AC DC Power Supply
Power supplies are electrical circuits and devices that are designed to convert mains power or electricity from any electric source to specific values of voltage and current for the target device...
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This article will give an in-depth view at electric switches.
The article will discuss topics such as:
This chapter will discuss what electric switches are and their operation.
An electric switch is a device – usually electromechanical – that is used to open and close an electric circuit. This disables and enables the flow of electric current, respectively.
Switches are synonymous with the interruption, or some form of manipulation, of the flow of electric current, or more elementarily, the flow of electrons.
The appreciation of the underlying principle behind a switch comes along with electricity and use of conducting material for electric current to flow. A discontinuity in the conductor implies the current does not flow. Needing a switch is as old as the discovery of the first practical application of electricity and the construction of the first electrical circuit.
By inference, the switch described above effectively puts an air gap on some parts of the circuit. The air gap has different electrical properties to the conducting material building the circuit and, if the air gap is big enough, it stops current flow. Fundamentally, that is what a switch does i.e. alter electrical properties of some part of a circuit to alter the flow of electrons in that circuit. Appreciating this would be important taking cognizance of switches whose operation never involves physical movement.
Switches provide a means to control electrical current supply to electric loads. One crucial aspect is to conduct electric current. The other aspect is the ability to break the current. This should happen as desired.
For many electric switches, breaking the circuit involves putting an air gap between two contacts. The contacts must be opened fast enough to ensure the desired operation.
For most electronic switches, the state of the connection is changed by changing the effective resistance of that connection. The resistance can be made very high to effectively make it an open circuit or made low enough to close the circuit. It is common to have no physically moving components on the switch itself.
Another important aspect is the ability to respond to the actuator. The actuator may be automatic or manual and is intended to make or break the circuit. The purpose of the actuator is to initiate some change in the state of the connection. Actuation may take the form of physical movement e.g., a lever or a slide. It can also take the form of some other event, e.g., some overvoltage or light intensity variation.
In the case that equipment to be connected through that switch may not be protected, a fuse is often incorporated as part of the switch.
This chapter will discuss the ratings and categories of electric switches.
Electric switches have ratings that need to be considered in electric switch selection and application. These ratings are mainly:
Current Rating is the maximum electric current that the switch is designed to carry. The switch might start incurring physical damage when this limit is exceeded. Such damage includes overheating, deformation, and melting of some of the components of that switch.
For a circuit breaker, the rating is the current at above which the breaker "trips". Tripping, in this context, means breaking up the circuit. There often is an allowance from the rated current before the breaker actually trips. This allowance is often expressed as a percentage of the rated current.
For some electronic switches, the current rating may show the value above which the actuator can no longer switch the connection off. Alternatively, it is the maximum current that the switch is designed to handle.
Voltage Rating is the maximum voltage that the switch is designed to withstand. Higher voltages cause more sparks than desirable.
For some electronic switches, such as the thyristor, the voltage rating is the maximum voltage that the switch can block. Any higher voltage will switch the thyristor on without the intended actuation. The intended actuation is often the presence of a gate voltage.
The same electrical switch can have different ratings for AC and DC circuits.
AC Circuits currents fluctuate. It follows that the voltage is zero at some point, twice every cycle (the current behaves similarly). Therefore, there are times when the electric field is zero (whenever the voltage is zero). To some extent, this behavior helps extinguish arching, especially when breaking the circuit.
DC Circuits has a steadier unidirectional current. Thus, arching can be more prolonged. This calls for faster switching speeds (time taken to open and/or close the connection). It follows that the maximum DC voltage that a switch can handle is smaller than the maximum AC voltage that the same switch can handle.
Electric switches can be divided into various categories. The main categories are detailed below.
Momentary means having one state that it always maintains as long as it is not energized. It only changes to the other state when energized and reverts back to the initial state when de-energized. One of the earliest famous examples of momentary switches was used on the telegraph machine. Momentary switches usually take two forms:
This switch only closes when energized, otherwise it stays open. The telegraph mentioned earlier used a normally open switch.
This switch is always closed, unless energized. It is often used for ‘stop‘ switches – that is, switches used to stop a task on a machine, or shut the machine down.
This switch maintains any given state and is energized to change state. Domestic light switches and car ignition switches traditionally worked this way.
The previously discussed categories of switches can be distinguished and discussed according to their types. These types are detailed below.
These connect electrical circuits. Distinguishable from electronic switches by the latter‘s usage of electrical signals in sending instructions and informing decision making.
