Product Introduction
A fuse protects a circuit by using the Joule heat generated by the current flowing through a fusible metal element to melt itself and thus break the circuit. In the latter half of the 19th century, fuses emerged alongside the invention of various electrical devices, led by the electric lamp. Since then, the basic structure of fuses has remained largely unchanged and has been continuously used. Common types we often see include glass tube fuses and blade fuses used in automobiles. As protective components for electronic devices, glass tube fuses have been used for a long time, but due to their large size, fragility, and inability to be installed automatically, there has been increasing demand in the industry for miniature fuses.
Traditional fuses generally serve the primary purpose of protecting the power input section. Nowadays, the situation has changed somewhat, and fuses have evolved to serve many new purposes, such as protecting internal printed circuit boards and ICs, as well as input and output circuits. This has led to a yearly increase in the use of miniature fuses. Miniature fuses are characterized by their small size, high sensitivity, and rapid protection. They are commonly used in switching power supplies, chargers, and small appliance control boards.
Classification
① Products can be divided by shape into square-type miniature fuses and cylindrical-type miniature fuses,
with sizes respectively: 8.5*7.2mm, 8.4*4.0*5.0mm, 8.4*4.0*7.7mm, 9.0*4.8*8.8mm, 12.4*6.4*9.2mm;
Voltage ratings: 65V/125V/250V/300V/400V/450V
Models include: VF370\VF372\VF373\VF382\VF389\VF391\VF392\VF393\VF395\VF396\VF400\VF804\VF807\VF808, etc.
② Miniature fuses are divided by blowing characteristics into fast-blow and slow-blow.
③ According to standards, they can be divided into American and European standards.
In practical applications, selecting a fuse solely based on UL specifications is far from sufficient.
1. Several issues that must be considered when selecting
Based on understanding the parameters of the circuit in use, select the fuse according to the following aspects: normal current, blowing current (the current and time at which you want the fuse to blow), open-circuit voltage, short-circuit current, surge current (current, time, and frequency not interrupted), and ambient temperature.
① Normal current: First, we must know the normal current flowing through the fuse in the circuit. Usually, a derating factor is preset (UL: 0.75, IEC: 0.9), and the selection principle is that the normal current must be less than the product of the rated current and the derating factor.
② Blowing current: According to UL specifications, the fuse should blow quickly at twice the rated current. However, in most cases, to ensure reliable blowing, we recommend the blowing current be greater than 2.5 times the rated current. If blowing time is important, the fuse characteristic curves provided by the manufacturer must also be referenced.
③ Open-circuit voltage: The open-circuit voltage should generally be selected to be less than the rated voltage. For example, using a fuse rated at DC 24V in an AC 100V circuit may cause the fuse to ignite or rupture.
④ Short-circuit current: The maximum current flowing through the fuse during a short circuit is called the short-circuit current. Each fuse has a specified rated breaking capacity, and when selecting a fuse, care must be taken not to exceed this rated breaking capacity. Choosing a fuse with a low breaking capacity may cause the fuse to rupture or even cause a fire.
⑤ Surge current: Observe the waveform of the surge current (pulse current waveform) and calculate its energy using the I²t value (Joule integral). Different magnitudes and frequencies of surge current have different effects on the fuse.
Fuse rated I 2 T > actual pulse I 2T/Pf
Pf: Pulse factor, varies depending on the number of pulses the fuse can withstand, specific values:
Surge cycle resistance capability
When the measured pulse I²t value is 22% of the fuse's melting I²t value, the fuse can withstand up to 100,000 pulse waves.
When the measured pulse I²t value is 29% of the fuse's melting I²t value, the fuse can withstand up to 10,000 pulse waves.
When the measured pulse I²t value is 38% of the fuse's melting I²t value, the fuse can withstand up to 1,000 pulse waves.
When the measured pulse I²t value is 48% of the fuse's melting I²t value, the fuse can withstand up to 100 pulse waves.
2. Other issues to note
In practical use, any special requirements or issues regarding installation conditions, chemical resistance, ultrasonic cleaning resistance, etc., for miniature fuses must be confirmed in advance with the manufacturer.
Application Cases
Power Adapter
The fuse used is of VF392 or VF808 specifications.
Charger
The fuse used is of VF392 or VF808 specifications.
The fuse used is of VF392 or VF808 specifications.
