ELECTRICAL CONTACTS

Electrical contacts are all around us, just consider

• the wall socket
• the flasher relay in the car
• the phone keypad
• the radio volume control
• the commutator brushes
in the drilling-machine
• the video SCART-connector
• the processor socket in the computer

Normally we consider the existence of electrical contacts only on the few occasions they are not working properly.

• The starting motor doesn't start when you turn the key, of course this doesn't happen to the mechanic at the garage!
• The radio makes a noise when you turn the volume control knob.
• The computer turns off by itself when you happen to bump against it.

An electrical contact is needed whenever electric energy is to be transmitted. Examples are power supply to electrical lighting or to a motor, signal transmission between and within electrical apparatus, grounding of cabinets for electrical appliances.

For transmitting a signal between two points, for example in a computer, several electrical contacts in series are required.

A contact is basically two surface pressed against each other. The surfaces are normally metal, one other example is graphite in carbon brushes (commutator brushes for electrical motors). There are however contacts working by another principle, e.g. solder joints. Contact of this type, where the metal is melted when closing the contact, will not be included in the following discussion.

A force pressing the contact surfaces together, a contact force, is required to produce a contact in the first place. The force flattens the hills of the surfaces, the area of the flattened hills equals the contact area.

It is however not sufficient to close the contact and then remove the force. In order to preserve the contact spots there must be a permanent force normal to the surface. If not, the elastic forces of the surface will press the asperities apart.

A force is also needed for wiping insulating surface films, oxides, dust etc., always present on surfaces exposed to the atmosphere. Films are formed by corrosion, the atmosphere attacking the metal producing oxides, sulphides and other corrosion products. Contamination such as dust, oil films etc. depositing on the contact surfaces also form surface films.
As the contact ages the films gets thicker and more difficult to wipe.

The contact force is obtained from a spring. The spring force, giving the contact force, decreases in the course of time, reducing the contact area. A lower contact force further makes wiping of surface films more difficult. Consequently the contact resistance increases and the risk for intermittent contact opens when disturbing contact surfaces (exposing contacts to bumbs, vibration, changes in temperature etc.) increases.

Ageing of contacts is however a slow process. Increases in contact resistance is normally not noticed until the main part of the active contact area has been lost. Contact problems show as noise, intermittent contact opens (bit errors), or as temperature rise, finally as permanent contact opens.

Considering the small size of the contact area, and the stress contacts often are exposed to, from wear, dust, humidity, air contaminants etc., it is more surprising that contacts work as well as they do, than that they don't.