Fuse blowing time standard_What is the main function of the fuse?

Fuses are commonly known as fuses or fuses. The earliest fuse was invented by Edison more than 100 years ago. Due to the undeveloped industrial technology at that time, incandescent lamps were very expensive. Therefore, fuses were originally used to protect expensive incandescent lamps.

Fuses protect electronic equipment from overcurrent damage, and also prevent serious damage to electronic equipment caused by internal faults.

Therefore, there is a rating on each fuse, and when the current exceeds the rating, the fuse will blow. When a current between the conventional non-fuse current and the rated breaking capacity (current) specified in the relevant standard acts on the fuse, the fuse shall work satisfactorily without endangering 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 fuse is blown when the fault occurs, there will be continuous arcing, ignition, fuse burning, melting together with contacts, fuse Marks cannot be recognized, etc. Of course, the breaking capacity of inferior fuses does not meet the requirements of the standard, and harm will also occur during use.

In addition to fuse resistors, the protection components used in electronic equipment also include ordinary fuses, thermal fuses and self-recovery fuses. The protection element is generally connected in series in the circuit. When an abnormal phenomenon such as overcurrent, overvoltage or overheating occurs in the circuit, it will immediately fuse and play a protective role, which can prevent further expansion of the fault.

(1) Ordinary fuse

Ordinary fuses are commonly known as fuses or fuses, which are non-recoverable fuses. It is represented by “F” or “FU” in the circuit.

Structural characteristics of ordinary fuses

Ordinary fuses are usually composed of glass tubes, metal caps and fuses. Two metal caps are sleeved on both ends of the glass tube, the fuse (made of low melting point metal material) is installed in the glass tube, and the two ends are respectively welded to the center holes of the two metal caps. When using, put the fuse into the fuse holder and connect it in series with the circuit.

The fuses of fuses are mostly straight, and only the delay fuses used in color TVs and computer monitors are spiral fuses.

The main parameters of ordinary fuse

The main parameters of ordinary fuses 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 working 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 maximum regulated operating voltage of the fuse, which is divided into four specifications: 32V, 125V, 250V and 600V. The actual working voltage of the fuse should be lower than or equal to the rated voltage. If the working voltage of the fuse exceeds the rated voltage, it will blow quickly.

The test of the current carrying capacity of the fuse was carried out at an ambient temperature of 25°C. The service life of the fuse is inversely proportional to the operating ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse and the shorter its life.

Response speed refers to how quickly the fuse responds to various electrical loads. Fuses can be divided into normal response type, delayed disconnection type, quick action type and current limiting type according to the response speed and performance.

(2) Thermal fuse

Thermal fuse, also known as thermal fuse, is a non-recoverable overheating fuse element, which is widely used in various electronic products such as electric cookers, electric motors, washing machines, electric fans, and power transformers. Thermal fuses can be divided into low melting point alloy thermal fuses, organic compound thermal fuses and plastic-metal thermal fuses according to different temperature sensing materials.

Low melting point alloy type thermal fuse 

The temperature sensing body of the low melting point alloy thermal fuse is processed from an alloy material with a fixed melting point. When the temperature reaches the melting point of the alloy, the temperature sensing body will automatically fuse and will be disconnected by the protection circuit. According to their different structures, low melting point alloy thermal fuses 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 consists of a temperature sensing body, a movable electrode, a spring, and the like. The temperature sensor is processed from organic compounds with high purity and low melting temperature range. When normal, the movable electrode is in contact with the fixed terminal, and the circuit is connected by the fuse; when the temperature reaches the melting point, the temperature sensing body automatically fuses, the movable electrode is disconnected from the fixed terminal under the action of the spring, and the circuit is disconnected. to protect.

Plastic-Metal Type Thermal Fuses

The plastic-metal thermal fuse adopts a surface tension structure, and the resistance value of its temperature sensing body is almost 0. When the working temperature reaches the set temperature, the resistance value of the temperature sensing body will suddenly increase, preventing the current from passing through.

(3) Self-recovery fuse

Self-recovery fuse is a new type of fuse element with overcurrent and overheating protection functions, which can be reused many times.

The structure principle of resettable fuse

The self-recovery fuse is a PTC thermal element with a positive temperature coefficient. It is made of a mixture of high molecular polymers and conductive materials. It is connected in series in the circuit and can replace the traditional fuse.

When the circuit is working 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 making the circuit enter a protection state. When the fault disappears and the self-recovery fuse cools, it is in a low-resistance conduction state and the circuit is automatically connected.

The action speed of the self-recovery fuse is related to the magnitude of the abnormal current and the ambient temperature. The larger the current and the higher the temperature, the faster the action speed.

Commonly used resettable fuses

Self-recovery fuses have plug-in, surface-mounted, chip and other structural shapes. Commonly used plug-in resettable fuses include RGE series, RXE series, RUE series, RUSR series, etc., which are used in computers and general electrical appliances.

Post time: Feb-17-2022