Miniature snap-action switch
A miniature snap-action switch trademarked and known as a micro switch, is an electric switch, actuated by little physical force, through the use of a tipping-point mechanism, sometimes called an "over-center" mechanism. Switching happens reliably at specific and repeatable positions of the actuator, not true of other mechanisms, they are common due to their low cost and durability, greater than 1 million cycles and up to 10 million cycles for heavy duty models. This durability is a natural consequence of the design; the defining feature of micro switches is that a small movement at the actuator button produces a large movement at the electrical contacts, which occurs at high speed. Most successful designs exhibit hysteresis, meaning that a small reversal of the actuator is insufficient to reverse the contacts. Both of these characteristics help to achieve a clean and reliable interruption to the switched circuit; the first micro switch was invented by Phillip Kenneth McGall in 1932 in Freeport, patent 1,960,020.
McGall was an employee of the Burgess Battery Company at the time. In 1937 W. B. Schulte, McGall's employer, started the company MICRO SWITCH; the company and the Micro Switch trademark has been owned by Honeywell Sensing and Control since 1950. The name has become a generic trademark for any snap-action switch. Companies other than Honeywell now manufacture miniature snap-action switches. In one type of microswitch, internally there are two conductive springs. A long flat spring has electrical contacts on the other. A small curved spring, preloaded so it attempts to extend itself, is connected between the flat spring near the contacts and a fulcrum near the midpoint of the flat spring. An actuator nub presses on the flat spring near its hinge point; because the flat spring is anchored and strong in tension the curved spring cannot move it to the right. The curved spring presses, or pulls, the flat spring upward, away, from the anchor point. Owing to the geometry, the upward force is proportional to the displacement which decreases as the flat spring moves downward.
As the actuator depresses it flexes the flat spring while the curved spring keeps the electrical contacts touching. When the flat spring is flexed enough it will provide sufficient force to compress the curved spring and the contacts will begin to move; as the flat spring moves downward the upward force of the curved spring reduces causing the motion to accelerate in the absence of further motion of the actuator until the flat spring impacts the normally-open contact. Though the flat spring unflexes as it moves downward, the switch is designed so the net effect is acceleration; this "over-center" action produces a distinctive clicking sound and a crisp feel. In the actuated position the curved spring provides some upward force. If the actuator is released this will move the flat spring upward; as the flat spring moves, the force from the curved spring increases. This results in acceleration. Just as in the downward direction, the switch is designed so that the curved spring is strong enough to move the contacts if the flat spring must flex, because the actuator does not move during the changeover.
Common applications of micro switches include the door interlock on a microwave oven and safety switches in elevators, vending machines, to detect paper jams or other faults in photocopiers. Micro switches are used in tamper switches on gate valves on fire sprinkler systems and other water pipe systems, where it is necessary to know if a valve has been opened or shut. Micro switches are widely used, they are rated to carry current in control circuits only, although some switches can be directly used to control small motors, lamps, or other devices. Special low-force versions air flow. Micro switches may be directly operated by a mechanism, or may be packaged as part of a pressure, flow, or temperature switch, operated by a sensing mechanism such as a Bourdon tube. In these latter applications, the repeatability of the actuator position when switching happens is essential for long-term accuracy. A motor driven cam and one or more micro switches form a timer mechanism; the snap-switch mechanism can be enclosed in a metal housing including actuating levers, plungers or rollers, forming a limit switch useful for control of machine tools or electrically-driven machinery.
Mercury switch Reed switch Honeywell celebrates the 75th anniversary of the Micro Switch
In electrical engineering a limit switch is a switch operated by the motion of a machine part or presence of an object. They are used for controlling machinery as part of a control system, as a safety interlocks, or to count objects passing a point. A limit switch is an electromechanical device that consists of an actuator mechanically linked to a set of contacts; when an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection. Limit switches are used in a variety of applications and environments because of their ruggedness, ease of installation, reliability of operation, they can determine the presence or absence, passing and end of travel of an object. They were first used to define the limit of travel of an object. Standardized limit switches are industrial control components manufactured with a variety of operator types, including lever, roller plunger, whisker type. Limit switches may be directly mechanically operated by the motion of the operating lever.
