00210030| Datasheet

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General Technical Information General technical information
1 Introduction PTC thermistors are ceramic components whose electrical resistance rapidly increases when a certain temperature is exceeded. This feature makes them ideal for use in countless applications of modern electrical and electronic engineering, for example as resettable fuses against current overload or as shortcircuit protection in motors. PTC thermistors are used in electronic lamp ballasts and switch-mode power supplies for delayed switching, and to degauss shadow masks in picture tubes. You find special motor starter PTC thermistors in the compressors of refrigerators for instance. Thermal protection of motors and transformers is another example of the versatility of PTC thermistors. The applications extend to measurement and control engineering, to entertainment, household and automotive electronics, plus data systems and telecommunications of course. PTC thermistors are also suitable as self-regulating heating elements, in auxiliary heating, nozzle heating and carburetor preheating in automobil
es, as well as in many domestic appliances such as door locks for washing machines, or glue guns and hair curlers. The different models of PTC thermistors from EPCOS are equally diverse, offering the matching solution for virtually every application. If you are unable to find the right PTC in this data book, contact one of our sales offices. They will help you, together with the EPCOS development department for PTCs, to find the right solution for your application. 2 Definition

A PTC thermistor is a thermally sensitive semiconductor resistor. Its resistance value rises sharply with increasing temperature after a defined temperature (reference temperature) has been exceeded. The very high positive temperature coefficient (PTC) of the resistance above the reference temperature has given the PTC thermistor its name. Applicable standards are EN 60738-1, IEC 60738-1, DIN 44081 and DIN 44082. 3 Structure and function

PTC thermistors are made of doped polycrystalline ceramic on the basis of barium titanate. Generally, ceramic is known as a good insulating material with a high resistance. Semiconduction and thus a low resistance are achieved by doping the ceramic with materials of a higher valency than that of the crystal lattice. Part of the barium and titanate ions in the crystal lattice is replaced with ions of higher valencies to obtain a specified number of free electrons which make the ceramic conductive. The material structure is composed of many individual crystallites (figure 1). At the edge of these monocrystallites, the socalled grain boundaries, potential barriers are formed. They prevent free electrons from diffusing into adjacent areas. The result is high resistance of the grain boundaries. However, this effect is neutralized at low temperatures. High dielectric constants and sudden polarization at the grain boundaries prevent the formation of potential barriers at low temperatures enabling a smooth flow of f
ree electrons. Above the ferroelectric Curie temperature, dielectric constant and polarization decline so far that there is strong growth of the potential barriers and thus of resistance. In a certain range of temperature above the Curie temperature TC, the resistance of the PTC thermistor rises exponentially. Beyond the range of the positive temperature coefficient the number of free charge carriers is in-

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