Thermal Fuse
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A thermal cutoff is an
electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...
safety Safety is the state of being "safe", the condition of being protected from harm or other danger. Safety can also refer to the control of recognized hazards in order to achieve an acceptable level of risk. Meanings There are two slightly di ...
device (either a thermal fuse or thermal switch) that interrupts
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...
when
heat In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
ed to a specific
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
. These devices may be for one-time use (a thermal fuse), or may be reset manually or automatically (a thermal switch).


Thermal fuse

A thermal fuse is a cutoff which uses a one-time fusible link. Unlike a thermal switch which may automatically reset itself when the temperature drops, the thermal fuse is more like an electrical fuse: a single-use device that cannot be reset and must be replaced when it fails or is triggered. A thermal fuse is used when the overheating is a result of a rare occurrence, such as failure requiring
repair The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure, and supporting utilities in industrial, business, and residential installa ...
(which would also replace the fuse) or replacement at the end of
service life A product's service life is its period of use in service. Several related terms describe more precisely a product's life, from the point of manufacture, storage, and distribution, and eventual use. Service life has been defined as "a product's ...
. One mechanism is a small meltable pellet that holds down a spring. When the pellet melts, the spring is released, separating the contacts and breaking the circuit. The Tamura LE series, NEC Sefuse SF series, Microtemp G4A series, and Hosho Elmwood D series, for example, may use thermoplastic pellets that lose strength or melt at specific temperature. Another mechanism is more similar to electrical fuse, a fusible element that melts when subjected to temperature above its threshold. The difference against electrical fuse is using the surrounding temperature, instead of the temperature generated by ohmic heating of the fusible element. The elements are conductive and usually consist of binary or ternary
fusible alloy A fusible alloy is a metal alloy capable of being easily fused, i.e. easily meltable, at relatively low temperatures. Fusible alloys are commonly, but not necessarily, eutectic alloys. Sometimes the term "fusible alloy" is used to describe alloys ...
of tin,
bismuth Bismuth is a chemical element with the symbol Bi and atomic number 83. It is a post-transition metal and one of the pnictogens, with chemical properties resembling its lighter group 15 siblings arsenic and antimony. Elemental bismuth occurs ...
,
antimony Antimony is a chemical element with the symbol Sb (from la, stibium) and atomic number 51. A lustrous gray metalloid, it is found in nature mainly as the sulfide mineral stibnite (Sb2S3). Antimony compounds have been known since ancient ti ...
,
indium Indium is a chemical element with the symbol In and atomic number 49. Indium is the softest metal that is not an alkali metal. It is a silvery-white metal that resembles tin in appearance. It is a post-transition metal that makes up 0.21 parts ...
,
lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cut, ...
, and other metals. Thermal fuses are usually found in heat-producing electrical appliances such as
coffeemaker A coffeemaker, coffee maker or coffee machine is a cooking appliance used to brew coffee. While there are many different types of coffeemakers the two most common brewing principles use gravity or pressure to move hot water through coffee gr ...
s and hair dryers. They function as safety devices to disconnect the current to the
heating element A heating element converts electrical energy into heat through the process of Joule heating. Electric current through the element encounters resistance, resulting in heating of the element. Unlike the Peltier effect, this process is indepen ...
in case of a malfunction (such as a defective thermostat) that would otherwise allow the temperature to rise to dangerous levels, possibly starting a
fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. At a certain point in the combustion reaction, called the ignition point, flames ...
. Unlike electrical fuses or
circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...
s, thermal fuses only react to excessive temperature and not to excessive current (unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature). For example, in a
surge protector A 'surge protector'' (or spike suppressor, surge suppressor, surge diverter, surge protection device (SPD) or transient voltage surge suppressor (TVSS) is an appliance or device intended to protect Electronics, electrical devices from voltage s ...
thermal fuses may be wired in series with the varistors; when the varistors conduct, the fuse heats up and disconnects the power, which eliminates the risk of fire which can occur when the varistors are overloaded.


