How to choose the fuse correctly - Solutions - Huaqiang Electronic Network

LC03-3.3 SOP8 TVS Static Protection 3.3V

The choice of fuse involves the following factors:
1. Normal operating current.
2. The applied voltage applied to the fuse.
3. An abnormal current that requires the fuse to be disconnected.
4. The shortest and longest time allowed for abnormal current to exist.
5. The ambient temperature of the fuse.
6. Pulse, inrush current, inrush current, starting current and circuit transients.
7. Is there a special requirement that exceeds the fuse specification?
8. Size limitations of the mounting structure.
9. The required certification body.
10. Fuse holder: fuse clip, mounting box, panel mounting, etc.
The following describes some of the parameters and terminology commonly used in fuse selection.
Normal operating current: Operating at 25 ° C, the fuse's current rating is typically reduced by 25% to avoid harmful fuses. Most conventional fuses use materials that have a lower melting temperature. Therefore, this type of fuse is sensitive to changes in ambient temperature. For example, a fuse with a current rating of 10A typically cannot operate at currents greater than 7.5A at 25 °C ambient temperature.
Voltage Rating: The voltage rating of the fuse must be equal to or greater than the effective circuit voltage. The general standard voltage rating series is 32V, 125V, 250V, 600V.
Resistance: The resistance of the fuse is not critical throughout the circuit. However, for a fuse with an amperage of less than one, there are several ohms of resistance, so this should be considered when using a fuse in a low voltage circuit. Most of the fuses are made of a material with a positive temperature coefficient, so there are also cold resistance and thermal resistance.
Ambient temperature: The current carrying capacity of the fuse. The experiment was carried out at an ambient temperature of 25 ° C. This experiment was affected by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, and the lower the current carrying capacity of the fuse, the shorter the life. Conversely, allowing at lower temperatures will extend the life of the fuse.
Fuse rated capacity: Also known as breaking capacity. The blown rated capacity is the maximum allowable current that the fuse can actually fuse at the rated voltage. In the event of a short circuit, the fuse will repeatedly pass a transient overcurrent that is greater than the normal operating current. Safe operation requires the fuse to remain intact (no burst or break).
Fuse performance: The performance of a fuse is how quickly the fuse reacts to various current loads. Fuses are often classified into four types: normal response, delayed disconnection, fast action, and current limit.
Harmful disconnection: often due to incomplete analysis of the designed circuit. Of all the factors involved in the fuse selection listed above, special attention must be paid to normal operating current, ambient temperature and overload. When using, it is not possible to select the fuse only based on the normal operating current and ambient temperature, and also pay attention to other conditions of use. For example, a common cause of a harmful disconnection of a conventional power supply is that the fuse's nominal heat of fusion is not adequately considered, and it must also meet the fuse requirements imposed by the surge current generated by the power supply smoothed input capacitor. If the fuse is to work safely and reliably, then the fuse's melting heat should be no more than 20% of the fuse's nominal heat of fusion.
Nominal melting heat: refers to the energy required to melt a dissolved part, expressed in I2t, read as "ampere square seconds." Generally, in the authoritative certification body, the melting heat test is performed: a current increment is applied to the fuse and the time at which the melting occurs is measured. If the melting does not occur in about 0.008 seconds or longer, the intensity of the pulse current is increased. The experiment was repeated until the fuse had a fuse time of less than 0.008 seconds. The purpose of this test is to ensure that the heat generated does not have enough time to run away from the fuse components by heat transfer, that is, all of the heat is used to blow the fuse.
Therefore, when selecting a fuse, in addition to considering the normal operating current, reducing the rated value, and the ambient temperature, the I2t value should also be considered. Also note that since most fuses have solder joints, special care must be taken when soldering these fuses. Too much solder heat causes the solder inside the fuse to reflow and change its rating. The fuse is similar to a semiconductor thermal element, so it is best to use a heat sink when soldering the fuse.