Resettable fuse

Last updated
Resettable fuses - PolySwitch devices Photo-Polyswitch.jpg
Resettable fuses - PolySwitch devices

A resettable fuse or polymeric positive temperature coefficient device (PPTC) is a passive electronic component used to protect against overcurrent faults in electronic circuits. The device is also known as a multifuse or polyfuse or polyswitch. They are similar in function to PTC thermistors in certain situations but operate on mechanical changes instead of charge carrier effects in semiconductors. These devices were first discovered and described by Gerald Pearson at Bell Labs in 1939 and described in US patent #2,258,958. [1]

Contents

Operation

A polymeric PTC device is made up of a non-conductive crystalline organic polymer matrix that is loaded with carbon black particles [2] to make it conductive. While cold, 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"), [3] it will pass a current. If too much current is passed through the device, the device will begin to heat. As the device heats, the polymer will expand, changing from a crystalline into an amorphous state. [4] The expansion separates the carbon particles and breaks the conductive pathways, causing the device to heat faster and expand more, further raising the resistance. [5] This increase in resistance substantially reduces the current in the circuit. A small (leakage) 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. Leakage current can range from less than a hundred mA at rated voltage up to a few hundred mA at lower voltages. The device can be said to have latching functionality. [6] The hold current is the maximum current at which the device is guaranteed not to trip. The trip current is the current at which the device is guaranteed to trip. [7]

When power is removed, the heating due to the leakage current will stop and 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. [6] This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, unless the power in it is weaker, or has been often used, slowly approaching the initial resistance value. The resetting will often not take place even if the fault alone has been removed with the power still flowing as the operating current may be above the holding current of the PPTC. The device may not return to its original resistance value; it will most likely stabilize at a significantly higher resistance (up to 4 times initial value). It could take hours, days, weeks or even years for the device to return to a resistance value similar to its original value, if at all. [8]

A PPTC device has a current rating and a voltage rating. [9]

Applications

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.[ citation needed ] Another application for such devices is protecting audio loudspeakers, 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. While a fuse could also offer similar protection, if the fuse is blown, the tweeter cannot operate until the fuse is replaced. [10]

See also

Related Research Articles

<span class="mw-page-title-main">Resistor</span> Passive electrical component providing electrical resistance

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, or chemical activity.

<span class="mw-page-title-main">Thermistor</span> Type of resistor whose resistance varies with temperature

A thermistor is a semiconductor 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.

<span class="mw-page-title-main">Ground (electricity)</span> Reference point in an electrical circuit from which voltages are measured

In electrical engineering, ground or earth is a reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth.

<span class="mw-page-title-main">Short circuit</span> Electrical circuit with negligible impedance

A short circuit 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 circuit. The opposite of a short circuit is an open circuit, which is an infinite resistance between two nodes.

<span class="mw-page-title-main">Circuit breaker</span> Automatic circuit protection device

A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by overcurrent. Its basic function is to interrupt current flow to protect equipment and to prevent the risk of fire. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation.

<span class="mw-page-title-main">Electrolytic capacitor</span> Type of capacitor

An electrolytic capacitor is a polarized capacitor whose anode or positive plate is made of a metal that forms an insulating oxide layer through anodization. This oxide layer acts as the dielectric of the capacitor. A solid, liquid, or gel electrolyte covers the surface of this oxide layer, serving as the cathode or negative plate of the capacitor. Due to their very thin dielectric oxide layer and enlarged anode surface, electrolytic capacitors have a much higher capacitance-voltage (CV) product per unit volume than ceramic capacitors or film capacitors, and so can have large capacitance values. There are three families of electrolytic capacitor: aluminum electrolytic capacitors, tantalum electrolytic capacitors, and niobium electrolytic capacitors.

<span class="mw-page-title-main">Fuse (electrical)</span> Electrical safety device that provides overcurrent protection

In electronics and electrical engineering, a fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit. Its essential component is a metal wire or strip that melts when too much current flows through it, thereby stopping or interrupting the current. It is a sacrificial device; once a fuse has operated it is an open circuit, and must be replaced or rewired, depending on its type.

In electronics, derating is the operation of a device at less than its rated maximum capability to prolong its life. Typical examples include operations below the maximum power rating, current rating, or voltage rating.

<span class="mw-page-title-main">Electronic component</span> Discrete device in an electronic system

An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components and elements.

<span class="mw-page-title-main">Thermal runaway</span> Situation where an increase in temperature causes a further increase in temperature

Thermal runaway describes a process that is accelerated by increased temperature, in turn releasing energy that further increases temperature. Thermal runaway occurs in situations where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result. It is a kind of uncontrolled positive feedback.

