as the process cools down. The temperature difference between the two switching points is the switching hysteresis or reset value/switch back value.
A very precise control result is achieved with temperature controllers. These measure the temperature with a temperature sensor and, as PID controllers, they regulate the temperature exactly to the setpoint value. In the case of binary control, precise adjustment is possible by changing the relative switch-on time of the actuator. For example, the heat output can be varied from 0 to 10 kW.
In contrast, temperature switches are assigned to the discontinuous two-state controllers – the process temperature fluctuates with them around the setpoint value. In many applications, such temperature fluctuations are acceptable, so that temperature switches can be used to set up very robust temperature control at low cost.
As the name suggests, a bimetal is used in bimetal switches. The 2 metals that are rolled onto each other (e.g. copper and nickel) have different expansion coefficients. As the temperature rises, the metals expand to different degrees so that a deflection occurs and a contact is opened (NC) or closed (NO).
Bimetal switches in industrial environments have a very compact design and are usually screwed into the process like a sensor. Their switching point is fixed during production. They are also very resistant to vibration – a vibration resistance of up to 10 g is not uncommon. The switches are connected in the direction of the evaluation system via a connection line. Temperature switches with a bimetal are used in such devices as compressors – they switch off the compressor in the event of excess temperature or start active cooling (e.g. fan).
This type of temperature switch is usually referred to as a thermostat and consists of a probe filled with a gas or liquid that is placed in the process. If the temperature at the probe rises, the system pressure increases. Because the membrane is connected to the sensor via the capillary line, the same pressure is created in it. The membrane expands and a snap-action switch is triggered. The inner diameter of the capillary line with 0.2 mm is just twice the diameter of a human hair (up to 0.1 mm).
The probe and the thermostat housing are connected to each other via the capillary line. Maximum capillary line lengths of up to 5 m can be regarded as practical. The switching point can usually be set on the housing.
The figure shows a temperature switch for controlling the room temperature. In this version, the capillary is only routed to a spiral probe, which is located directly on the thermostat housing.
In many cases, for example, a compact controller regulates the process temperature, which is measured by an electrical thermometer. A setpoint value is set on the controller so that the temperature in the process is controlled exactly to the setpoint value by activating the heater accordingly via the controller output. A temperature switch also measures the temperature and switches the heating off at a limit temperature (e.g. 1000 °C).
A temperature that is too high can be caused by a technical defect or incorrect operation. There are systems for which the described switch-off is mandatory. Examples are heat transfer oil systems or grain dryers. Often, however, it is simply a matter of common sense to protect the goods to be treated or the system from overheating by switching it off to prevent damage.
In the case of temperature switches for control, a simple setpoint specification can be made - usually via a rotary knob. These components are known as temperature controllers.
Temperature switches for switching off at a defined limit temperature are called temperature monitors. With these temperature switches, the setpoint can usually only be set using a tool.
There are also temperature switches for switch-off for which a reset must be carried out. If the contact was opened due to an excess temperature, it does not close automatically when the temperature returns to the good range. A reset button must also be pressed.These so-called temperature limiters are often used in unmonitored systems where there are no technical personnel on site.
Temperature monitors and limiters are normally equipped with an additional safety function and are then referred to as safety temperature monitors or safety temperature limiters.