Resettable Fuse: Basic Introduction

Resettable fuse resettable fuse is an electronic device for overcurrent protection that is processed through an exclusive process following the addition of particles with conductive properties under conditions of high temperature, high pressure and vulcanization reactions of the high molecular organic polymers. The standard fuse is only able to protect one time in the overcurrent protection, and must be replaced if it gets blowing, whereas the resettable fuse serves a dual function of protecting against overcurrent as well as thermal protection as well as automatic recovery.

Resettable Fuse Working Principle

It is comprised of an specially treated polymer resin (Polymer) and the conductive elements (Carbon Black) which are scattered throughout.

When operating normally the polymer resin bonds the conductive particles to the crystal structure, creating an electrical path that resembles a chain. In this moment the resettable fuse is in a state of low resistance (a) and the heat generated by the current that flows via the resettable fuse the line is low and does not alter its crystal’s structure.

If you short circuit the lines, or overloaded, the heat produced by the huge flow of current via the resettable fuse is going to cause the melting of the polymer resin. the volume will rise quickly, creating a high-resistance state (b) while the operating current drop quickly, thus limiting the circuit’s risk.

Once the fault has been eliminated the resettable fuse cools and crystallizes. As the volume decreases, and the conducting particles form an electrical path while the resettable fuse reverts to a low resistance state which completes protecting the circuit with no replacing it manually.

Resettable Fuse Action Principle

The principle that drives the fuse that resets is a type equilibrium of energy. Because of the thermal effects of the current, the flow of current into the fuse produces some heat energy (resettable fuse has resistance) which is completely or partially released to the environment. Any heat generated that hasn’t been released will increase its temperature in the fuse that is resettable.

In normal use the temperature is low, and there’s an equilibrium between the heat produced and the heat lost.

If the resettable fuse is in a low-resistance condition the fuse is not in a state of operation. If the current that flows through it raises the temperature in the environment, and when the energy that is generated and dissipated attains a certain level that the resettable fuse will not work. When the temperature increases it will rise to an extremely high temperature.

If the temperature or current increases and the energy that is produced will be greater than the energy that is dissipated and will result in a significant rise in the temperature of the fuse that is resettable. In this case any slight change in temperature will result in an extreme increase in resistance. In this moment, it is in the high resistance safety state. The increased resistance is a limitation on the current and the current drops rapidly in a short period of duration, thus preventing the circuit’s devices from damage. So long as the energy produced by the voltage applied is enough to eliminate the heat that is generated from the resettable fuse The fuse in the operational condition (high resistance) will remain in that state.

If the voltage that is applied disappears the resettable fuse will be reset automatically.

Resettable Fuse Symbol

Ih – The highest operating current that can be achieved at an ambient temperature of 25 degrees Celsius.

It is the minimum current that provides protection at an ambient temperature of 25 degrees Celsius.

Imax is the maximum current in which the resettable fuse is able to be able to withstand.

Pdmax – The power consumption

Vmax – The highest operating voltage

Vmaxi is the maximum amount of voltage the fuse that resettable will stand in the state of blocking

Rmin is the initial minimum resistance prior to the operation

Rmaxi – The resistance that is the maximum prior to the operation

Blown Fuse Vs Resettable Fuse

The most significant distinction between them is the fact that the resettable fuse has the ability to reset itself. Of course, there are other distinctions between the two models.

  1. Leakage current: When it’s overloaded the resettable fuse shifts to a lower resistance to an extremely high resistance state this is commonly referred to as “tripping”. The fuse limits the current to a specified leakage amount to protect the user. The leakage current may rise from around 100 milliamps when operating at the maximum voltage up to several hundred milliamps for lower voltages. In the event of a damaged fusethat is overloaded the fuse will be blowing to totally cut off the current. As a result, the leakage current created by the circuit that is disconnected can be “0”.
  2. Breaking current refers to the maximum short-circuit voltage determined by the fuse in the recommended voltage. This is the maximum amount of current the device can handle and resettable fuse fuses usually don’t actually stop the flow of current (see “Leakage Current” above.) Resettable fuses that are standard have an rated short circuit of 40A. A fuse that is blown actually stops the current when it detects an overload. The rated breaking current can range in the hundreds up to 10,000 amps at voltages rated.
  3. Rated current nominal work current for the resettable fuse may be as high as 11A. However, the maximum operating current of the fuse can be greater than 20A.
  4. Rated voltage nominal voltage of resettable fuses that are used in the conventional setting is not more than 60V, however the rated voltage of blowing fuses can exceed 600V.
  5. Resistance: In the specifications of the product It is apparent that resettable fuses’ resistance fuse is double (sometimes more) than those blown fuse with similar ratings.
  6. Rated temperature traditional upper temperature limit for resettable fuses typically is 85degC. However, the maximum operating temperature for fuse blows is at 125degC. Both devices must be derated whenever operating at or above 20degC.
  7. Time-current characteristics: When comparing the time-current curves for resettable fuse or fuses is able to be seen that the time to respond of resettable fuses is similar to the Slo-Blo fuses.
  8. The Resettable Fuse are certified by the Component Program of Underwriters Laboratories, Inc. according to UL Standard 1434 for Thermistors. It is also certified by the CSA Component Acceptance Program. With the help of certifications like TUV as well as VDE The resettable fuse could be considered to have met standards of IEC Standard 730-1 (automatic electronic control). Certifications for blown fuse include Underwriters Laboratories’ Component Program Certification as well as the CSA’s Component Acceptance Program. In addition many fuses are made available with complete “Listing” certification based on the latest fuse additional standards UL 248-14.

Resettable Fuse Versus Traditional Fuse

If you decide to go with an old-fashioned fuse, or PTC resettable fuse will depend on the particular product circuit each fuse comes with distinct advantages.

For instance, a lot of designs for peripherals, computers as well as portable devices (such as tablets, smartphones etc.) require PTCs because they are able to reset themselves. If conventional fuses are employed they will need to be replaced each when an overcurrent occurs which is not acceptable to the user.

In other situations it is possible to utilize conventional fuses that are capable of cutting off current in situations of failure. This is an appropriate option when security is the top priority or to prevent damage to other circuits downstream. Fuse use is an excellent tool for troubleshooting, which assists the designers and those who use equipment to identify the root of the overcurrent fault.

Overcurrent circuit protection

In the case of providing protection against overcurrent the circuit designer has options in which technology that they can use. Traditional fuses as well as polymer-based PTC (positive temperature coefficient) devices are the most popular solutions. Knowing the difference between these two components eases the selection process of selecting the right protection device for your needs.

A fuse has been thought of as a “one-off” device due to the fact that in the case of an overload it blows up once, providing protection against overcurrent and must be replaced. The basic component of a fuse is a wire that is heated up to their melting point when it is at a temperature that exceeds. If the wire is blowing and the current flowing through the circuit decreases to zero.

PTC will also react to current that is too high, but it’s not a “self-healing” system. Polymer-based components will automatically reset once the overload is gone which allows multiple circuits to be used for protection. If a conductive polymer gets overheated by an overcurrent it will experience an increase in its resistance and limit the circuit current.


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