What is fuse?
The fuse is a kind of protective electrical and electronic components. It is usually connected in series in a circuit. When the fault current increases to a certain value, it fuses itself and cuts off the circuit to achieve the purpose of protecting other devices in the circuit. The most commonly used part of the circuit overcurrent protection component is the fuse. The traditional fuse is mainly composed of two parts: the tube with metal connection terminals at both ends and the metal melt in the tube. Most of the fuses are cylindrical in shape, which is called tubular structure. But now with the rapid development of high-tech, some special materials have been well used. The fuse industry has begun to develop toward micro-products. A series of new products have appeared, such as chip fuse, surface-attached fuse and so on.

Two. Fuse function
The earliest fuse was invented by Edison more than a hundred years ago, and was originally used to protect expensive incandescent lamps because the industrial technology at that time was underdeveloped and expensive. The fuse protects the electronic equipment from overcurrent, and also avoids the serious damage caused by the internal fault of the electronic equipment. Therefore, each fuse has a rating, and the fuse will break when the current exceeds the rating. When the current between the conventional uninterruptible current and the rated breaking capacity specified in the relevant standards acts on the fuse, the fuse should act satisfactorily and not endanger the surrounding environment. The expected fault current of the circuit in which the fuse is placed must be less than the rated breaking capacity current specified in the standard. Otherwise, when the fault occurs, the fuse will be fused with continuous arcing, ignition, fuse burning, fusing together with the contacts, fuse marking can not be recognized and so on. Of course, the breaking ability of the inferior fuse can not meet the requirements of the standard, the use of the same hazards will occur.

Three, related instructions
1. Rated current: also known as the nominal working current of the fuse. The rated current values are usually 100mA, 160mA, 200mA, 315mA, 400mA, 500mA, 630mA, 800mA, 1A, 1.6A, 2A, 2.5A, 3.15A, 4A, 5A, 6.3A.
2. Rated voltage: the nominal working voltage of the fuse, the standard voltage rating of the general fuse is 24V, 32V, 63V, 125V, 250V, etc. A fuse can be used at a voltage equal to or less than the rated voltage, but it can not be used in a circuit whose circuit voltage is greater than the rated voltage of the fuse.
3. Voltage drop: On the fuse rated current, when the fuse reaches the thermal balance, that is, the temperature stability of the measured voltage at both ends. Because of the voltage drop at the two ends of the fuse, the circuit will be certain.
Therefore, there is a clear regulation on voltage drop in the European regulation, but no requirement in the American regulation.
4. Overload current: overload current refers to the current flowing through the circuit which is higher than normal. If the overload current can not be cut off in time, it may cause damage to electronic or electrical components in the circuit. Short-circuit current refers to the current generated by partial or total short-circuit in a circuit. The short-circuit current is usually large and larger than the overload current.
5, fuse characteristic: time / current characteristic. There are usually two ways of expression, that is, I-T diagram and test report. I-T diagram is a curve formed by the average fuse fuse fusing time coordinates under different current loads in a coordinate system composed of load current x coordinates and fusing time y coordinates. Each type of fuse has a corresponding curve representing its fusing characteristics. This curve can be used for reference when selecting fuse. The test report is the test data record of the test items done according to the standard requirements.
6. Breaking capacity: also known as rated short-circuit capacity, that is, under rated voltage, the fuse can safely disconnect the maximum current of the circuit (AC is the effective value), it is an important safety parameter of the fuse.
7, the melting energy value: the energy value of fuse melting, its code is I2t, A2Sec. It is the product of the square of the corresponding current and the fusing time when the fuse is disconnected in 8ms or less. The time limit is less than 8ms, so that all the heat generated by the fuse is used to fuse and not to dissipate heat. It is a constant for each different fuse component, a parameter of the fuse itself, determined by the design of the fuse.
8. Temperature Rise: Temperature Rise refers to the difference between the temperature of the fuse and that of the room temperature when the temperature is stabilized under the condition of the specified current value.

