In electronics, a switch
is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another. [1] [2] The most familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts. Each set of contacts can be in one of two states: either 'closed' meaning the contacts are touching and electricity can flow between them, or 'open', meaning the contacts are separated and nonconducting.
Since the advent of digital logic in the 1950s, the term has spread to a variety of digital active devices such as transistors and logic gates whose function is to change their output state between two logic levels or connect different signal lines, and even computers, network switches, whose function is to provide connections between different ports in a computer network. [3] The term 'switched' is also applied to telecommunications networks, and signifies a network that is circuit switched, providing dedicated circuits for communication between end nodes, such as the public switched telephone network. The common feature of all these usages is they refer to devices that control a binary state: they are either on
or off
, closed
or open
, connected
or not connected
.
|
SWITCH TICKETS
EVENT | DATE | AVAILABILITY |
---|
Switch Tickets 8/10 | Aug 10, 2024 Sat, 7:30 PM | | Switch Tickets 8/11 | Aug 11, 2024 Sun, 7:00 PM | | Switch Tickets 8/12 | Aug 12, 2024 Mon, 7:30 PM | | Switch Tickets 8/13 | Aug 13, 2024 Tue, 8:00 PM | | Switch Tickets 8/15 | Aug 15, 2024 Thu, 8:30 PM | |
|
Contacts
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 [4].
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).
Arcs and quenching
When the wattage being switched is sufficiently large, the electron flow across opening switch contacts is sufficient to
ionize the air molecules across the tiny gap between the contacts as the switch is opened, forming a
gas plasma, also known as an
electric arc. The plasma is of low resistance and is able to sustain power flow, even with the separation distance between the switch contacts steadily increasing. The plasma is also very hot and is capable of eroding the metal surfaces of the switch contacts.
Where the voltage is sufficiently high, an arc can also form as the switch is closed and the contacts approach. If the voltage potential is sufficient to exceed the
breakdown voltage of the air separating the contacts, an arc forms which is sustained until the switch closes completely and the switch surfaces make contact.
In either case, the standard method for minimizing arc formation and preventing contact damage is to use a fast-moving switch mechanism, typically using a spring-operated
tipping-point mechanism to assure quick motion of switch contacts, regardless of the speed at which the switch control is operated by the user. Movement of the switch control lever applies tension to a spring until a tipping point is reached, and the contacts suddenly snap open or closed as the spring tension is released.
As the wattage being switched increases, other methods are used to minimize or prevent arc formation. A plasma is hot and will rise due to
convection air currents. The arc can be quenched with a series of nonconductive blades spanning the distance between switch contacts, and as the arc rises its length increases as it forms ridges rising into the spaces between the blades, until the arc is too long to stay sustained and is extinguished. A
puffer
may be used to blow a sudden high velocity burst of gas across the switch contacts, which rapidly extends the length of the arc to extinguish it quickly.
Extremely large switches in excess of 100,000 watts capacity often place the switch contacts in something other than air to increase the resistance against arc formation, such as enclosing the switch contacts in a vacuum, or immersing the switch contacts in mineral oil.
Contact arrangements
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 are classified according to the arrangement of their contacts in electronics.
Electricians installing building wiring 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 described 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.
A switch with both types of contact is called a
changeover switch. These may be "make-before-break" which momentarily connect both circuits, or may be "break-before-make" which interrupts one circuit before closing the other.
The terms
pole
and
throw
are also used to describe switch contact variations. A
pole
is a set of contacts and terminals that are connected to a single circuit. A
throw
is one of two or more positions that the switch can adopt. A single-throw switch has one position that closes contacts, a double-throw switch has two position, and so on.
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.
Electronics specification and abbreviation
| Expansion of abbreviation
| British mains wiring name
| American electrical wiring name
| Description
| Symbol
| IEC 60617
|
SPST
| Single pole, single throw
| One-way
| Two-way
| A simple on-off switch: The two terminals are either connected together or not connected to anything. An example is a light switch.
|
|
|
SPDT
| Single pole, double throw
| Two-way
| Three-way
| A simple changeover switch: C (COM, Common) is connected to L1 or to L2.
|
|
|
SPCO
SPTT, c.o.
| Single pole changeover or
Single pole, centre off or
Single Pole, Triple Throw
|
|
| Similar to SPDT
. Some suppliers use SPCO/SPTT
for switches with a stable off position in the centre and SPDT
for those without.
|
|
|
DPST
| Double pole, single throw
| Double pole
| Double pole
| Equivalent to two SPST
switches controlled by a single mechanism
|
|
|
DPDT
| Double pole, double throw
|
|
| Equivalent to two SPDT
switches controlled by a single mechanism: A is connected to B and D to E, or A is connected to C and D to F.
|
|
|
DPCO
| Double pole changeover or
Double pole, centre off
|
|
| Equivalent to DPDT
. Some suppliers use DPCO
for switches with a stable off position in the centre and DPDT
for those without.
|
|
| Intermediate switch
| Four-way switch
| DPDT
switch internally wired for polarity-reversal applications: only four rather than six wires are brought outside the switch housing; with the above, B is connected to F and C to E; hence A is connected to B and D to C, or A is connected to C and D to B.
|
|
|
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".
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" (or normally-open or NO) switch, which makes contact when the button is pressed and breaks when the button is released. Each key of a 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.