These switches only have an actuator and electrical conduct. They do not have any kind of protection.
It is often the case that protection mechanisms are part of the circuits where plain switches are installed.
These switches add a fuse to a plain switch. This is done when the intended load is not protected any other way.
These switches have a mechanism to break the circuit in the event of a surge. Current breakers are more common than voltage breakers.
The actuator in a breaker is mounted with a recoil mechanism, such as a recoil spring. It has another mechanism to maintain the switch once it is reset. The operation is such that the recoil spring breaks the circuit when the mechanism that maintains the switch in the on position is disengaged. The said disengagement is achieved by a third mechanism, which is actuated by a surge.
This switch compares the difference between the current being sent to the load and the current coming from the load. Such differences imply leakage of current, in the presence of which the switch would cut the current to the load.
These switches connect electronic circuits or use electronic signals to connect electrical circuits. Electronic switches use certain current and/or voltage signals, as either inputs or outputs, to actuate certain actions in the operation of the circuit.
Usually, the actuation is achieved by sending an electrical current to a "gate" of the switch. This gate current is often much smaller than the current it is meant to control, and the former is usually a direct current, whereas the latter can be either AC or DC.
These switches connect one circuit only at a time.
This switch has one input and one output. This implies one circuit. The switch makes and breaks the circuit, as the actuator may intend.
This switch has one input but two different outputs. However, the two outputs cannot be connected to the input at the same time. It follows that there are two possible circuits that the switch may make or break but never make the two circuits at the same time.
These switches connect multiple circuits simultaneously.
This is a single switch that connects two circuits simultaneously. It has two inputs and two outputs. It makes or breaks both circuits at the same time. It is technically equivalent to two rocker switch levers mechanically joined together such that they move together. Such an arrangement can be visible on some switching panels.
These have two inputs and four outputs. They are like two single pole double throw switches being operated by the same actuator.
These are an extension of the double pole switch where the number of circuits is more than two and, at least in theory, can be any number.
This is not a common setup, despite it being technically feasible. It is often replaced by an array of parallel switches such as the common branches, with each branch having its own switch and outlet.
Having a single switch with a single input and many outlets comes with the disadvantages associated with a common point of failure. It also inhibits the isolation of any one outlet.
This is an array of switches physically configured onto the same casing. It is often found on the supply mains, be it domestic or commercial. Panels often contain switches that control related circuits. It may also feature multiway switching.
This is a set of switches and fuses and similar equipment. The basic underlying principle (and intent) is that of a switch. However, the term "switchgear" is associated with high voltage applications, power generation and distribution. Given the difference in the behavior and effect of electricity when the voltage becomes very high, certain distinct features will be required that may not necessarily be associated with household switches.
Switchgear is often mounted outdoors and can be operated either manually or remotely. Of particular importance in the operation of switchgear is the diffusion of arcing during circuit breaking. Arcing in switchgear demands considerable attention given the high voltages that the switchgear breaks. Switchgear breakers use various insulation media, including:
Such switches are intended for outdoor, and in some cases sub aqua, applications. The casing of the switch is watertight to protect the conducting elements of the circuitry from water.
This type of switch is so named because of its physical configuration – it is in a dual in line package (DIP).
This chapter will discuss the design of electric switches.
Relays are electrical constructions which are used in cases such as when it is necessary to control a circuit using an independent power signal. The latter is often a low power one. Briefly explained, a relay is a switch that is operated electrically.
An electromagnetic coil has been the basis of the traditional relay switch. An electrical signal would energize the coil such that it attracts some metal contact placed in a particular arrangement. On being attracted, the metal contact moves, making or breaking a certain contact in the process. There usually is a mechanism to restore contact to its normal position (that is, the de-energized state) once the attraction from the coil discontinues. The metal conduct remains attracted for as long as the coil is energized. In such an application, the relay would be working as a momentary switch.
The latching relay is a variant of the switch described above and works more like a maintained switch. It usually takes signals of opposite polarity to open and close the circuit. The absence of power does not function like an instruction (it has no effect on the controlled switch regardless of the state the latter might be in). These are used in cases where the controlled circuit needs both states for significant amounts of time each – so that the coil would not need power to maintain any one state.
In an electrical power supply, relays are used to open and/or close circuit breakers. Telecommunication systems made extensive use of electromagnetic switches until the not-so-distant past in applications like analog switching. Other applications are in railway signaling and transceiver selection.
Solid state relays are the electronic version of the electromagnetic relay. They make use of semiconductor components to achieve control of isolated circuits. An example is an opto-coupler, which couples a light emitting diode and a photodiode.
