Principles
Switching is part of a sensor action for many devices, so that the topics relating to switch contacts are of considerable importance in specifying some types of sensors. In addition, switches, as an electromechanical component, are a vital part of any sensing or transducing equipment.
Switches are almost as old as present-day electricity applications, which is probably why their action is taken so much for granted. The function of a switch is to make or break current in a circuit, and in the early 19th century this would have meant only a DC circuit. Later, switching had to be applied to AC, later still to audio and radio frequencies and video signals, and most recently to digital signals. Early switches were simple mechanical devices, the knife switches that can still be seen in old factories. This century has seen the development of much more elaborate mechanical principles, and of switches with no mechanical moving parts at all. In this book, however, we are mainly concerned with mechanically-operated switches, and although some electronic types are dealt with, relays and contactors have been excluded.
The switch, then, is a device with a circuit path that can be made or broken. When the circuit path is made, the circuit resistance through the switch is low, of the order of mO for most switches. With the circuit path broken, the circuit path through the switch has a high resistance, several MO or higher. In addition, the resistance between the switch circuit path and the body of the switch, including the operating mechanism and the mountings, is generally required to be high. This resistance, and the maximum voltage that can be applied to this insulation, is often of major importance in mains voltage switches, and is a vital feature of legislation regarding switch suitability. For some applications to high-frequency signals the capacitance between the switch circuits and the switch body is of considerable importance, and will decide the design of the switch.
Contact resistance
The resistance of the (mechanical) switch circuit when the switch is on (made) is determined by the switch contacts, i.e., the moving metal parts in each part of the circuit which will touch when the switch is on. The amount of the contact resistance depends on the area of contact, the contact material, the amount of force that presses the contacts together, and also the way that this force has been applied. For example, if the contacts are scraped against each other (a wiping action) as they are forced together, then the contact resistance can often be much lower than can be achieved when the same force is used simply to push the contacts straight together. In general, large contact areas are used only for high-current operation, and the contact areas for low-current switches (as used for elec- tronics circuits) will be small.
The true area of electrical connection will not be the same as the physical area of the contacts, because it is generally not possible to construct contacts that are precisely flat, or have surfaces that are perfectly parallel when the contacts come together. This problem will be familiar to anyone who has renewed the ignition points in a car in the days before electronic ignition circuits. The actual area of contact of the switch is revealed by the contact burning when the switch is used in inductive circuits, but is not necessarily obvious in other cases. Since the size of the switch to a large extent determines the amount of contact pressure that can be used, and the area of contact is rather indeterminate, the main factor that affects contact resistance is the material used to make the contacts themselves.
WIPING CONTACTS
One method of reducing contact resistance in difficult conditions is to use a wiping action. As the name suggests, the mechanical action of the switch is such that the contacts are rubbed against each other as they make connec- tion. This will normally remove any thin films of oxide or other contaminants, and so ensure a much lower contact resistance than could be obtained by bringing the contacts together in a more straightforward manner. The disadvantage of a wiping action is that it can abrade the contacts, and if these consist of steel or nickel-alloy plated with gold, the gold plating can be removed by this abrasion. Wiping contacts are best suited to switches that are infrequently used and are located in contaminating environments, and those in which silver is used as the contact material. The switch designer has the choice of making the whole of a contact from one material, or of using electroplating to deposit a more suitable contact
SWITCH PRINCIPLES
material. By using electroplating, the bulk of the contact can be made from any material that is mechanically suitable, and the plated coating will provide the material whose resistivity and chemical action is more suitable to the electrical function of the switch. Plating also makes it possible to use materials such as gold and platinum, which would make the switch impossibly expensive if used as the bulk material for the contacts. It is normal, then, to find that contacts for switches are constructed from steel or from nickel alloys, with a coating of material that will supply the necessary electrical and chemical properties for the contact area.
The choice is never easy, because the materials whose contact resistance is lowest are, in general, those that are most vulnerable to chemical attack by the atmosphere. In addition, some materials that can provide an acceptably low contact resistance exhibit sticking, so that the contacts do not part readily and may weld shut.
The other main problems are burning and oxidation. The spark current that passes at the time when contacts are opening can induce melting of the contacts, or cause the metal to combine chemically with the oxygen in the atmosphere (oxidation or burning). Oxidation, rather than melting, is the more severe problem, because the oxides of most metals are non-conduc- tors. This means that oxidation causes a large rise in contact resistance, even to the point of making the switch useless.
The usual choice of materials is illustrated in Table 10.1. From the point of view of resistance alone, silver is the preferred material, since silver has the lowest resistivity of all metals. Unfortunately, silver is very badly corroded by the atmosphere, particularly if any traces of sulphur dioxide exist (where coal or oil is burned, for example), and silver contacts will
have a short life if there is any arcing at the contacts (see below). Silver contacts are desirable if the lowest contact resistance is needed for high- current use, but the action of the switch will have to be such that the contact pressure is high, and the contacts are wiped as they are brought together. Silver-coated contacts can be used with fewer problems when the switch is sealed in such a way as to exclude atmospheric contamination, or if the contacts are surrounded by inert gases, but these are not simple solutions to the corrosion problem.
Gold plating is a very common solution to the contact problem, particularly in switches for low-current electronics use. The contact resistance can be moderately low, and the gold film is soft, so that moderate pressure can result in a comparatively large area of contact. Of all metals, gold is about the most resistant to corrosion, although the combination of hydrochloric acid and electric current can cause gold to be attacked quite rapidly, making this material unsuitable if the atmosphere contains traces of chlorine or hydrochloric acid.
Since passing electric current through sea-water causes chlorine to be generated, gold-plated contacts can be severely corroded when the switches are used at sea. The switches are suitable only for the lower currents, but since most switch applications in electronics are for low currents this is no handicap. Gold plating of contacts is often mandatory in the specification of switches for military contracts.
Contact coatings based on the ‘noble’ metals are often employed. These metals are so named because, like gold, they exhibit a high resistance to chemical attack; typical metals in this group are platinum, palladium, iridium and rhodium. All of them have rather high contact resistance, but are very stable and resist chemical attack. Platinum is particularly useful for contacts that will be used for low currents and for high voltage levels. Rhodium and iridium platings provide a high level of resistance to corrosion along with stable contact resistance, and are suitable for medium-voltage, medium-current applications. Some advantages can be gained by using thick films of contact materials that are alloys of the noble metals with silver. For example, palladium-silver has a much better resistance to contamination than silver itself, although with a higher contact resistance than silver. It is one of the general contact materials that can be used in switches for various types of application.
The metals tungsten and molybdenum, although not of the platinum group, are also used as contact materials for special purposes. Tungsten in particular is very resistant to burning caused by contact arcing, and is used for high-power switching. Its disadvantage is that the surface will oxidize easily, causing contact-resistance problems. The most common general- purpose contact alloy material, however, is nickel silver. The cost of nickel silver is such that it can be used as a bulk material rather than as a coating, and although its contact resistance is higher than that of pure silver, it is much more resistant to chemical attack than the pure metal. It also resists burning, and the contacts do not tend to stick together.
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