A reed switch may be used to indicate proximity of a magnet mounted on some moving part. Proximity switches operate by the disturbance of an electromagnetic field, by capacitance, or by sensing a magnetic field. A final operating device such as a lamp or solenoid valve will be directly controlled by the contacts of an industrial limit switch, but more the limit switch will be wired through a control relay, a motor contactor control circuit, or as an input to a programmable logic controller. Miniature snap-action switch may be used for example as components of such devices as photocopiers, computer printers, convertible tops or microwave ovens to ensure internal components are in the correct position for operation and to prevent operation when access doors are opened. A set of adjustable limit switches are installed on a garage door opener to shut off the motor when the door has reached the raised or lowered position. A numerical control machine such as a lathe will have limit switches to identify maximum limits for machine parts or to provide a known reference point for incremental motions
A spark gap consists of an arrangement of two conducting electrodes separated by a gap filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the potential difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its electrical resistance. An electric current flows until the path of ionized gas is broken or the current reduces below a minimum value called the "holding current"; this happens when the voltage drops, but in some cases occurs when the heated gas rises, stretching out and breaking the filament of ionized gas. The action of ionizing the gas is violent and disruptive leading to sound and heat. Spark gaps were used in early electrical equipment, such as spark gap radio transmitters, electrostatic machines, X-ray machines, their most widespread use today is in spark plugs to ignite the fuel in internal combustion engines, but they are used in lightning arresters and other devices to protect electrical equipment from high-voltage transients.
The light emitted by a spark does not come from the current of electrons itself, but from the material medium fluorescing in response to collisions from the electrons. When electrons collide with molecules of air in the gap, they excite their orbital electrons to higher energy levels; when these excited electrons fall back to their original energy levels, they emit energy as light. It is impossible for a visible spark to form in a vacuum. Without intervening matter capable of electromagnetic transitions, the spark will be invisible. Spark gaps are essential to the functioning of a number of electronic devices. A spark plug uses a spark gap to initiate combustion; the heat of the ionization trail, but more UV radiation and hot free electrons ignite a fuel-air mixture inside an internal combustion engine, or a burner in a furnace, oven, or stove. The more UV radiation is produced and spread into the combustion chamber, the further the combustion process proceeds. Spark gaps are used to prevent voltage surges from damaging equipment.
Spark gaps are used in high-voltage switches, large power transformers, in power plants and electrical substations. Such switches are constructed with a large, remote-operated switching blade with a hinge as one contact and two leaf springs holding the other end as second contact. If the blade is opened, a spark may keep the connection between spring conducting; the spark ionizes the air, which becomes conductive and allows an arc to form, which sustains ionization and hence conduction. A Jacob's ladder on top of the switch will cause the arc to rise and extinguish. One might find small Jacob's ladders mounted on top of ceramic insulators of high-voltage pylons; these are sometimes called horn gaps. If a spark should manage to jump over the insulator and give rise to an arc, it will be extinguished. Smaller spark gaps are used to protect sensitive electrical or electronic equipment from high-voltage surges. In sophisticated versions of these devices, a small spark gap breaks down during an abnormal voltage surge, safely shunting the surge to ground and thereby protecting the equipment.
These devices are used for telephone lines as they enter a building. Less sophisticated spark gaps are made using modified ceramic capacitors. A voltage surge causes a spark that jumps from lead wire to lead wire across the gap left by the sawing process; these low-cost devices are used to prevent damaging arcs between the elements of the electron gun within a cathode ray tube. Small spark gaps are common in telephone switchboards, as the long phone cables are susceptible to induced surges from lightning strikes. Larger spark gaps are used to protect power lines. Spark gaps are implemented on Printed Circuit Boards in mains power electronics products using two spaced exposed PCB traces; this is an zero cost method of adding crude overload protection to electronics products. Transils and trisils are the solid-state alternatives to spark gaps for lower-power applications. Neon bulbs are used for this purpose. A triggered spark gap in an air-gap flash is used to produce photographic light flashes in the sub-microsecond domain.