Thermal switch

A thermal switch (sometimes thermal reset or thermal cutout (TCO)) is a device which normally opens at a high temperature (often with a faint "plink" sound) and re-closes when the temperature drops. The thermal switch may be a
bimetallic strip A bimetallic strip is used to convert a temperature change into mechanical displacement. The strip consists of two strips of different metals which expand at different rates as they are heated. The different expansions force the flat strip to be ...
, often encased in a tubular glass bulb to protect it from
dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ...
or
short circuit A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circu ...
. Another common design uses a bimetallic shallow dome-shaped cap which "clicks" to an inside-out inverted cap shape when heated, such as the "Klixon" brand of thermal cutouts.Alan R. Earls, Robert E. Edwards, ''Raytheon Company: The First Sixty Years''Arcadia Publishing, 2005 , page 9 Unlike a thermal fuse, a thermal switch is usually reusable and is therefore suited to protecting against temporary situations which are common and user-correctable. Thermal switches are used in power supplies in case of overload, and also as thermostats, and overheat protection in some heating and cooling systems. They are found in virtually every refrigerator, microwave, clothes dryer, space heater, and many more appliances found throughout the home. Another type of thermal switch is a PTC (Positive Temperature Coefficient)
thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...
; these thermistors have a "cutting off" temperature at which the resistance suddenly rises rapidly, limiting the current through the circuit. When used in conjunction with a thermistor relay, the PTC will switch off an electrical system at a desired temperature. Typical use is for motor overheat protection. These devices are not a true “switch” as they do not disengage completely and there is always some resistance across the switch. Devices also apply PTC tech as a PTC thermistor sensor, this is different from a cutout and can confuse amateur electronic technicians. Thermal switches are included in
turn signals The lighting system of a motor vehicle consists of lighting and signalling devices mounted to or integrated into the front, rear, sides, and in some cases the top of a motor vehicle. They illuminate the road ahead for the driver and increase th ...
on older vehicles, some
light fixture A light fixture (US English), light fitting (UK English), or luminaire is an electrical device containing an electric lamp that provides illumination. All light fixtures have a fixture body and one or more lamps. The lamps may be in sockets fo ...
s, particularly with
recessed light A recessed light or downlight (also pot light in Canadian English, sometimes can light (for canister light) in American English) is a light fixture that is installed into a hollow opening in a ceiling. When installed it appears to have light shi ...
s, where excessive heat is most likely to occur. This may lead to "cycling" ( flashing), where a light turns off and back on every few minutes. Flashing incandescent
Christmas lights Christmas lights (also known as fairy lights, festive lights or string lights) are lights often used for decoration in celebration of Christmas, often on display throughout the Christmas season including Advent and Christmastide. The custom goe ...
take advantage of this effect. Some flasher bulbs interrupt power when heated, while other twinkle/sparkle mini-bulbs momentarily shunt current around the filament. Thermal switches are part of the normal operation of older
fluorescent light A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet lig ...
fixtures, where they are the major part of the starter module. GE trademarked the name "Guardette" for the thermal protection switches used on their refrigeration compressors. Thermal switches on
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...
s often stop only the fetching of instructions to execute, reducing the
clock rate In computing, the clock rate or clock speed typically refers to the frequency at which the clock generator of a processor can generate pulses, which are used to synchronize the operations of its components, and is used as an indicator of the pr ...
to zero until a lower temperature is reached, while maintaining power to the
cache Cache, caching, or caché may refer to: Places United States * Cache, Idaho, an unincorporated community * Cache, Illinois, an unincorporated community * Cache, Oklahoma, a city in Comanche County * Cache, Utah, Cache County, Utah * Cache County ...
to prevent data loss (although a second switch, with a higher triggering temperature, usually turns off even the cache and forces the computer to reboot). This mitigates the impact of programs resembling
power virus Power most often refers to: * Power (physics), meaning "rate of doing work" ** Engine power, the power put out by an engine ** Electric power * Power (social and political), the ability to influence people or events ** Abusive power Power may a ...
es on the processor's longevity, while still accommodating their possible legitimate uses; it can also make
overclocking In computing, overclocking is the practice of increasing the clock rate of a computer to exceed that certified by the manufacturer. Commonly, operating voltage is also increased to maintain a component's operational stability at accelerated spe ...
possible with less risk.


Manual reset

Some thermal switches must be reset manually after having tripped. This type is used when an automatic and unattended restart would create a hazardous condition, such as sudden startup of a powerful motor without warning. These types of thermal cutouts are usually reset by pressing a
push-button A push-button (also spelled pushbutton) or simply button is a simple switch mechanism to control some aspect of a machine or a process. Buttons are typically made out of hard material, usually plastic or metal. The surface is usually flat or sh ...
by hand or with a special tool.


See also

*
Thermistor A thermistor is a type of resistor whose resistance is strongly dependent on temperature, more so than in standard resistors. The word thermistor is a portmanteau of ''thermal'' and ''resistor''. Thermistors are divided based on their conduction ...


References


External links


Explanation of thermal cutoff switch
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Smart Thermostat for Electric Baseboard Heaters
{{DEFAULTSORT:Thermal Cutoff Electric power systems components Safety switches Fuses