FUSE is a key component of many electronic devices. Since Edison's invention of the first plug fuse in the 1890s, which enclosed thin wires in a lamp holder, fuses have grown in variety and use. Here are some fuse parameters, selection and application common sense.
Fuse ratings and performance indicators are determined according to laboratory conditions and acceptance specifications. There are a number of authoritative testing and authentication institutions in the world, such as the United States Underwriters Experimental company UL certification, Canadian Standards Association CSA certification, Japan's Ministry of International Trade and Industry MTTI certification and International Electrical Technology Commission ICE certification.
The selection of a fuse involves the following factors:
1. Normal working current.
2. The applied voltage applied to the fuse.
3. The abnormal current that requires the fuse to be disconnected.
4. Minimum and maximum time for allowing abnormal current to exist.
5. Ambient temperature of the fuse.
6. Pulse, impulse current, surge current, starting current and circuit transient value.
7. Whether there are any special requirements beyond the fuse specification.
8. Size limitation of mounting structure.
9. Certification bodies required.
10. Fuse holder: fuse clip, mounting box, panel mounting, etc.
The following is the fuse selection of common parameters and terms for some description.
Normal operating current: When operating at 25℃, the current rating of the fuse is usually reduced by 25% to avoid harmful blowout. Most conventional fuses are made of materials with low melting temperatures. Thus, the fuse is more sensitive to changes in ambient temperature. For example, A fuse with a current rating of 10A will not normally operate at a current greater than 7.5A at an ambient temperature of 25℃.
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 very important in the whole circuit. However, the resistance of fuses with amps less than 1 will be several ohms, so this should be considered when using fuses in low-voltage circuits. Most fuses are made of positive temperature coefficient materials, so there are cold resistance and hot resistance.
Ambient temperature: the current carrying capacity of the fuse. The experiment is carried out at the ambient temperature of 25℃, which is affected by the change of the ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, the lower the current carrying capacity of the fuse, and 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 fuse rating capacity is the maximum allowable current that the fuse can actually blow at the rated voltage. In short circuit, a transient overload current larger than the normal working current will pass through the fuse several times. Safe operation requires that fuses remain intact (no blowouts or breaks).
Fuse performance: Fuse performance refers to the speed with which the fuse responds to various current loads. Fuses are usually divided into four types according to their performance: normal response, delay break, fast action and current limit.
Harmful circuit breaks: Often caused by incomplete analysis of the designed circuit. Of all the factors involved in the selection of fuses listed earlier, special attention must be paid to normal operating current, ambient temperature, and overloading. In use, not only according to the normal operating current and ambient temperature to choose the fuse, but also pay attention to other conditions of use. For example, a common cause of harmful circuit breaks in conventional power sources is insufficient consideration of the nominal melting heat energy rating of fuses, which must also meet the requirements placed on fuses by the surge current generated by the input capacitors of the power supply, which are smoothly filtered. If the fuse is to work safely and reliably, the fusion heat energy of the fuse shall be selected to be no more than 20% of the nominal melting heat energy rating of the fuse.
Nominal melting heat: refers to the energy required to melt broken parts, expressed in I2t, read as "amperes squared seconds". In general, a fusion heat test is performed at a certified authority: a current increment is applied to the fuse and the time for melting to occur is measured. If no melting occurs for about 0.008 seconds or longer, the intensity of the pulse current is increased. Repeat the experiment until the fuse is blown within 0.008 seconds. The purpose of this test is to ensure that the heat generated does not have enough time to escape from the fuse component through thermal conduction, that is, all the heat is used to break the fuse.
Therefore, when choosing a fuse, in addition to considering the normal working current, reduced rating, ambient temperature, but also consider the I2t value. Also note: Since most fuses have welded joints, special care should be taken when welding these fuses. Too much welding heat will cause the solder in the fuse to backflow and change its rating. Fuses are similar to semiconductor thermal components, so it is best to use heat absorbers when welding fuses