A spark radiates energy throughout the electromagnetic spectrum. Nowadays, this is regarded as illegal radio frequency interference and is suppressed, but in the early days of radio communications, this was the means by which radio signals were transmitted, in the unmodulated spark-gap transmitter. Many radio spark gaps include cooling devices, such as the rotary gap and heat sinks, since the spark gap becomes quite hot under continuous use at high power. A calibrated spherical spark gap will break down at a repeatable voltage, when corrected for air pressure and temperature. A gap between two spheres can provide a voltage measurement without any electronics or voltage dividers, to an accuracy of about 3%. A spark gap can be used to measure high voltage AC, DC, or pulses, but for short pulses, an ultraviolet light source or radioactive source may be put on one of the terminals to provide a source of electrons. Spark gaps may be used as electrical switches because they have two states with
A mercury switch is an electrical switch that opens and closes a circuit when a small amount of the liquid metal mercury connects metal electrodes to close the circuit. There are several different basic designs but they all share the common design strength of non-eroding switch contacts; the most common is the mercury tilt switch. It is in one state when tilted one direction with respect to horizontal, the other state when tilted the other direction; this is what older style thermostats used to turn a air conditioner on or off. The mercury displacement switch uses a'plunger' that dips into a pool of mercury, raising the level in the container to contact at least one electrode; this design is used in relays in industrial applications that need to switch high current loads frequently. These relays use electromagnetic coils to pull steel sleeves inside hermetically sealed containers. Mercury switches have one or more sets of electrical contacts in a sealed glass envelope that contains a small quantity of mercury.
The envelope may contain air, an inert gas, or a vacuum. Gravity pulls the drop of mercury to the lowest point in the envelope; when the switch is tilted in the appropriate direction, the mercury touches a set of contacts, thus completing an electrical circuit. Tilting the switch in the opposite direction moves the mercury away from that set of contacts, breaking that circuit; the switch may contain multiple sets of contacts, closing different sets at different angles, for example, single-pole, double-throw operation. Mercury switches offer several advantages over other switch types: The contacts are enclosed, so oxidation of the contact points is unlikely. In hazardous locations, interrupting the circuit does not emit a spark that could ignite flammable gases. Contacts stay clean, if an internal arc occurs, the contact surfaces renew on every operation, so they don't wear out. A small drop of mercury has low resistance, so switches can carry useful amounts of current in a small size. Sensitivity of the drop to gravity provides a unique sensing function, lends itself to simple, low-force mechanisms for manual or automatic operation.
The switches are quiet. The mass of the moving mercury drop provides an over center effect to avoid chattering as the switch tilts; the envelope can include contacts for two or more circuits. Mercury switches have several disadvantages: Their slow operating rate makes them unsuitable for applications that require many operating cycles per second. Glass envelopes and wire electrodes may be fragile and require flexible leads to prevent damage to the envelope; the mercury drop forms a common electrode, so circuits are not isolated from each other in a multi-pole switch. Their sensitivity to gravity may make them unsuitable in portable or mobile devices that can change orientation or vibrate. Mercury compounds are toxic and accumulate in any food chain, so safety codes exclude mercury in many new designs. Tilt switches provide a rollover or tip over warning for applications like construction equipment and lift vehicles that operate in rugged terrain. There are several non-mercury types, but few are implemented due to sensitivity to shock and vibration, causing false tripping.
However, devices resistant to shock and vibration do exist. Automobile manufacturers once used mercury switches for lighting controls, ride control, anti-lock braking systems. Scrapped automobiles can leak mercury to the environment. Since 2003, new American-built cars no longer use mercury switches. Work performed in confined space raises special safety concerns. Tilt switches sound an alarm. Electrically driven attitude indicators use mercury switches to keep the gyro axis vertical; when the gyro is off vertical, mercury switches trigger torque motors that move the gyro position back to the correct position. Mercury switches were once common in bimetal thermostats; the weight of the movable mercury drop provided some hysteresis by a degree of over-center action. The bimetal spring had to move further to overcome the weight of the mercury, tending to hold it in the open or closed position; the mercury provided positive on-off switching, could withstand millions of cycles without contact degradation.