<span class="mw-page-title-main">Capacitor types</span> Manufacturing styles of an electronic device

Capacitors are manufactured in many styles, forms, dimensions, and from a large variety of materials. They all contain at least two electrical conductors, called plates, separated by an insulating layer (dielectric). Capacitors are widely used as parts of electrical circuits in many common electrical devices.

Current limiting is the practice of imposing a limit on the current that may be delivered to a load to protect the circuit generating or transmitting the current from harmful effects due to a short-circuit or overload. The term "current limiting" is also used to define a type of overcurrent protective device. According to the 2020 NEC/NFPA 70, a current-limiting overcurrent protective device is defined as, "A device that, when interrupting currents in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having compatible impedance."

<span class="mw-page-title-main">Thermal cutoff</span>

A thermal cutoff is an electrical safety device that interrupts electric current when heated to a specific temperature. These devices may be for one-time use, or may be reset manually or automatically.

<span class="mw-page-title-main">Solid</span> State of matter

Solid is one of the four fundamental states of matter. The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural rigidity and resistance to a force applied to the surface. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire available volume like a gas. The atoms in a solid are bound to each other, either in a regular geometric lattice, or irregularly. Solids cannot be compressed with little pressure whereas gases can be compressed with little pressure because the molecules in a gas are loosely packed.

<span class="mw-page-title-main">Thinking Electronic</span>

Thinking Electronic Industrial Co., Ltd. is one of the major circuit protection component manufacturer in Taiwan. It was established in 1979 and the headquarters are in Kaohsiung, Taiwan.

<span class="mw-page-title-main">Tantalum capacitor</span> Type of electrolytic capacitor

A tantalum electrolytic capacitor is an electrolytic capacitor, a passive component of electronic circuits. It consists of a pellet of porous tantalum metal as an anode, covered by an insulating oxide layer that forms the dielectric, surrounded by liquid or solid electrolyte as a cathode. Because of its very thin and relatively high permittivity dielectric layer, the tantalum capacitor distinguishes itself from other conventional and electrolytic capacitors in having high capacitance per volume and lower weight.

<span class="mw-page-title-main">Failure of electronic components</span> Ways electronic components fail and prevention measures

Electronic components have a wide range of failure modes. These can be classified in various ways, such as by time or cause. Failures can be caused by excess temperature, excess current or voltage, ionizing radiation, mechanical shock, stress or impact, and many other causes. In semiconductor devices, problems in the device package may cause failures due to contamination, mechanical stress of the device, or open or short circuits.

<span class="mw-page-title-main">Polymer capacitor</span> Solid conductive electrolyte

A polymer capacitor, or more accurately a polymer electrolytic capacitor, is an electrolytic capacitor (e-cap) with a solid conductive polymer electrolyte. There are four different types:

<span class="mw-page-title-main">Aluminum electrolytic capacitor</span> Type of capacitor

Aluminum electrolytic capacitors are polarized electrolytic capacitors whose anode electrode (+) is made of a pure aluminum foil with an etched surface. The aluminum forms a very thin insulating layer of aluminum oxide by anodization that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode (cathode) (-) of the capacitor. A second aluminum foil called “cathode foil” contacts the electrolyte and serves as the electrical connection to the negative terminal of the capacitor.

Overheating is a phenomenon of rising temperatures in an electrical circuit. Overheating causes damage to the circuit components and can cause fire, explosion, and injury. Damage caused by overheating is usually irreversible; the only way to repair it is to replace some components.

References

  1. U.S. Patent 2258958.: "Conductive device", filed 13 Jul 1939, retrieved 7 mar 2017.
  2. Herman F. Mark (16 October 2013). Encyclopedia of Polymer Science and Technology, Concise. John Wiley & Sons. pp. 274–. ISBN   978-0-470-07369-8.
  3. Gianfranco Pistoia (25 January 2005). Batteries for Portable Devices. Elsevier. pp. 183–. ISBN   978-0-08-045556-3.
  4. Ming Qiu Zhang; Min Zhi Rong (28 June 2011). Self-Healing Polymers and Polymer Composites. John Wiley & Sons. pp. 391–. ISBN   978-1-118-08258-4.
  5. A. Wright; P.G. Newbery (January 2004). Electric Fuses . IET. pp.  15–. ISBN   978-0-86341-399-5.
  6. 1 2 Institute of Electrical and Electronics Engineers. San Francisco Bay Area Council (1995). WESCON Conference Record. Western Electronic Show and Convention.
  7. Machine Design. Penton/IPC. 1997.
  8. "PolySwitch Resettable Devices Fundamentals" (PDF). TE Connectivity . Archived from the original (PDF) on 22 January 2015. Retrieved 31 August 2014.
  9. Henning Wallentowitz; Christian Amsel (27 June 2011). 42 V-PowerNets. Springer Science & Business Media. pp. 80–. ISBN   978-3-642-18139-9.
  10. Loudspeaker application note