Contact resistance

The contact resistance , transition resistance or contact transition resistance is the electrical resistance of an electrical contact surface , for example between the contacts of an electrical switch . The contact resistance is made up of the sum of the two components of narrow resistance and foreign layer resistance .

Tight resistance

The tight resistance arises from the microscopic unevenness of a contact surface. The effective contact area is therefore smaller and the current flow is restricted. The tightness resistance depends on the specific resistance of the material used, the surface unevenness (e.g. caused by burning) and the number of effective contact surfaces. The size of the contact points results from the normal contact force and the hardness or strength of the surface material. For the electrical conductivity of the contact, expressed in the Siemens unit , the following results in simplified form:

{\ displaystyle \ Lambda \ approx {\ frac {3 {,} 7} {E ^ {*} \ cdot \ rho \ cdot l}} \ cdot F_ {N}}

The effective modulus of elasticity denotes ${\ displaystyle E ^ {*}}$

{\ displaystyle E ^ {*} = {\ frac {E} {2 \ cdot (1- \ nu ^ {2})}}}

,

${\ displaystyle E}$ the modulus of elasticity , the Poisson's number ( for metallic materials), the specific electrical resistance of the contact material, the root mean square value of the roughness-height distribution (if not known, well approximated with (2 µm)) and the normal force on the contact. ${\ displaystyle \ nu}$ ${\ displaystyle \ nu \ approx {\ tfrac {1} {3}}}$ ${\ displaystyle \ rho}$ ${\ displaystyle l}$ ${\ displaystyle 2 \ cdot 10 ^ {- 6} \, \ mathrm {m}}$ ${\ displaystyle F_ {N}}$

Like the frictional force, the contact area is proportional to the normal force and does not depend on the (apparent) contact area. The conductivity depends only on the height topography of the rough surface, not on the detailed surface topography. As soon as the contact length reaches the order of magnitude of the linear dimension of the body, the conductivity no longer increases; is in saturation . ${\ displaystyle D}$ ${\ displaystyle \ Lambda _ {\ mathrm {max}} \ approx D / \ rho}$

Example: Contact resistance of two flat copper washers with mm, which are pressed against each other with a force of 2.7 N. For copper at room temperature : ${\ displaystyle D = 1}$

{\ displaystyle \ rho \ approx 1 {,} 8 \ cdot 10 ^ {- 8} \, \ Omega \ mathrm {m}, \ E ^ {*} \ approx 5 {,} 6 \ cdot 10 ^ {10} \, \ mathrm {Pa}}

and thus:

{\ displaystyle R = {\ frac {1} {\ Lambda}} = 0 {,} 1 \, \ mathrm {m} \ Omega}

The saturation force is 56 kN.

Impurity resistance

By corrosion (. E.g., oxidation ) is formed on the contact surface of a third-layer, which increases the resistance. To avoid this, precious metals such as gold , silver , palladium or platinum are used, often only in thin layers. Switches and relays can also be designed in such a way that the contact surfaces briefly rub against each other at the moment of switching and the foreign layer is removed again. The foreign layer resistance is particularly disturbing at very low voltages. Some of these very thin layers will break down again when switching somewhat higher voltages. This effect is called fritting , the voltage required for it fritting voltage . Constant and / or low resistances are important because of the influence on the signals (e.g. electroacoustics or measurement technology) or because of power losses.

The choice of surface materials is also important when plugging in or switching when live. In addition, mechanical loads act on the surfaces. Both can easily destroy thin layers of precious metal. The contact surface must therefore be designed differently depending on the application.

In order to reduce the contact resistance in terminal contacts, interfering foreign layers can or must be removed before connection. Particularly known for this is aluminum , which forms hard, insulating oxide layers after only a short storage period. It is used for large conductor cross-sections because of its weight advantages and is brushed and greased for contact. For smaller conductor cross-sections such as in house installations , conductors made of copper or in the form of copper-clad aluminum (CCA) are used.

Effects

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

Avoidance of corrosion (precious metals, grease, contact anti-corrosion oils)
high contact pressure (e.g. terminals, spring-type terminals, areas that are not too large)
Moving against each other ( sliding contact , step switch, potentiometer ), (note wear)

The last two - annoying with contacts - effects are a. Used in the case of the carbon microphone to convert mechanical mini-movements or changes in contact pressure (sound pressure) into an alternating voltage via a change in resistance. Foot-operated starting resistors on older sewing machines also use this effect - they consist of a stack of graphite disks exposed to pedal pressure.

In the case of the coherer , however, the contact resistance is influenced by the high-frequency voltage and is therefore dependent on the amplitude. He was able to detection of radio signals are used.

literature

Ragnar Holm: Electrical Contacts Handbook . 3. Edition. Springer-Verlag, 1958.
Valentin L. Popov: Contact Mechanics and Friction. A text and application book from nanotribology to numerical simulation . Springer-Verlag, 2009, ISBN 978-3-540-88836-9 .
Eduard Vinaricky, A. Keil, WA Merl: Electrical contacts, materials and applications . Springer-Verlag, 2002, ISBN 978-3-540-42431-4 .

Web links

Contact resistance. Retrieved May 16, 2015 .