Electrical contacts

An electrical contact is used in electrical engineering to establish an electrical connection between electrical components, between or within circuits or within components such as contactors, relays or buttons / switches.

A distinction is insoluble ( solid ), detachable, switch contacts and sliding contacts:

Irreconcilable contacts / connections

Insoluble compounds are not absolutely intractable to solve them, but it is necessary that at least partial destruction of the compound.

  • Solder joint ( for example, printed circuit boards, with solder terminals, with solder lugs )
  • Welding connection ( for example, bonding)
  • Press connection ( press sleeve, connecting cable lugs to wires)
  • Rivet
  • Winding connection
  • Adhesive bond

With tool detachable connections

  • Clamp connection
  • Screw connection ( for example, busbars, screw cable lugs )

In a broader sense also includes soldering and winding connections to the tool with detachable connections (caused permanent connections ).

Detachable connections

Connectors, screw with knurled or Edison screw (bulbs, screw type ) can be solved by hand. Detachable connections must endure less actuations than switching contacts, among them the preservation of the low contact resistance is in the foreground. Therefore, they are often made ​​of silver or gold coated base materials. Other common coatings are tin and chrome. Uncoated, detachable plug and screw connections for example consist of copper, bronze or brass.

A stable contact resistance is an important characteristic for a good plug connection. Changes in the contact resistance can have various causes. One of them is the electric breakthrough of high resistance to corrosion and contamination layers, also called frit. In order to eliminate the influence of the fritting can be determined for the testing of connectors test and the maximum allowable voltage drop. A clear sign of the frit is a kink in the voltage and current characteristic curve, which indicates a jump of the contact resistance. With respect to the change in the contact resistance due to the fritting, the voltage at the contact. In general, testing of connections, it may, by the use of a large measuring current and a high open circuit voltage, come to the frits. Therefore, the standard prescribes IEC512 Part 2 for testing of connectors a maximum measuring current of 100mA and a maximum open circuit voltage of 20mV before.

Switching contacts

Switching contacts can be found in relays, relays, pushbuttons / switches. They are the most demanding electrical contacts because they will have their mechanical and electrical properties often maintained over many millions of cycles:

  • To avoid oxidation, they are corrosion resistant (precious metals ) or operate under vacuum, inert gas or oil
  • To avoid contact wear, they have high performances at a high melting point ( tungsten)
  • To achieve a low contact resistance, they must be well- conductive ( copper, silver )
  • They may not tend to weld (formerly usual: cadmium)

All these properties can not be in a switching contact unite, therefore, consist switching contacts with high breaking capacity of materials other than switching contacts for signals or low performance. Often switching contacts also consist of combinations of materials (for example, for low power coated with noble metals copper or bronze contacts as well as for large switching capacities silver filled porous tungsten contacts).

Are also common gold flashed silver contacts for low switching capacities of their low contact resistance reserve ( the gold layer ) and as soon as they are used for large switching capacity, lose their gold layer and expose a robust silver contacts. Switching contacts in relays and small switches are often designed to be able to use them both for signaling purposes as well as for high switching capacities can.

Silver is in spite of its high conductivity limited for small switching operations, since it forms silver sulfide layers.

The most important characteristics specified by the manufacturer of a switching contact are:

  • Switching capacity
  • Maximum switching voltage
  • The thermally tolerable continuous current
  • The maximum switch-on and switch-off current at a given load

Switching contacts ( buttons, keyboards ) for high impedance signals often consist of a pair of conductive rubber on the one hand and gold or graphite conducting paste on the other. They are very reliable and show little bounce.

The bouncing of switch contacts is a periodic closing and re - opening in moment of switching. It leads with switching contacts for high switching capacity for increased contact wear or even for welding ( " sticking" ) of the contacts. Command switching for digital circuits have to be debounced.

Sliding contacts

Sliding contacts (also sliding contacts ) are used for contacting moving parts. Examples are commutators, pantograph and contact rings. It can be used pairings of Kupfer-/Kupferlegierungen and graphite as well as combinations of precious metals.

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