Temperature Sensors - Thermostats - Mechanical

A 310000011051 application field and working principle is used in the context of a temperature-controlled system, such as an automated home HVAC system or industrial cooling system. It typically consists of a temperature sensor(s), a control unit, a mechanical thermostat, and a switch or valve. The thermostat regulates the temperature in the controlled space by comparing a target temperature to the sensed temperature. If the sensed temperature is lower than the target temperature, the thermostat commands the control unit to turn on a heating or cooling system. The same logic works for when the sensed temperature is higher than the target temperature. The thermostat can be mechanically-actuated or electronically-controlled.

Mechanical Thermostats

Mechanical thermostats typically contain a bimetallic strip, a component formed by combining two metal strips with different expansion coefficients. As the temperature changes, the bimetallic strip bends in one direction, due to the different expansion coefficients of the two metals. This bending motion is typically used to actuate a mechanical switch or to adjust the position of a needle on a temperature dial. Mechanical thermostats are slow to respond to temperature changes, but are relatively inexpensive and require no electrical connection to the control system. In temperature-controlled automated home heating systems, mechanical thermostats are typically used for low-cost applications.

Temperature Sensor Selection

The functioning of a thermostat relies on the temperature sensor placed in the controlled space. RTD (Resistance Temperature Detector) sensors, thermistors, and thermocouples are the three most common temperature sensors used in mechanical thermostats. RTDs and thermistors provide a variable resistance that changes with temperature. RTDs consist of a metal, such as platinum, copper, or nickel, and are typically accurate to tenths of a degree. In contrast, thermistors are typically made of polycrystalline semiconductor material and can be accurate to hundredths of a degree. Thermocouples produce a continuous voltage output that depends on the temperature difference between two junctions. Unlike RTDs and thermistors, thermocouples can measure temperatures up to 1,800 degrees Celsius. RTDs and thermocouples are more accurate than thermistors and can measure wider temperature ranges, but their cost is typically two to three times higher.

Control Unit

The control unit is the element that connects the thermostat and temperature sensor to the heating/cooling system. It contains a circuit board with electrical components that regulate the power to the system. Depending on the manufacturer, the exact components may differ but the general purpose is the same. The core component is the relay, which is typically a solid-state switch that connects or breaks an electrical circuit, based on the input signal from the thermostat. For the 310000011051 application field, the contact closure of the relay turns the electric heater or cooling system on or off, as needed to achieve the desired temperatures in the controlled space.