By EECTECH | 21 June 2020 | 0 Comments
General introduction of fuse
The fuse protects the electronic equipment from over-current damage, and can also avoid the serious injury caused by internal failure of the electronic equipment. Therefore, each fuse has a rated specification. When the current exceeds the rated specification, the fuse will blow. When the current between the conventional non-fusing current and the rated breaking capacity (current) specified in the relevant standard acts on the fuse, the fuse should work satisfactorily and will not endanger the surrounding environment.
The expected fault current of the circuit where the fuse is placed must be less than the rated breaking capacity current specified by the standard, otherwise, when the fault occurs, the fuse will continue to arc, ignite, the fuse burns, melts together with the contact, the fuse Unrecognizable signs and other phenomena. Of course, the breaking capacity of inferior fuse can not meet the requirements of the standard, and it will also cause harm when used.
In addition to the fuse resistors, the protective s used in electronic equipment include ordinary fuses, thermal fuses, and self-recovery fuses. The protection is generally connected in series in the circuit. When an abnormal phenomenon such as overcurrent, overvoltage or overheat occurs in the circuit, it will immediately melt and play a protective role, which can prevent the fault from further expanding.
(1) Ordinary fuse
Ordinary fuses are commonly known as fuses or fuses. They are fused non-recoverable fuses. Only new fuses can be replaced after fusing. It is represented by "F" or "FU" in the circuit.
Structure characteristics of ordinary fuse
Common fuses are usually composed of glass tubes, metal caps and fuses. Two metal caps are sleeved on both ends of the glass tube, fuses (made of low melting point metal materials) are installed in the glass tube, and both ends are welded to the center holes of the two metal caps, respectively. When in use, install the fuse in the safety seat and connect it with the circuit in series.
The fuses of fuses are mostly linear, and only the delay fuses used in color televisions and computer monitors are spiral fuses.
The main parameters of ordinary fuse
The main parameters of ordinary fuse are rated current, rated voltage, ambient temperature and response speed. The rated current is also called the breaking capacity, which refers to the current value that the fuse can fuse under the rated voltage. The normal operating current of the fuse is 30% lower than the rated current. The rated current value of the domestic fuse is usually marked directly on the metal cap, and the imported fuse is marked on the glass tube with a color ring.
The rated voltage refers to the most regulated operating voltage of the fuse, which is divided into four specifications of 32V, 125V, 250V and 600V. The actual operating voltage of the fuse should be lower than or equal to the rated voltage. If the working voltage value of the fuse exceeds the rated voltage value, it will quickly blow.
The experiment of the current carrying capacity of the fuse is carried out under the condition of 25℃ ambient temperature. The service life of the fuse is inversely proportional to the working ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse and the shorter its life.
The reaction speed refers to how quickly the fuse responds to various electrical loads. The fuse can be divided into normal response type, delay opening type, fast acting type and current limiting type according to the reaction speed and performance.
(2) Thermal fuse
Thermal fuse, also known as temperature fuse, is a non-recoverable overheating insurance component, which is mostly used in various electronic products such as electric cookers, motors, washing machines, electric fans, power transformers and so on. The thermal fuse can be divided into low melting point alloy thermal fuse, organic compound thermal fuse and plastic-metal thermal fuse according to different temperature-sensitive materials.
Low melting point alloy thermal fuse
The temperature-sensing body of the low-melting-point alloy thermal fuse is made of an alloy material with a fixed melting point. When the temperature reaches the melting point of the alloy, the temperature-sensing body will automatically melt and will be disconnected by the protection circuit. According to the different structure, low melting point alloy type thermal low melting point alloy thermal fuse can be divided into three types: gravity type, surface tension type and spring reaction type.
Organic compound type thermal fuse
The organic compound type thermal fuse is composed of temperature sensing body, movable electrode, spring and so on. The temperature-sensitive body is made of organic compounds with high purity and low fusing temperature range. Normally, the movable electrode is in contact with the fixed end point, and the circuit is connected by the fuse; when the temperature reaches the melting point, the temperature sensing body is automatically fused, the movable electrode is disconnected from the fixed end point under the action of the spring, and the circuit is disconnected. Protect it.
Plastic-metal thermal fuse
The plastic-metal thermal fuse adopts a surface tension type structure, and the resistance value of the temperature sensing body is almost zero. When the working temperature reaches the set temperature, the resistance of the temperature-sensing body will suddenly increase, preventing the current from passing.
(3) Self-recovery fuse
Self-recovery fuse is a new type of fuse with over-current and over-temperature protection function, which can be used repeatedly.
Structure principle of self-recovery fuse
Self-recovery fuse is a PTC thermal with positive temperature coefficient. It is made of a mixture of polymer and conductive material. It is connected in series in the circuit and can replace the traditional fuse.
When the circuit works normally, the self-recovery fuse is in a conducting state. When an overcurrent fault occurs in the circuit, the temperature of the fuse itself will rise rapidly, and the polymer material will quickly enter a high-resistance state after being heated, and the conductor will become an insulator, cutting off the current in the circuit and putting the circuit into a protected state. When the fault disappears and the self-recovery fuse cools, it assumes a low-resistance conduction state and automatically turns on the circuit.
The operating speed of the resettable fuse is related to the size of the abnormal current and the ambient temperature. The higher the current and the higher the temperature, the faster the operating speed.
Commonly used resettable fuses
Self-recovery fuses have plug-in type, surface mount type, chip type and other structural shapes. Commonly used plug-in self-recovery fuses are RGE series, RXE series, RUE series, RUSR series, etc., used in computers and general electrical appliances.
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