Actuator mechanisms on a switch turn a circuit on and off manually. Examples of actuator mechanisms can be found in:
These switches come in the form of a button, or a similar key. They can either be a maintained or momentary switch. The push button is the most common type of momentary switch. The normally closed push button switch is often called the push to break, and the normally open pushbutton switch is often called the push to make a switch. The maintained pushbutton switch is attached to a mechanism that alternately holds and releases with consecutive pushes.
Push buttons are usually two-state switches. Though it is technically feasible to have more than two states with a push button mechanism, this is not a popular choice.
These switches are so named because of how they move – in a rocking motion. It has two main parts, a moving part – that makes the back-and-forth rocking motion – and a fixed part, relative to which the rocking motion is made. The switch stays at either of the two extreme ends of the rocking motion, one of which is an on, and the other an off. The rocker switch is the most common wall mounted switch, particularly for domestic purposes. They can also be mounted to the particular device on which power supply is to be controlled. For the latter application, however, they compete with maintained push button switches.
As implied by the rocking motion, moving one extreme end in one direction moves the opposite end in the reverse direction. Such movements are achieved by simply pressing the desired end. The rocking component on the switch is attached to a contact, such that rocking in one direction closes a connection and rocking in the reverse direction opens it.
This switch is most commonly used where more than two states are required, or more is required than a simple on and off. The name ‘rotary‘ refers to the manner in which the states are selected, rather than to imply any number of states. Most applications with two states make use of rocker and push button switches.
A knob, or an equivalent component, rotates about some axis, such that it moves a contact around a circular array of connectors – alternately connecting them in the process. The rotation can be either smooth or stepped. It can have a definitive number of states, or have an ‘off‘ state and a continuously varying ‘on‘ state. Rotary switches are usually maintained switches. They can be wall mounted – particularly for industrial machinery or mounted to the respective devices such as an electric oven.
Another common example of a rotary switch is the traditional car ignition switch – the key and socket one.
This kind of switch uses a lever to change the state of a connection. It is practicable as either a momentary or maintained switch. It is usually a two-state switch. The toggle switch is the most common type of actuation for household circuit breakers. Compared to other electrical switches, they are capable of having high switching speeds.
For this switch, a slide is moved from one position to another, moving a contact with it and therefore changing the states of the connection. This is often a two-state switch, but it is practicable in more than two states. It is the most viable alternative to a rotary switch, though the latter remains preferable in most multi-state applications.
The motion of the slider can be smooth or stepped.
A contactor is a component of an electrical circuit, particularly a switch. Its purpose is to make and break contact. The circuit is closed when the contactors are in contact and open when they are not. Such making and breaking of contact provides a means to control the flow of electrons – and therefore electrical power and signals – in a circuit.
They are usually made up of metals, though any conductive material can be used.
The following materials are often used in the manufacture of contactors.
Copper is a very good conductor of electricity (and heat), with its conductivity being behind only that of silver. A common copper alloy used in the making of contactors is brass.
Silver is a good conductor of electricity, probably the very best. Its resistance to oxidation is good, so is that of its alloys.
Gold is also a good conductor with its conductivity being behind only copper and silver. Its resistance to corrosion is also high. Amongst switch contactors, those made of gold are not very common because of the (un)availability of gold and its cost.
Platinum is possibly the most expensive material (per unit weight) used in circuitry. The cost becomes more apparent given that volume is often more important than weight and platinum has a particularly high specific weight.
For a given material, the electrical resistance is dependent, in part, on the spatial dimensions of the conductors, and not necessarily on the weight. It may also be noted, however, that mass also depends on spatial dimensions. It follows that there (maybe) correlation between weight and resistance, but there never is dependency.
A non-metal, but a conductor nonetheless (in some of its forms). It does not compare well with metals and is mainly used in experimentation and recreational applications. There are a few specialized industrial applications however.
Most metals can be conductors and are used in an assorted array of applications. They may be used where resistance may not be a very significant consideration.
These metals include
This chapter will discuss contactor properties in switches and heat regulation in switches.
There are certain contactor properties that need to be considered in the selection and application of electrical switches. These are:
This is one of the most important characteristics of the contactor. Low resistance aids in reducing the heating up of the contactors, and subsequently, the switch.
Contacts may be exposed to high humidity and/or high temperatures during their service lives. They would need to be resistant to corrosion to retain their electrical and mechanical properties.
