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NORDLUX electrical components are pyroelectric infrared sensors, devices constructed from thermocouples. They are widely used as temperature detection devices in fields such as ear thermometers, radiation thermometers, electric ovens, and food temperature testing. The article details the basic unit of a thermopile—the thermocouple; and thoroughly introduces the structure, working principle, main parameters, and applications of thermopiles.

NORDLUX electrical components principle

When two different conductors or semiconductors A and B form a loop with their ends connected, as long as the temperatures at the two junctions differ, one junction at temperature T is called the working end or hot end, and the other junction at temperature T0 is called the free end or cold end. An electromotive force (EMF) is generated in the loop; the direction and magnitude of this EMF depend on the conductor materials and the temperatures of the two junctions. This phenomenon is called the "thermoelectric effect," the loop formed by the two conductors is called a "thermocouple," these two conductors are called "thermoelectrodes," and the generated EMF is called "thermoelectric EMF." Thermoelectric EMF consists of two parts: the contact EMF between the two conductors and the thermal gradient EMF within a single conductor. The magnitude of the thermoelectric EMF in a thermocouple loop depends only on the conductor materials forming the thermocouple and the temperatures of the two junctions, and is independent of the shape and dimensions of the thermocouple. Once the electrode materials of the thermocouple are fixed, the thermoelectric EMF is the functional difference between the temperatures t and t0 of the two junctions. This relationship is widely applied in practical temperature measurement. Since the cold end t0 is constant, the thermoelectric EMF generated by the thermocouple changes only with the temperature of the hot end (measurement end), meaning a specific thermoelectric EMF corresponds to a specific temperature. Temperature measurement can thus be achieved by measuring the thermoelectric EMF. The basic principle of thermocouple temperature measurement is that two conductors of different compositions form a closed loop; when a temperature gradient exists between the ends, current flows through the loop, creating an EMF—thermoelectric EMF—between the ends, known as the Seebeck effect. Two homogeneous conductors of different compositions serve as thermoelectrodes; the end with the higher temperature is the working end, and the end with the lower temperature is the free end, which is usually maintained at a constant temperature. Based on the functional relationship between thermoelectric EMF and temperature, thermocouple reference tables are created; these tables are derived under the condition that the free end temperature is 0 °C, and different thermocouples have different reference tables. When a third metal material is introduced into the thermocouple loop, as long as the temperatures at its two junctions are identical, the thermoelectric potential generated by the thermocouple remains unchanged, i.e., it is unaffected by the inclusion of the third metal. Therefore, during thermocouple temperature measurement, measuring instruments can be connected; once the thermoelectric EMF is measured, the temperature of the measured medium can be determined. Thermocouple temperature measurement requires the cold end temperature (the measurement end is the hot end; the end connected to the measurement circuit via lead wires is called the cold end) to remain constant so that the thermoelectric potential maintains a proportional relationship with the measured temperature. If the cold end (ambient) temperature changes during measurement, it will seriously affect measurement accuracy. Taking measures at the cold end to compensate for the effects of cold end temperature changes is called thermocouple cold junction compensation. Special compensation cables are used for connection to measuring instruments. Thermocouple cold junction compensation calculation methods: (1) From millivolts to temperature: Measure the cold end temperature, convert it to the corresponding millivolt value, add it to the thermocouple's millivolt value, and convert the sum to temperature; (2) From temperature to millivolts: Measure the actual temperature and the cold end temperature, convert them separately to millivolt values, subtract them to obtain the net millivolt value, which yields the temperature.

NORDLUX electrical components composition

A diode is formed by a PN junction plus corresponding electrode leads and package encapsulation. Using different doping processes and diffusion, P-type semiconductor and N-type semiconductor are fabricated on the same semiconductor substrate (usually silicon or germanium), forming a space charge region at their interface called the PN junction. The electrode led from the P region is called the anode, and the electrode led from the N region is called the cathode. Due to the unidirectional conductivity of the PN junction, when the diode conducts, current flows from the anode through the interior of the tube to the cathode. A diode has two electrodes; the electrode led from the P region is the positive terminal, also called the anode; the electrode led from the N region is the negative terminal, also called the cathode.

NORDLUX electrical components principle

The construction of a mercury switch is very simple. The exterior is a sealed cylindrical insulating housing containing approximately one-fifth mercury. At one end of the housing, two wires leading out from the inside are fixed. This constitutes a mercury switch. Due to gravity, the mercury bead flows to the lower part of the container; if it simultaneously contacts both electrodes, the switch closes the circuit, turning the switch on. The shape of the container also affects the conditions under which the mercury bead contacts the electrodes.

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