Some old doorbells, for example, the Soviet ZM-1U4, use mercury switches as current interrupters. Some pressure switches use a mercury switch; the small force generated by the tube reliably operates the switch. Mercury switches are still used in electro-mechanical systems where physical orientation of actuators or rotors is a factor, they are commonly used in vending machines for tilt alarms that detect when someone tries to rock or tilt the machine to make it vend a product. A tilt switch can trigger a bomb. Mercury tilt switches can be found in some bomb and landmine fuzes in the form of anti-handling devices, for example, a variant of the VS-50 mine. Since mercury is a poisonous heavy metal, devices containing mercury switches must be treated as hazardous waste for disposal; because it is now RoHS restricted, most modern applications have eliminated it. A metal ball and contact wires can directly replace it, but may require additional circuitry to eliminate switch bounce. Low-precision thermostats use a switch contact.
Precision thermostats use a silicon temperature sensor. Low-cost accelerometers replace the mercury tilt switch in precision applications. In the United States, the En
An inertial switch is a switch mounted upon a vehicle or other mobile device, that triggers in the event of shock or vibration. It is a part of electrical circuits that may either disable some function; the switch shown to the right is intended to disable an electric fuel pump in automotive applications. This functionality is required in some vehicle racing applications, since an electric fuel pump may otherwise continue operating after a collision or rollover. If the fuel line is broken or the vehicle is inverted, fuel may be spilled. A small loose weight is trapped within a spring-loaded cage. A shock in any direction will cause movement of the mass relative to the cage. If sufficiently shocked, the cage will spring open; the switch is reset by pressing. These switches are used to open a contactor to disable the high power circuit of a battery electric vehicle upon collision. A open switch is used to activate passenger safety equipment early in a collision to pre-tension seat belts and/or to activate air-bags to protect the occupant from collision with the vehicle interior or the steering wheel.
Inertia switches are used in ordnance applications for both safety and fuzing
The cryotron is a switch that operates using superconductivity. The cryotron works on the principle; this simple device consists of two superconducting wires with different critical temperature. The cryotron was invented by Dudley Allen Buck of the Massachusetts Institute of Technology Lincoln Laboratory; as described by Buck, a straight wire of tantalum is wrapped around with a wire of niobium in a single layer coil. Both wires are electrically isolated from each other; when this device is immersed in a liquid helium bath both wires become superconducting and hence offer no resistance to the passage of electric current. Tantalum in superconducting state can carry large amount of current as compared to its normal state. Now when current is passed through the niobium coil it produces a magnetic field, which in turn reduces the superconductivity of the tantalum wire and hence reduces the amount of the current that can flow through the tantalum wire. Hence one can control the amount of the current that can flow in the straight wire with the help of small current in the coiled wire.
We can think of the tantalum straight wire as a "gate" and the coiled niobium as a "control". The article by Buck includes descriptions of several logic circuits implemented using cryotrons, including: one stage of a binary adder, carry network, binary accumulator stage, two stages of a cryotron stepping register. A planar cryotron using thin films of lead and tin was developed in 1957 by John Bremer at GE’s General Engineering Lab in Schenectady, New York, USA; this was one of the first integrated circuits, although using superconductors rather than semiconductors. In the next few years a demonstration computer was made and arrays with 2000 devices operated. A short history of this work is in the November 2007 newsletter of the IEEE History Center. Matisoo developed a version of the cryotron incorporating a Josephson junction switched by the magnetic field from a control wire, he explained the shortcomings of traditional cryotrons in which the superconductive material must transition between superconducting and normal states to switch the device, thus switch slowly.
