Parameter Selection Of Fuse

In many electronic devices, fuses are indispensable. Since Edison invented the first plug-in fuse that sealed the thin wire in the lamp holder in the 1990s, there are more and more kinds of fuses and their applications are more and more widely. This paper introduces the parameters, selection and application of fuse. I hope you can benefit.

The rated values and performance indexes of fuses are determined according to laboratory conditions and acceptance specifications. There are many authoritative testing and certification institutions in the world, such as UL certification of Underwriters Laboratories in the United States, CSA certification of Canadian Standards Association, MTTI certification of the Ministry of international trade and industry of Japan and IEC certification of the International Electrical Technical Committee.

The selection of fuses involves the following factors:

1. Normal working current.

2. Applied voltage applied to fuse.

3. Abnormal current required for fuse disconnection.

4. The shortest and longest time allowed for abnormal current.

5. Ambient temperature of fuse.

6. Pulse, impulse current, surge current, starting current and circuit transient value.

7. Whether there are special requirements beyond the fuse specification.

8. Size limit of installation structure.

9. Required agency certification.

10. Fuse base parts: fuse clip, mounting box, panel installation, etc.

The following describes the common parameters and terms in fuse selection.

1. When the normal working current operates at 25 ℃, the current rating of the fuse shall be reduced by 25% to avoid harmful fusing. Most traditional fuses use materials with low melting temperatures. Therefore, this kind of fuse is sensitive to the change of ambient temperature. For example, a fuse with a current rating of 10A is generally not recommended to operate at an ambient temperature of 25 ℃ at a current greater than 7.5A.

2. 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 are 32V, 125V, 250V and 600V.

3. The resistance of resistance fuse is not important in the whole circuit. Since the resistance of fuses with amperage less than 1 is only a few ohms, this problem should be considered when using fuses in low-voltage circuits. Most fuses are made of materials with positive temperature coefficient. Therefore, there are cold resistance and thermal resistance.

4. The current carrying capacity of the ambient temperature fuse is tested at an ambient temperature of 25 ℃, which is affected by the change of ambient temperature. The higher the ambient temperature, the higher the working temperature of the fuse and the shorter its service life. On the contrary, operating at a lower temperature will prolong the life of the fuse.

5. Rated fusing capacity is also called breaking capacity. The rated fusing capacity is the maximum allowable current that the fuse can indeed fuse under the rated voltage. In case of short circuit, the instantaneous overload current greater than the normal working current will pass through the fuse for many times. Safe operation requires fuses to remain intact (without bursting or breaking) and eliminate short circuits.

6. Fuse performance the performance of fuse design refers to the rapidity of fuse response to various current loads. According to the performance, fuses are often divided into four main types: normal response, delayed disconnection, fast action and current limit.

7. Harmful open circuit is often caused by incomplete analysis of the designed circuit. Among all the factors involved in fuse selection listed above, special attention must be paid to normal operating current, ambient temperature and overload increment (item 6). When in use, the fuse should not be selected only according to the normal working current and ambient temperature, but also pay attention to other service conditions. For example, a common cause of harmful open circuit of conventional power supply is that the rated value of the nominal melting heat energy of the fuse is not fully considered, and it must also meet the requirements of various surge currents generated by the input capacitor of the power supply for the fuse. If you want the fuse to be safe, reliable and long service life, the melting heat energy of the selected fuse should not be greater than 20% of the nominal melting heat energy rating of the fuse.

8. The nominal melting heat energy is the energy required to melt the fused parts, expressed in i2t and read as "ampere square second". Generally, in the authoritative certification body, the nominal melting heat energy shall be tested: apply a current increment to the fuse and measure the melting time. If the melting does not occur within about 0.008 seconds or even less, increase the intensity of the pulse current. Repeat this experiment until the melting of the fuse is limited to about 0.008 seconds. The purpose of this test is to ensure that the generated heat energy does not have enough time to run away from the fuse components through heat conduction, that is, all the heat energy is used for melting.

Therefore, when selecting fuses, in addition to the normal working current, reduced rating and ambient temperature mentioned above, i2t value should also be considered. In addition, we should pay attention to one thing: during welding, because most fuses have welded joints, we should be very careful when installing these fuses by welding. Excessive welding heat will reflow the solder in the fuse and change its rating. A fuse is a thermal element similar to a semiconductor. Therefore, it is best to use a heat absorbing device when welding a fuse.