Switch Topics

• bright illumination
• rotary switches
• moving contact
• single pole single throw
• single pole double throw
• electrical terminals
• electronics industry
• actuator
• electrical power
• nomenclature
• parentheses
• rotor
• abbreviations
• match
• modes of operation
• contacts
• lead

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Switch, Switch

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.

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.

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 most common type is a "push-to-make" (or normally-open or NO) switch, which makes contact when the button is pressed and breaks when the button is released. Each key of your computer keyboard, for example, is a normally-open "push-to-make" switch. A "push-to-break" (or normally-closed or NC) 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.

These crossover switches only have four terminals rather than six. Two of the terminals are inputs and two are outputs. When connected to a battery or other DC source, the 4-way switch selects from either normal or reversed polarity. Intermediate switches are also an important part of multiway switching systems with more than two switches (see next section).

Through one switch A connects to B or C, through the other also to B or C; the light is on if B connects to C, i.e. if A connects to B with one switch and to C with the other.

In the first method one of the three wires just has to pass through the switch, which tends to be less convenient than being connected. When multiple wires come to a terminal they can often all be put directly in the terminal. When wires need to be joined without going to a terminal a crimped joint, piece of terminal block, wirenut or similar device must be used and the bulk of this may require use of a deeper backbox.

Another problem with this method is that in both switches there will be hot and neutral wires entering a single switch, which can lead to a short circuit in the event of switch failure, unlike the other methods.

In nearly any and all applications, neutral conductors should never be switched. Not only is this a shock hazard due to mistakenly believing that a hot conductor is switched off; it is also a fire hazard and can destroy sensitive equipment due to excessive and unbalanced current on hot conductors that would otherwise return to ground on the neutral conductor.

When a switch is in the on state its resistance is near zero and very little power is dropped in the contacts; when a switch is in the off state its resistance is extremely high and even less power is dropped in the contacts. However when the switch is flicked the resistance must pass through a state where briefly a quarter (or worse if the load is not purely resistive) of the load's rated power is dropped in the switch.

A regular on-off switch (such as on a flashlight) has a constant on-off feature. Dual-action switches incorporate both of these features.

When switched off, the current cannot drop instantaneously to zero; a spark will jump across the opening contacts. Switches for inductive loads must be rated to handle these cases. The spark will cause electromagnetic interference if not suppressed; a snubber network of a resistor and capacitor in series will quell the spark. Exact values can be optimised for the particular application, but for many cases a 100 ohm resistor in series with a 100 nanofarad capacitor will do.

In Europe the converse is true; the up position generally corresponds to a switch being off. A purely hypothetical argument for using the USA convention is that the switch should move to its off position if its spring fails, and so this convention provides a safe failure mode. However, this hypothesis has not been validated, and there is little evidence that suggests this hypothesis the reason for the USA convention. So because there is no significant technical reason for either preference, the conventions likely developed due to chance and some degree of cultural isolation.

In a 3-way configuration, whether lights are on or off will correspond to whether the switches are in the same state (both up or down) or a different state (one up and one down). Multiway switching conventions will often vary from installation to installation; some electricians may not have a multiway switching convention.

Switch and relay contacts are usually made of springy metals that are forced into contact by an actuator. When the contacts strike together, their momentum and elasticity act together to cause bounce. The result is a rapidly pulsed electrical current instead of a clean transition from zero to full current. The waveform is then further modified by the parasitic inductances and capacitances in the switch and wiring, resulting in a series of damped sinusoidal oscillations. This effect is usually unnoticeable in AC mains circuits, where the bounce happens too quickly to affect most equipment, but causes problems in some analogue and logic circuits that respond fast enough to misinterpret the on-off pulses as a data stream Walker, PMB, Chambers Science and Technology Dictionary , Edinburgh, 1988, ISBN 1852961503.

The voltage waveform produced by switch bounce usually violates the amplitude and timing specifications of the logic circuit. The result is that the circuit may fail, due to problems such as metastability, race conditions, runt pulses and glitches.

However, mercury wetted switches, along with mercury-wetted relay and mercury switches are prohibited by RoHS because of mercury's toxicity.

But when the switch is opened (generally released) the capacitor takes some time to recharge. Therefore contact bounce will have negligible effect on the output. The slow edges can be cleaned up with a Schmitt trigger if necessary. This method has the advantage of fast response to the initial press but the current surges through the switch may be undesirable. Other RC based systems are also possible with various responses and such systems are probably the easiest method when constructing with simple logic gates and discrete components.

As long as the bounces are small enough not to take the switch between these positions, bounce problems will be eliminated. Hysteresis can be mechanical or electronic (e.g. a Schmitt trigger).

When the switch is bouncing around in the middle no change is detected. To get a single logic signal from such a setup a simple SR latch can be used.

Source: Wikipedia > Switch