Resettable fuse
Encyclopedia
A polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuse) is a passive electronic component used to protect against overcurrent
faults in electronic circuit
s. They are actually non-linear thermistor
s, however, and cycle back to a conductive state after the current is removed, acting more like circuit breaker
s, allowing the circuit to function again without opening the chassis
or replacing anything. These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user never has to open), and in aerospace/nuclear applications where replacement is difficult.
Another application for such devices is protecting audio speakers, particularly tweeters, from damage when over driven: by putting a resistor or light bulb in parallel with the PPTC device it is possible to design a circuit that limits total current through the tweeter to a safe value instead of cutting it off, allowing the speaker to continue operating without damage when the amplifier is delivering more power than the tweeter could tolerate. A fuse will also protect the speaker but when a fuse blows the speaker cannot operate until the fuse is replaced.
These devices are sold by different companies under various trademarks, including PolySwitch (TE Connectivity), OptiReset (OptiFuse), Everfuse (Polytronics), “Way-on",Polyfuse (Littelfuse
) and Multifuse (Bourns, Inc.
).
A PPTC device has a current rating
. When the current flowing through the device, (which has a small resistance
in the on state) exceeds the current limit, the PPTC device warms up above a threshold temperature and the electrical resistance
of the PPTC device suddenly increases several orders of magnitude to a "tripped" state where the resistance will typically be hundreds or thousands of ohms, greatly reducing the current. The rated trip current can be anywhere from 20 mA to 100 A.
A polymeric PTC device comprises a non-conductive crystalline organic polymer matrix that is loaded with carbon black
particles to make it conductive. While cool, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conductive chains. Since it is conductive (the "initial resistance"), it will pass a given current, called the "hold current". If too much current is passed through the device, the "trip current", the device will begin to heat. As the device heats the polymer will expand, changing from a crystalline into an amorphous state. The expansion separates the carbon particles and breaks the conductive pathways, causing the resistance of the device to increase. This will cause the device to heat faster and expand more, further raising the resistance. This increase in resistance substantially reduces the current in the circuit. A small current still flows through the device and is sufficient to maintain the temperature at a level which will keep it in the high resistance state. The device can be said to have latching functionality. The critical material, which is a carbon black and plastic mix, used in the automatically resetting devices described in this article was serendipitously discovered and then described by Fred Kohler (author)
and scientist and patented as patent
#3,243,753 on March 29, 1966.
When the power and fault are removed, the PPTC device will cool. As the device cools, it regains its original crystalline structure and returns to a low resistance state where it can hold the current as specified for the device. This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value.
Since a PPTC device has an inherently higher resistance than a metallic fuse or circuit breaker at ambient temperature, it may be difficult or impossible to use in circuits that cannot tolerate significant reductions in operating voltage, forcing the engineer to choose the latter in a design.
In case of potted (hard resin or even soft silicone-based) assemblies, manufacturers recommend leaving an open space around the device, to allow expansion. This can be achieved by placing a small box over the PPTC before pouring.
The Bourns, Inc.
Transient Blocking Unit (TBU) is faster than a polyswitch, but requires a higher voltage to trip.
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
faults in electronic circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
s. They are actually non-linear thermistor
Thermistor
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor...
s, however, and cycle back to a conductive state after the current is removed, acting more like circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s, allowing the circuit to function again without opening the chassis
Chassis
A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...
or replacing anything. These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user never has to open), and in aerospace/nuclear applications where replacement is difficult.
Another application for such devices is protecting audio speakers, particularly tweeters, from damage when over driven: by putting a resistor or light bulb in parallel with the PPTC device it is possible to design a circuit that limits total current through the tweeter to a safe value instead of cutting it off, allowing the speaker to continue operating without damage when the amplifier is delivering more power than the tweeter could tolerate. A fuse will also protect the speaker but when a fuse blows the speaker cannot operate until the fuse is replaced.
These devices are sold by different companies under various trademarks, including PolySwitch (TE Connectivity), OptiReset (OptiFuse), Everfuse (Polytronics), “Way-on",Polyfuse (Littelfuse
Littelfuse
Littelfuse Inc. is an American multinational electronics manufacturing company based in Chicago, Illinois.Littelfuse, Inc. provides circuit protection products to companies in the areas of over-voltage, over-current and integrated technologies in the automotive, electronics, electrical and...
