2008年11月10日星期一

Switch

The switch is referred to as a "gate" when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i.e. a computer is a system of electronic switches which function as logical gates.

Three tactile switches. Major scale is inches.
A railroad switch is not electrical, but a mechanical device to divert a train from one track to another.


Contacts

A toggle switch in the "on" position.
In the simplest case, a switch has two pieces of metal called contacts that touch to make a circuit, and separate to break the circuit. The contact material is chosen for its resistance to corrosion, because most metals form insulating oxides that would prevent the switch from working. Contact materials are also chosen on the basis of electrical conductivity, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity[1].

Sometimes the contacts are plated with noble metals. They may be designed to wipe against each other to clean off any contamination. Nonmetallic conductors, such as conductive plastic, are sometimes used.


Actuator
The moving part that applies the operating force to the contacts is called the actuator, and may be a toggle or dolly, a rocker, a push-button or any type of mechanical linkage (see photo).


Contact arrangements


Triple Pole Single Throw (TPST or 3PST) knife switch used to short the windings of a 3 phase wind turbine for braking purposes. Here the switch is shown in the open position.
A pair of contacts is said to be 'closed' when there is no space between them, allowing electricity to flow from one to the other. When the contacts are separated by an insulating air gap, an air space, they are said to be 'open', and no electricity can flow at typical voltages.

Switches can be and are classified according to the arrangement of their contacts in electronics fields— but electricians in the electrical wiring service business and their electrical supplier industries use different nomenclature, such as "one-way", "two-way", "three-way" and "four-way" switches—which have different meanings in North American and British cultural regions as is delineated in the table below.

Some contacts are normally open (Abbreviated "n.o." or "no") until closed by operation of the switch, while others are normally closed ("n.c. or "nc") and opened by the switch action, where the abbreviations given are commonly used on electronics diagrams for clarity of operation in assembly, analysis or troubleshooting. The serve to synchronize meaning with possible mistakes in wiring assembly, where wiring part of switch one way and part another (usually opposite) way will pretty much guarantee things won't work as designed.

A switch with both types of contact is called a changeover switch or "make-before-break" switch contact, whereas most switches have a spring loaded action which momentarily disconnect the load and so are "break-before-make" types by contrast— which type is used could be important, if for example, the switch selects two different power sources instead of switching circuit loads, or the circuit load will not and cannot tolerate any interruption in applied power.

The terms pole and throw are also used to describe switch contact variations. A pole is a set of contacts, the switch's electrical terminals that are connected to and belong to a single circuit, usually a load. A throw is one of two or more positions (the nomenclature is also applied to rotary switches, which can have many 'throw' positions) that the switch can adopt, which normally, but not always correspond to the number positions the switch handle or rotor can take when connecting between the common lead of the switch and a pole or poles. A throw position which connects no terminals (poles), has a mis-match between positions and positions which connect terminals, but are quite useful to turn things "Off" or for example, alternatively select between two scaled modes of operation. (e.g. Bright illumination, moderate illumination, no illumination.)

These terms give rise to abbreviations for the types of switch which are used in the electronics industry such as "single-pole, single-throw" (SPST) (the simplest type, "on or off") or "single-pole, double-throw" (SPDT), connecting either of two terminals to the common terminal. In electrical power wiring (i.e. House and building wiring by electricians) names generally involving the suffixed word "-way" are used; however, these terms differ between British and American English and the terms two way and three way are used in both with different meanings.

Switches with larger numbers of poles or throws can be described by replacing the "S" or "D" with a number or in some cases the letter "T" (for "triple"). In the rest of this article the terms SPST, SPDT and intermediate will be used to avoid the ambiguity in the use of the word "way".


Make-before-break, break-before-make
In a multi-throw switch, there are two possible transient behaviors as you move from one position to another. In some switch designs, the new contact is made before the old contact is broken. This is known as make-before-break, and ensures that the moving contact never sees an open circuit (also referred to as a shorting switch). The alternative is break-before-make, where the old contact is broken before the new one is made. This ensures that the two fixed contacts are never shorted to each other. Both types of design are in common use, for different applications.


Biased switches
A biased switch is one containing a spring that returns the actuator to a certain position. The "on-off" notation can be modified by placing parentheses around all positions other than the resting position. For example, an (on)-off-(on) switch can be switched on by moving the actuator in either direction away from the centre, but returns to the central off position when the actuator is released.

The momentary push-button switch is a type of biased switch. The most common type is a push-to-make switch, which makes contact when the button is pressed and breaks when the button is released. A push-to-break switch, on the other hand, breaks contact when the button is pressed and makes contact when it is released. An example of a push-to-break switch is a button used to release a door held open by an electromagnet. Changeover push button switches do exist but are even less common.


Special types
Switches can be designed to respond to any type of mechanical stimulus: for example, vibration (the trembler switch), tilt, air pressure, fluid level (the float switch), the turning of a key (key switch), linear or rotary movement (the limit switch or microswitch), or presence of a magnetic field (the reed switch).


Mercury tilt switch
The mercury switch consists of a drop of mercury inside a glass bulb with 2 contacts. The two contacts pass through the glass, and are connected by the mercury when the bulb is tilted to make the mercury roll on to them.

This type of switch performs much better than the ball tilt switch, as the liquid metal connection is unaffected by dirt, debris and oxidation, it wets the contacts ensuring a very low resistance bounce free connection, and movement and vibration do not produce a poor contact.


Knife switch
Knife switches are unique, because rather than employing an enclosed circuit connection area with a rubber- or plastic-insulated section for the user, the contacts and bridge are fully exposed.

The "knife", a flat metal swinging arm, is moved by the user between two or more contact areas. The knife and contacts are typically formed of copper, steel, or brass, depending on the application.

The primary advantage of a knife switch is the extremely high current capability inherent to the design. The amount of surface area on the "knife" that shorts the contacts is also extremely high, allowing a wide range of high voltage or high amperage applications with no circuit degradation, choke, or arcing during the switch throw. Thicker components need only be accompanied by wider contacts to conduct higher currents, which allows the design to scale extremely well with size.

Although knife switches are inferior to traditional switches in applications where user safety is paramount, knife switches are still commonly employed in everyday high-voltage applications such as building transformers, large power relays, and air-conditioning units.

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