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. These types can be used for precision works.
It can also be used where arcing is dangerous (such as in the presence of explosive vapour) as the entire unit is sealed.
Knife switch
Knife switches consist of a flat metal blade, hinged at one end, with an insulating handle for operation, and a fixed contact. When the switch is closed, current flows through the hinged pivot and blade and through the fixed contact. Such switches are usually not enclosed. The parts may be mounted on an insulating base with terminals for wiring, or may be directly bolted to an insulated switch board in a large assembly. Since the electrical contacts are exposed, the switch is used only where people cannot accidentally come in contact with the switch.
The knife and contacts are typically formed of copper, steel, or brass, depending on the application. Fixed contacts may be backed up with a spring. Several parallel blades can be operated at the same time by one handle.
Knife switches are made in many sizes from miniature switches to large devices used to carry thousands of amperes. In electrical transmission and distribution, gang-operated switches are used in circuits up to the highest voltages.
The disadvantages of the knife switch are the slow opening speed and the proximity of the operator to exposed live parts. Metal-enclosed safety disconnect switches are used for isolation of circuits in industrial power distribution. Sometimes spring-loaded auxiliary blades are fitted which momentarily carry the full current during opening, then quickly part to rapidly extinguish the arc.
Others
A simple
semiconductor switch is a
transistor.
Other types of switch include:
- Centrifugal switch
- DIP switch
- Hall-effect switch
- Inertial switch
- Membrane switch
- Toggle switch
- Transfer switch
- Time switch
- Vandal resistant switch
- Latching switch
Intermediate switch
A DPDT switch has six connections, but since polarity reversal is a very common usage of DPDT switches, some variations of the DPDT switch are internally wired specifically for polarity reversal. 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).
Light switches
In building wiring, light switches are installed at convenient locations to control lighting and occasionaly other circuits. By use of multiple-pole switches, control of a lamp can be obtained from two or more places, such as the ends of a corridor or stairwell.
Power switching
When a switch is designed to switch significant power, the transitional state of the switch as well as the ability to stand continuous operating currents must be considered. 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.
For this reason, most power switches (most light switches and almost all larger switches) have spring mechanisms in them to make sure the transition between on and off is as short as possible regardless of the speed at which the user moves the rocker.
Power switches usually come in two types. A momentary on-off switch (such as on a
laser pointer) usually takes the form of a button and only closes the circuit when the button is depressed. A regular on-off switch (such as on a
flashlight) has a constant on-off feature. Dual-action switches incorporate both of these features.
Inductive loads
When a strongly
inductive load such as an
electric motor is 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.
Contact bounce
Contact bounce (also called
chatter
) is a common problem with mechanical switches and
relays. 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
[5].
Sequential digital logic circuits are particularly vulnerable to contact bounce. 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.
There are a number of techniques for debouncing (mitigating the effects of switch bounce). They can be split into wet contacts, timing based techniques and
Hysteresis based techniques.
Wet contacts
Mercury wetted switch contacts do not suffer from bounce, as once the connection is made the
mercury maintains a conductive path despite the mechanical bounce. However, mercury wetted switches, along with
mercury-wetted relays and
mercury switches are prohibited by
RoHS because of mercury's toxicity.
Timing based
Resistor and capacitor
If an on/off switch is used with a pull up (or pull down) resistor and a single capacitor is placed over the switch (or across the resistor, but this can cause nasty spikes of current on the power supply lines) then when the switch is closed (generally pressed) the capacitor will almost instantly discharge through the switch. 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. A compromise that is preferred by most electrical design engineers is to replace the capacitor in the diagram with a resistor plus capacitor in series. The series resistor serves to protect the contact from a large surge of current. A resistance of one or two ohms is commonly chosen, but the RC time is calculated with reference to the external resistance, not the small resistor inserted. For TTL level signals good results are obtained by using a 10K ohm (external) resistor and a one microFarad capacitor in series with a 2 ohm (internal) resistor.
Sampling
Arguably the simplest way to debounce a switch transition, either in hardware or software, is merely to sample the switch state at intervals longer than any possible train of bounces. This guarantees that any bouncing affects at most one sample, which must agree either with the previous sample or with the following sample. Either case results in only one clean transition in the sampled data. A simple hardware implementation is a single D-type flip-flop clocked at a suitable rate; and software sampling is easy to program. For most switches, a suitable sampling rate can easily introduce less latency than a human being can perceive.
Hysteresis
Alternatively, it is possible to build in
hysteresis by making the position where a press is detected separate from that where a release is detected. 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).
Changeover switch
A changeover switch provides two distinct events, the making of one contact and the breaking of the other. These can be used to feed the inputs of a
flip-flop. This way the press will only be detected when the pressed contact is made and the release will only be detected when the released contact is made. 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.
Variable resistance
Normal switches are designed to give a hard on-off but it is also possible to design one that varies more gradually between the hard-on and hard-off states. This keeps the output changes caused by bouncing small. Then by feeding the output to a
Schmitt trigger the effect of those bounce based changes can be eliminated.
References
- Switch
- Switch
- 'Switch'
- General Electric Contact Materials
- Walker, PMB, ''Chambers Science and Technology Dictionary'', Edinburgh, 1988, ISBN 1852961503