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Current fuse, small fuse, tubular fuse application selection guide part 1

1. The basic principle of fuse
1. structure:
The most commonly used in circuit overcurrent protection components is the small tubular fuse, which is composed of two main parts: a tube body with metal connection terminals at both ends and the metal melt in the tube. The function of the shell part It is support and connection. Most fuses are cylindrical in shape, which is called tubular; the key function is determined by the internal melt.

2. Function:
The fuse is connected in series in the circuit, and its resistance is generally required to be small (low power consumption). Therefore, when the circuit is working normally, the fuse is only equivalent to a wire and can be used stably for a long time; it occurs due to power supply or external interference When the current fluctuates, the fuse can also withstand a certain range of overload; only when there is a large overload current in the circuit-fault or short circuit -, the fuse will act, and protect the safety of the circuit by breaking the current.

3. principle:
When the fuse is energized, the temperature of the melt will rise due to the heat converted by the current. When the load is normal working current or allowable overload current, the heat generated by the current is radiated, convection and conduction through the melt, the shell and the surrounding environment. The heat dissipated by other methods can gradually reach equilibrium; if the heat dissipation speed cannot keep up with the heat, the heat will be accumulated in the melt step by step, so that the temperature of the melt will rise. Once the temperature reaches and exceeds the melting point of the melt material, it will cause It melts, thereby disconnecting the current, playing a role of safety protection.

4. Terminology:

Rated current: the nominal working current of the fuse, code: In

Rated voltage: the nominal working voltage of the fuse, code: Un

Voltage drop: the voltage drop across the fuse at rated current, code: Ud

Cold resistance: the resistance value of the fuse itself when it is not working, code: Rn
Overload capacity: the overload current that the fuse can work for a long time (some varieties can work under high temperature conditions)

Fusing characteristics: the performance index of fuse operation-the functional relationship between load current and fusing time, that is, time/current characteristics (also known as ampere-second characteristics). There are usually two ways of expression:

----Fusing characteristic curve: the load current is the X coordinate, and the fusing time is the Y coordinate. The curve is connected by the coordinate points of the average fusing time of the fuse under different current loads. Each type of fuse has a corresponding curve to represent its fusing characteristics, this curve can be used as a reference when selecting a fuse.

----Fusing characteristic table: a table composed of several representative load current values and corresponding fusing time. Each type of fuse has a fusing characteristic table, which can be used as a basis for testing the fuse.

Breaking capacity: the most important safety index of the fuse-the maximum current value that the fuse can safely break when a large overload current (short-circuit) occurs. Safe breaking refers to the phenomenon that no splashing, burning, explosion, etc. endanger the surrounding components and even the personal safety when the fuse breaks the circuit. Code: Ir

Melting heat value: The nominal energy value required to melt the melt of the fuse is a parameter of the fuse itself. Code: I2 t

2. Classification of tubular fuses

1. By area of use:
Due to the different starting points and experiences of industrial development in various countries and regions in the world, there are still great differences in the design and application of small tubular fuses. At present, the main internationally recognized ones are: European specifications; North American specifications; Japanese specifications: In addition There are other specifications that are only used in a limited range.

2. According to fusing characteristics:
According to different application requirements, the same type of fuse is designed to have many different fusing characteristics. There are two types of fuses: fast fusing and slow fusing. There are also very fast fusing; medium-speed fusing and special fusing. Slow blow, etc.

3. According to breaking capacity:
From the size of the maximum current that the fuse can safely break, fuses can be divided into two categories: high breaking and low breaking, and the enhanced breaking capacity fuse between the two.

4. According to the dimensions:
There are many types of tubular fuses, the most commonly used ones are: Φ6X30(6G/6C) Φ5X20(5G/5C); Φ4X15(4G); Φ3X10(3G/3C); Φ2.4X7(2F/2T), etc.

5. According to the structure type:
There are two main types of welding connection between the end cap of the tubular fuse and the melt. They are: welding inside the tube and welding outside the tube.

6. According to the connection method:
There are two types of fuse connection to the circuit: directly soldered on the circuit board (called PGT type) and connected through other connectors.

7. Other categories:
According to the scope of application, tubular fuses can be divided into industrial electrical appliances and household appliances; according to the application industry, tubular fuses can be divided into instrumentation, communication, power supply, lighting, automotive, etc. According to the position where the fuse is connected in the circuit, there are primary and secondary fuses.
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