December 1953 The magnetically controlled switch is proposed in Dudley Allen Buck notebook. July 1955 Dudley A. Buck application for U. S. Patent 2,832,897 Magnetically Controlled Gating Element August 1955 Lincoln Laboratory Memorandum 6M-3843The Cryotron - A Superconductive Computer Component 1956 A Cryotron Catalog Memory System by Al Slade and Howard McMahon 1957 James W. Crowe application for U. S. patent 3,100,267 Superconducting Gating Devices Oxford dictionary of science,4th edition,1999,ISBN 0-19-280098-1
Silicon controlled rectifier
A silicon controlled rectifier or semiconductor controlled rectifier is a four-layer solid-state current-controlling device. The principle of four-layer p–n–p–n switching was developed by Moll, Tanenbaum and Holonyak of Bell Laboratories in 1956; the practical demonstration of silicon controlled switching and detailed theoretical behavior of a device in agreement with the experimental results was presented by Dr Ian M. Mackintosh of Bell Laboratories in January 1958; the name "silicon controlled rectifier" is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W. "Bill" Gutzwiller in 1957. Some sources define silicon-controlled rectifiers and thyristors as synonymous, other sources define silicon-controlled rectifiers as a proper subset of the set of thyristors, those being devices with at least four layers of alternating n- and p-type material. According to Bill Gutzwiller, the terms "SCR" and "controlled rectifier" were earlier, "thyristor" was applied as usage of the device spread internationally.
SCRs are unidirectional devices as opposed to TRIACs. SCRs can be triggered only by currents going into the gate as opposed to TRIACs, which can be triggered by either a positive or a negative current applied to its gate electrode. There are three modes of operation for an SCR depending upon the biasing given to it: Forward blocking mode Forward conduction mode Reverse blocking mode In this mode of operation, the anode is given a positive voltage while the cathode is given a negative voltage, keeping the gate at zero potential i.e. disconnected. In this case junction J1 and J3 are forward-biased, while J2 is reverse-biased, due to which only a small leakage current exists from the anode to the cathode until the applied voltage reaches its breakover value, at which J2 undergoes avalanche breakdown, at this breakover voltage it starts conducting, but below breakover voltage it offers high resistance to the current and is said to be in the off state. An SCR can be brought from blocking mode to conduction mode in two ways: Either by increasing the voltage between anode and cathode beyond the breakover voltage, or by applying a positive pulse at the gate.
Once the SCR starts conducting, no more gate voltage is required to maintain it in the ON state. There are two ways to turn it off: Reduce the current through it below a minimum value called the holding current, or With the gate turned off, short-circuit the anode and cathode momentarily with a push-button switch or transistor across the junction; when a negative voltage is applied to the anode and a positive voltage to the cathode, the SCR is in reverse blocking mode, making J1 and J3 reverse biased and J2 forward biased. The device behaves as two reverse-biassed diodes connected in series. A small leakage current flows; this is the reverse blocking mode. If the reverse voltage is increased at critical breakdown level, called the reverse breakdown voltage, an avalanche occurs at J1 and J3 and the reverse current increases rapidly. SCRs are available with reverse blocking capability, which adds to the forward voltage drop because of the need to have a long, low-doped P1 region; the reverse blocking voltage rating and forward blocking voltage rating are the same.
The typical application for a reverse blocking SCR is in current-source inverters. An SCR incapable of blocking reverse voltage is known as an asymmetrical SCR, abbreviated ASCR, it has a reverse breakdown rating in the tens of volts. ASCRs are used where either a reverse conducting diode is applied in parallel or where reverse voltage would never occur. Asymmetrical SCRs can be fabricated with a reverse conducting diode in the same package; these are known for reverse conducting thyristors. Forward-voltage triggering gate triggering dv/dt triggering temperature triggering light triggeringForward-voltage triggering occurs when the anode–cathode forward voltage is increased with the gate circuit opened; this is known as avalanche breakdown. At sufficient voltages, the thyristor changes to its on state with low voltage drop and large forward current. In this case, J1 and J3 are forward-biased. SCRs are used in devices where the control of high power coupled with high voltage, is demanded, their operation makes them suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control.
SCRs and similar devices are used for rectification of high-power AC in high-voltage dc power transmission. They are used in the control of welding machines GTAW processes similar, it is used as switch in various devices. A silicon-controlled switch behaves nearly the same way as an SCR. Unlike an SCR, an SCS can be triggered into conduction when a negative voltage/output current is applied to that same lead. SCSs are useful in all circuits that need a switch that turns on/off through two distinct control pulses; this includes power-switching circuits, logic circuits, lamp drivers, etc. A TRIAC resembles an SCR. Unlike an SCR, a TRIAC can pass curre