Four. Structure of fuse
The general fuse consists of three parts: first, the melt part, which is the core of the fuse, plays the role of cutting off the current when fusing, the geometric size should be the same, the resistance value should be as small as possible and consistent, the most important thing is to fuse the same characteristics; second, the electrode part, usually two, it is fused and circuit connection. Important components, it must have good conductivity, should not produce obvious installation contact resistance; third, the part of the support, fuse melt is generally slender and soft, the role of the support is to fix the melt and make the three parts into a rigid whole for easy installation and use, it must have good mechanical strength, insulation It should not cause fracture, deformation, combustion and short circuit in use.
Five. The working principle of fuse
When the fuse is electrified, the electric energy is transformed into Joule heat because of its own resistance, and the heat generated by the electric current radiates to the surrounding environment through the melt and shell, and dissipates heat through convection and conduction. When the fuse passes through the allowable working current, the heat emitted is balanced with the heat generated. The heat does not accumulate in the melt to raise the melt temperature, so the fuse does not reach its melting point and fuse. When the current through the fuse reaches a certain value, the heat transformed by electric energy increases, the heat dissipation rate can not keep up with the heating rate, the heat will gradually accumulate in the melt, so that the melt temperature rises, when the temperature reaches the melting point of the fuse, the fuse begins to melt and continues to absorb heat to further melt into liquid, with The arc is a kind of gas discharge phenomenon. The arc strength is related to the voltage of the circuit. The higher the voltage, the stronger the arc. The main difference of the rated voltage of the fuse is that the voltage of the circuit can be withstood when the fuse is fused. In a voltage circuit, the arc is not easily extinguished because the circuit voltage is higher than the fuse rated voltage. In addition, the strength of the arc is also related to the current in the circuit. The greater the current, the stronger the arc. If the arc can not be extinguished in time, not only the circuit can not be cut off, but also other components in the circuit may be burned out, resulting in fire, causing accidents. The high explosion-proof fuse is used to extinguish the arc by adding explosion-proof sand. After the arc is extinguished, the fuse can cut off the current and protect other equipment.

Six. Fuse classification
According to the form of protection, it can be divided into overcurrent protection and over heat protection. The fuse used for over current protection is the usual fuse (also known as current limiting fuse). The fuse used for overheating protection is commonly referred to as "temperature fuse". Temperature fuses can be divided into low melting point alloys, temperature sensitive triggers, memory alloys, etc. (Temperature fuses are designed to protect the heater from overheating, such as hair dryers, iron, rice cookers, electric stoves, transformers, motors, etc.); they affect the temperature rise of electrical appliances. Rise, do not care about the size of the circuit's working current. Its working principle is different from "fuse".
According to the scope of use, it can be divided into: power fuse, machine tool fuse, electrical instrument fuse (electronic fuse), automobile fuse.
According to body integral, it can be divided into large, medium, small and miniature. According to the rated voltage, can be divided into: high-voltage fuse, low-voltage fuse and safe voltage fuse.
According to the breaking capacity, it can be divided into high and low breaking capacity fuses.
According to the shape, it can be divided into: flat-headed tubular fuse (can also be divided into internal welding fuse and external welding fuse), pointed tubular fuse, raspberry fuse, screw fuse, insert fuse, flat fuse, wrapped fuse, patch fuse.
According to the fusing speed, it can be divided into: ultra-slow fuse (usually expressed by TT), slow fuse (generally expressed by T), medium-speed fuse (generally expressed by M), fast fuse (generally expressed by F), ultra-fast fuse (generally expressed by FF).
According to the standard: European Standard Fuse, American rule fuse and sundial fuse.
According to the type, it can be divided into: current fuse (patch fuse, micro fuse, insert fuse, tubular fuse), temperature fuse (RH [block type], RP [resistance type], RY [metal shell]), self-replicating fuse (plug-in, laminate, patch)
According to size, it can be divided into: patch type 0603, 0805, 1206, 1812, 2016, 2920; non-patch type phi 2.4*7, phi 3*7, phi 3.6*10, phi 4.5*15, phi 5.0*20, phi 6*25, phi 6*30, phi 6*32, phi 8.5*8, phi 8.5*8*4