The main function and working principle of fuses in circuits

Fuses are commonly referred to as fuses or fuses. Edison invented the earliest fuse more than 100 years ago, because fluorescent lamps with underdeveloped industrial technology were very expensive at that time, so fuse was initially used to maintain expensive fluorescent lamps. Fuses protect electronic products from current damage and prevent serious damage caused by internal faults. So, each fuse has a rated specification, and when the current exceeds the rated specification, the fuse may melt. When the current between the conventional non fusing current and the rated breaking capacity (current) specified in relevant standards is applied to a fuse, the fuse should work satisfactorily and not endanger the surrounding environment.

The predicted fault current of the power supply placed with the fuse must be lower than the rated separation capacity current specified in the standard. Otherwise, when the fuse blows, there will be continuous arcing, ignition, fuse burning, melting of connecting contact components, and indistinguishable fuse identification. Of course, the separation ability of counterfeit and inferior fuses cannot meet the requirements of the standard, and it can also cause harm in use.

In addition to fuse resistors, commonly used protective components in electronic products also include general fuses, thermal fuses, and self repairing fuses. Protective components are usually connected in series in a circuit. When abnormal situations such as overcurrent, overvoltage, or overheating occur in the circuit, it will immediately fuse to provide protection and prevent further expansion of the fault.

（1） General fuse

A general fuse, commonly known as a fuse or fuse, is an irreparable fuse that can only be replaced with a new one after it has blown. It is indicated by "F" or "FU" in the circuit.

Structural characteristics of general fuses

Generally, fuses are composed of glass tubes, metal caps, and fuses. Two metal caps are placed on both sides of the glass tube, and a fuse (made of low melting point metal material) is installed in the glass tube. Both sides are welded to the center holes of the two metal caps. Install the fuse on the fuse holder and connect it in series with the circuit.

Most fuses are straight, with only the commonly used delay fuses in color TV and computer displays being spiral fuses.

Basic parameters of general fuses

The basic parameters of a general fuse include rated voltage, rated current, operating temperature, and reaction speed. Rated voltage, also known as open circuit volume, refers to the current value at which a fuse can open circuit at rated current. The stable working current of the fuse is less than 30% of the rated voltage. The rated voltage value of domestic fuses is generally directly marked on the metal cap, while imported fuses are marked on the glass tube with a color ring.

The rated current refers to the most adjusted voltage of the fuse, which is divided into 32V, 125V, and four specifications: 250V and 600V. The specific working voltage of the fuse should be less than or equal to the rated current value. If the working voltage value of a fuse exceeds the rated current value, it will quickly blow.

The current carrying capacity test of the fuse should be conducted at a working temperature of 25 ℃. The service life of a fuse is inversely proportional to its operating temperature. The higher the ambient temperature, the higher the working temperature of the fuse, and the shorter its service life.

The reaction rate refers to the rapid level at which a fuse takes action on various electrical loads. According to the reaction rate and performance, fuses can be divided into normal response type, delayed opening type, fast action type, and current limiting type.

（2） Thermal fuse

Thermal fuse, also known as Thermal cutoff, is an irreparable overheat insurance element, which is widely used in various electronic devices such as electric cookers, motors, washing machines, fans, power transformers, etc. According to the different materials of temperature sensors, thermal fuses can be divided into low melting point alloy thermal fuses, organic thermal fuses, and plastic metal thermal fuses.

Low melting point alloy

The temperature sensor of low melting point alloy thermal fuses is made of alloy products with fixed melting points. When the temperature reaches the melting point of the alloy, the temperature sensor automatically fuses and disconnects the protected circuit. According to their structural differences, low melting point alloy thermal fuses can be divided into gravity, interfacial tension, and torsion spring reaction.

Organic thermal fuse

Organic thermal fuses are composed of temperature sensors, mobile electrical stages, torsion springs, etc. The temperature sensor should be made of organic matter with high purity and low melting ambient temperature. Under normal circumstances, the mobile power stage is in contact with a fixed node, and the circuit is connected by a fuse; When the temperature reaches the dissolution point, the temperature sensor automatically fuses, and under the action of the spring, the mobile electrical stage is disconnected from the fixed node, and the protection circuit is disconnected.

Plastic metal thermal fuse

The plastic metal thermal fuse adopts an interface tension structure, and the resistance value of the temperature sensor is almost zero. When the operating temperature reaches the set value, the resistance value of the temperature sensor will suddenly increase to prevent current from passing through.

(3) Self repairing fuse

Self repairing fuse is a new type of safety element with overcurrent and overheating protection functions, which can be reused multiple times.

The structural principle of self repairing fuses

A self-healing fuse is a positive temperature coefficient PTC thermistor composed of a mixture of polymer and conductive materials. It is connected in the circuit and can replace traditional fuses.

When the circuit operates normally, the self-healing fuse is in a conducting state. When an overcurrent fault occurs in a circuit, the temperature of the fuse itself will rapidly rise, and the polymer material will quickly enter a high resistance state after heating. From the conductor to the insulation, the current in the circuit will be disconnected, causing the circuit to enter a maintenance state. When the fault subsides and the self repairing fuse cools down, it is in a low resistance conduction state and automatically connects to the circuit.

The speed of self repairing fuses is related to the magnitude of abnormal current and environmental temperature. The higher the current, the higher the temperature, and the faster the speed.

Common self-healing fuses

Self repairing fuses have structural shapes such as plug-in type, surface mount type, and chip type. Common plug-in self repair fuses include the RGE series, RXE series, RUE series, RUSR series, etc., used in computers and general electrical appliances.

Working principle of fuse:

When current passes through a conductor, it generates heat due to its resistance. After confirming the material and shape of the fuse, determine the resistance. When an electric current flows through, it will generate heat. As time increases, its calorific value also increases. The magnitude of current and resistance determines the heating rate, while the structure and installation of fuses determine the heat consumption rate. If the generated heat rate is lower than the heat dissipation rate, the fuse will not melt.

If the rate of heat generated is equal to the rate of heat dissipation, it will not melt for a long time. If the rate of heat generated exceeds the rate of heat dissipation, more and more heat will be generated, and the increase in heat will lead to an increase in temperature. When the temperature rises above the melting point of the fuse, the fuse will melt.

With the continuous improvement and upgrading of many products, the requirements for fuses in current electronic devices are becoming increasingly high. In order to maintain the safety of various commodity circuits, fuses also need to be continuously improved. Nowadays, there are many types of fuse wires, such as disposable fuse wires, self recovery fuses, patches, inserts, etc. Different types of fuses can be applied to high and low voltage working atmospheres. Nowadays, fuse links are becoming increasingly important in many electronic circuits.

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