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Bimetallic strips

The principle behind a bimetallic strip is that different metals expand to different extents with temperature changes. By combining two different metals one on top of another into a strip, a bimetallic strip is formed. As the two metals expand or contract differently under the same temperature change, the strip bends. It can then be used to switch on or off a circuit at certain temperatures. Bimetallic strips are often found in ovens. The typical structure of this type of control is shown in Fig. 5.

Fig. 4   A typical bimetallic strip Fig. 5   The structure of a bimetallic strip

The device shown in Fig. 5 is typical of those used in ovens. The upper metal (blue) expands more when heated and contracts more when cooled than the lower metal. Thus, when the temperature inside the oven drops below a certain point, the bimetallic strip bends upwards enough to complete the circuit, switching on the heating element. In a refrigerator, the reverse set-up is used. When the temperature inside the refrigerator rises, the bimetallic strip bends to switch on the compressor which starts the cooling cycle.

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Thermistors

Fig. 6   A thermistor has a temperature-dependent resistance.

A thermistor changes its resistance according to the temperature. Unlike metal, the resistance of a thermistor usually decreases with increasing temperature. A typical thermistor has a resistance of a few hundred ohms at room temperature. This decreases continuously to less than a hundred ohms at 100 oC. In an electronically controlled domestic water boiler for example, a processor or circuit measures the resistance of the thermistor. When a resistance indicating a particular temperature is reached, the heating elements is switched on or off.

Thermistors make use of semiconductors to achieve the resistance changes. Many thermistors are made of a thin coil of semiconducting material such as a sintered metal oxide. The material has the property that, as the temperature increases, more electrons in the material are excited and able to move about for the conduction of electricity. As more charge carriers are available for conduction, the resistance of the material decreases with increasing temperature.

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Modern temperature controllers

Fig. 7   This temperature controller uses a thermocouple to measure temperature changes. When the measured temperature (22 oC) approaches a certain value (42 oC), the electrical power output to the socket will be automatically reduced.

Modern temperature controllers make use of thermocouples to measure the detailed temperature change of the object being monitored. The thermocouple converts the temperature data into electrical signals. The electronic components in the controller use this information to deduce future temperature change, and control the power output to an appliance (e.g. heater or air conditioner) accordingly to keep the temperature of the object within a preset range. Users can easily preset the temperature range according to their needs.

The thermocouples used in temperature controllers generally consist of two dissimilar metal/alloy wires attached together (e.g. by welding) at one end. The attached end is for measuring temperature and is called the hot junction. The other end of the thermocouple is connected to a voltage measuring device, and is called the cold junction. When the temperature of the two junctions is different, a potential difference will appear between the two dissimilar materials. The potential difference is approximately proportional to the temperature difference between the two junctions. This phenomenon is called Seebeck effect. Thermocouples are generally very durable, can be placed in tight spaces and can measure high temperatures, making them very versatile thermometers.