) and Multifuse (Bourns, Inc.
Bourns, Inc.
Bourns, Inc. is a private, global electronics corporation that develops, manufactures and supplies a wide range of electronic components for a variety of industries including automotive, industrial, instrumentation, medical electronics, consumer equipment, computers and peripherals,...
).
A PPTC device has a current rating
Ampacity
Ampacity is the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration.Also described as current rating or current-carrying capacity, ampacity is the RMS electric current which a device can continuously carry while remaining...
. When the current flowing through the device, (which has a small resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
in the on state) exceeds the current limit, the PPTC device warms up above a threshold temperature and the electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
of the PPTC device suddenly increases several orders of magnitude to a "tripped" state where the resistance will typically be hundreds or thousands of ohms, greatly reducing the current. The rated trip current can be anywhere from 20 mA to 100 A.
A polymeric PTC device comprises a non-conductive crystalline organic polymer matrix that is loaded with carbon black
Carbon black
Carbon black is a material produced by the incomplete combustion of heavy petroleum products such as FCC tar, coal tar, ethylene cracking tar, and a small amount from vegetable oil. Carbon black is a form of amorphous carbon that has a high surface-area-to-volume ratio, although its...
particles to make it conductive. While cool, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conductive chains. Since it is conductive (the "initial resistance"), it will pass a given current, called the "hold current". If too much current is passed through the device, the "trip current", the device will begin to heat. As the device heats the polymer will expand, changing from a crystalline into an amorphous state. The expansion separates the carbon particles and breaks the conductive pathways, causing the resistance of the device to increase. This will cause the device to heat faster and expand more, further raising the resistance. This increase in resistance substantially reduces the current in the circuit. A small current still flows through the device and is sufficient to maintain the temperature at a level which will keep it in the high resistance state. The device can be said to have latching functionality. The critical material, which is a carbon black and plastic mix, used in the automatically resetting devices described in this article was serendipitously discovered and then described by Fred Kohler (author)
Fred Kohler (author)
Fred Kohler is an inventor, author and lecturer who appears to be the first person to write about the human species becoming a "Societal Organism", or Super Organism as a further development in the human evolution of life.-Early years:Kohler was born in Nuremberg, Germany in 1920 into a secular,...
and scientist and patented as patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....
#3,243,753 on March 29, 1966.
When the power and fault are removed, the PPTC device will cool. As the device cools, it regains its original crystalline structure and returns to a low resistance state where it can hold the current as specified for the device. This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value.
Since a PPTC device has an inherently higher resistance than a metallic fuse or circuit breaker at ambient temperature, it may be difficult or impossible to use in circuits that cannot tolerate significant reductions in operating voltage, forcing the engineer to choose the latter in a design.
In case of potted (hard resin or even soft silicone-based) assemblies, manufacturers recommend leaving an open space around the device, to allow expansion. This can be achieved by placing a small box over the PPTC before pouring.
The Bourns, Inc.
Bourns, Inc.
Bourns, Inc. is a private, global electronics corporation that develops, manufactures and supplies a wide range of electronic components for a variety of industries including automotive, industrial, instrumentation, medical electronics, consumer equipment, computers and peripherals,...
Transient Blocking Unit (TBU) is faster than a polyswitch, but requires a higher voltage to trip.
Operating Parameters
- Initial resistance: The resistance of the device as received from the factory of manufacturing.
- Operating voltage: The maximum voltage a device can withstand without damage at the rated current.
- Holding current: Safe current through the device.
- Trip current: Where the device interrupts the current.
- Time to trip: The time it takes for the device to trip at a given temperature.
- Tripped state: Transition from the low resistance state to the high resistance state due to an overload.
- Leakage current: A small value of stray current flowing through the device after it has switched to high resistance mode.
- Trip cycle: The number of trip cycles (at rated voltage and current) the device sustains without failure.
- Trip endurance: The duration of time the device sustains its maximum rated voltage in the tripped state without failure.
- Power dissipation: Power dissipated by the device in its tripped state.
- Thermal duration: Influence of ambient temperature.
- Hysteresis: The range between where the device trips and where the device returns to a conductive state.