Bimetal relay

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A bimetal relay or thermal relay is a relay that is operated via a bimetal strip . With it, the control current is passed through a conductor loop that is wrapped around a bimetal strip (thermal coupling). At high currents, the bimetal strip itself can also serve as a heating resistor.

Layout and function

Thermal relay as fully electronic motor protection with a working range of 3.7 A to 12 A.
Thermal relay as used in electromechanical telephone systems

The higher the current, the more the conductor loop and thus the bimetal strip heats up. This bends when heated and thus triggers the switching mechanism when a defined and set temperature is reached. There are versions in which the current flows through the winding via the switching contact; these are mostly used to monitor electrical currents.

Precise bimetal relays for current monitoring contain an additional bimetal strip that compensates for differences in the ambient temperature. The two bimetal strips are arranged in opposite directions, so that when the temperature changes, both strips press against each other and room temperature differences are compensated. Only one of these strips has a heating coil. In this way, more precise compliance with the switching current is possible.

See also temperature switch and temperature fuse .

Bimetal relays designed for three-phase current contain a separate bimetal strip for each outer conductor .
They contain three contacts; in the case of motor protection switches, the bimetal
strips interrupt all three contacts via a mechanical OR link with a release. They are used in motor systems with 400 V three-phase current to protect the motor against overload and failure of individual phases. Newer devices measure the motor currents electronically, which means that the device's heat losses are lower. The electronic devices are characterized by larger current setting ranges, which simplifies storage, but they are more expensive than bimetal constructions.

With bimetallic working fuses (breakers) containing in addition to overload relays for overcurrents or an electromagnetic trip mechanism for the short circuit .

In motor vehicles , the flasher unit for the direction indicator used to work with a bimetal strip. Today, this type of flasher is only used in so-called vintage vehicles.

Bimetal relays were also used in electromechanical switching technology; their switching times lasted up to 50 seconds.

In fans , as they are often z. B. can be found in windowless bathrooms and toilets , bimetal relays are often used as time-delayed switching elements. Via a control input, which z. B. is switched with the room lighting, you get the heating of a PTC thermistor , which heats a bimetal contact and thus enables delayed switching (<1 min). After the lighting has been switched off and the PTC thermistor has been heated, the PTC thermistor slowly cools down for a few minutes. In this way it was possible to dispense with elaborately encapsulated electronics in damp rooms. The inherent switch-on delay is desirable here so that z. B. If the room is entered briefly, the fan is not switched on.

In night storage heating systems , thermal relays are often used to control nighttime charging. The value supplied by the outside temperature sensor is converted by the central control unit into a control signal in the form of intermittent mains voltage , provided that the control voltage is switched off in phases ( vibration packet control ). The lower the outside temperature, the shorter the interval until the next switch-off. The control signal acts on a control resistor after it has been weakened by the manual controller and the residual temperature sensor of the heating stove, if necessary. The control resistor in turn controls the switching process in the thermal relay.

See also


  • Günter Springer: Expertise in electrical engineering . 18th edition, Verlag Europa-Lehrmittel, Wuppertal 1989, ISBN 3-8085-3018-9 .
  • Harry Dittrich, Günther Krumm: Elektro-Werkkunde Volume 5 / Professional experience for telecommunications fitters and telecommunications mechanics. 4th edition, Winklers Verlag, Darmstadt 1971.

Individual evidence

  1. Kurt Pribich, Harald Gessinger, Helmut Haslinger: Components of communications technology: a textbook and reference book for all communications technology . 10., corrected u. erg. ed., Kohl and Noltemeyer Verlag, Dossenheim (Heidelberg) 1980, ISBN 3-88173-001-X .