This is the resistance of a component to the loss of its own material due to some mechanical action. In many switch designs, the contacts have to slide onto, and away from, each other as they make and break circuits. Such sliding creates friction and abrasion, both of which are agents of wear. Resistance to such wear is crucial to the service life of the contactors, and the switch.
Electronic switches are the ones that more commonly require temperature control. This is partly because of the sensitivity of the performance of the switch itself to temperature.
It is often the case that the rating of an electronic switch is valid for a certain temperature range.
The temperature may be regulated by attaching a heat sink to the switch. The heat sink is usually a piece of metal with high conductivity and emissivity that helps the switch dissipate heat faster than it would on its own.
Alternatively, the switch can be placed in a conventionally air-conditioned environment. This helps maintain the optimum performance of the switch.
This chapter will discuss deformation, malfunction, hazards in switches.
The operation of a switch depends on the need and ability to repeatedly alternate between several predetermined physical positions and sustain this behavior over a considerable lifetime. Doing so may involve deforming the contactor itself and requiring the same conductor to return to exact positions under predetermined mechanical loading situations as input by actuators. Such deformation is usually in the form of bending, which calls for some springiness in the contactor. Alternatively, the contactor is made rigid and then attached to a spring (or several such springs).
Contactors are often isolated (electrically) from the ground and from users of the switches. It follows that they are attached to some non-conductive (usually plastic) casing or bracket. The entire housing for the switch is often plastic. However, it may be metal for certain applications such as circuit breakers, though it is always the case that the housing and actuator mechanism is electrically separated from the contacts. The ratings of the switches are attained mostly by manipulating the characteristics of the contactors together with the geometrical construction of the switch as a whole.
Several malfunctions are possible with electric switches. Understanding these possibilities helps in troubleshooting the electric switches. Examples include when it:
It happens when a contactor is broken, or deformed, or melted such that it no longer closes the connection as intended by the input from the actuator. For most switches, this malfunction is rectified by replacing the switch. For fused switches, failure to close the circuit may result in a blown fuse. This is rectified by replacing the fuse
It may also be a result of deformation of the contactors, such that the actuator may not control the contactor as intended. Alternatively, the mechanism connecting the actuator to the contactors may be broken, resulting in free play on the part of the actuator.
Some electronic switches may fail to break the circuit in the event of over currents that cause latching.
This happens when either the actuator or the contactor are deformed so the contactors no longer separate adequately as intended by the input from the actuator. This effectively compromises the rating of the switch – it starts failing before the rated parameters are breached.
A circuit breaker might fail to trip in the event of an overload. This may be caused by wear or faulty manufacturing, such that the breaker fails to detect a surge in current. It may cause the breaker to allow more current to pass or may cause the breaker to fail without breaking. In any case, such a breaker needs replacing.
This happens when some of the current is leaking to the ground. It happens when there is a short circuit in the wiring of an appliance. The naked wiring touches a conducting part of an appliance in a manner that is not intended, and the current finds an alternative path to ground. If that short circuit happens before the electrical load, it can result in a surge that can trip a breaker or blow a fuse.
This happens when there is an opening between contacts, coupled with a high enough electric field to break the air gap down and ionize the air particles causing current to jump the air gap. This jumping current will appear as a spark. It is the same underlying principle behind lightning. Reducing the air gap or increasing the voltage, or both, increases the electric field. Thus sparks become more pronounced under overloads or deformed switches which can no longer open large enough air gaps.
A switch overheats when:
A short circuit occurs when the current in a circuit finds an alternative path, one that is not intended. This usually happens when there is a failure in the insulation on some part of the circuit. It is often the purpose of a switch to respond to some kinds of short circuits.
Electric switches are prone to different hazards just like all other equipment or tools. The most common hazards are:
It may be a result of excessive sparks, or failure of the switch to respond correctly to an eventuality in the power circuit, such as a surge or a leakage. It also happens when the incorrect rating is used and the switch explodes under certain conditions.
It happens when the switch allows surges to pass through to the equipment. It is most common for plain switches that do not have protection mechanisms. It also happens when the ratings of the switches are not suitable for the intended equipment, for example, when the switch cuts power when it is required or lets it pass when it is in excess.
Such malfunction also causes electrocution. That is when the switch cannot respond to short circuits and leakage currents.
Switches have evolved and diversified considerably since their invention. They also continue to fit into cross-cutting issues affecting the globe, including environmental sustainability and automation. In as much as it may not be far-fetched to expect switches to change form considerably in the distant future, the underlying principles are likely to remain salient and the current value chains may continue to stand a good chance to keep improving.
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