Contact resistance

The contact resistance is the resistance of an electrical contact surface. It consists of the constriction resistance and contamination resistance.

Constriction resistance

The constriction resistance is caused by the microscopic roughness of a contact surface. The effective contact area is small and thus, the current flow is concentrated. The constriction resistance is dependent on the resistivity of the material used, the surface irregularities (for example, caused by combustion ), and the number of the effective contact surfaces. The size of the contact points is derived from the normal contact force and the hardness or strength of the surface material. For the electrical conductivity, expressed in units of Siemens, the contact results in simplified:

With the effective elastic modulus

Young's modulus, Poisson's ratio for metallic materials, the electrical resistivity of the contact material, the quadratic mean of the distribution of height (if not specifically known to work well with approximately ) and the normal force of the contact.

As the frictional force is proportional to the contact surface and the normal force does not depend on the (apparent) contact surface. The conductivity depends only on the level of the rough surface topography, but not by the detailed surface topography. As soon as the contact length is reached, the magnitude of the linear dimension D of the body, the conductivity does not increase further; is saturated.

For example, contact resistance of two flat copper plates with D = 1 mm, which are pressed with the force of 2.7 N to each other. For copper at room temperature:

And thus:

The saturation power is found to be 56 kN.

Contamination resistance

By corrosion (for example oxidation ) is formed on the contact surface of a third layer, which increases the resistance. To avoid this, noble metals such as gold, silver, palladium or platinum is used, often only in thin layers. Switches and relays can also be constructed so that the contact surfaces in the switching moment just rub against each other and so the pollution layer is removed again. A foreign film resistor interfere, especially at very low voltages. Some of these very thin layers are penetrated again when shifting slightly higher voltages. This effect is called fritting, and the required voltage fritting. Constant and / or low resistance are important because of the influence of the signals ( for example electro-acoustic or measurement ) or due to power losses. The choice of surface materials is also important for mating cycles or circuits in current - carrying state. In addition, mechanical stress act on the surfaces. Through both thin noble metal layers can be easily destroyed. Thus, the contact surface must be interpreted differently depending on the application.

In order to reduce the contact resistance at terminal contacts, can or must be disposed of foreign layers before connecting. Known for being particularly aluminum, which forms hard insulating oxide layers after a short storage. It is only used for large conductor cross-sections and freely brushed and greased for contacting. For small cross sections ( domestic installation), it is no longer used, that too, because it loosens because of its creep behavior in terminals and oxide layers infiltrate the contact area. It uses a self -enhancing process of the contact - failure: an increased contact resistance due to the current heat leads to increased temperature and thus to further ease and oxidation.

Effects

Electrical contacts (terminals, relay and switch contacts, sliding contacts ) must be designed taking into account the above two effects:

• Prevention of corrosion (precious metals, grease, contact corrosion protection oils)
• Large contact pressure (eg clamps, spring-loaded clamps, not large areas )
• Against each other moving ( sliding contact, selector switches, potentiometers ), ( wear note )

The last two - annoying with contacts - effects are inter alia used in coal microphone to convert mechanical mini- movements or Anpressdruckänderungen (sound pressure ) through a change in resistance in an AC voltage. Also, foot operated starting resistors on older sewing machines made ​​use of this effect - it consisted of a pressure to the pedal exposed stack of graphite discs.

When Fritter, the contact resistance is, however, affected by the high-frequency voltage and thus amplitude -dependent. It could be used for the detection of radio signals.