Fuse selection

Aguilera

Fuse selection [Copy link]

1.
The following considerations must be made to correctly select the rated current value of the fuse: --- UL specification fuses have weak overload capacity, rated current In = Ir/ (Of *Ow) Here Ir is the circuit operating current, Of = 0.75 is the current reduction rate of the fuse, Ow is the temperature attenuation, the attenuation ratio is as shown in the figure below, for example, when the circuit operating current Ir = 1.5A, the operating temperature is 40°C, at least a fuse with a rated current value of >= 1.5/0.75/95% (A) should be selected


2. According to the surge waveform generated in the actual circuit, calculate the surge I2tvalue.
Then use the "pulse cycle withstand capacity" curve and the rated melting heat energy value of the fuse (i.e. the rated I2t value of the fuse).

For example, if I use a fuse that can withstand 10,000 surges, we first calculate the I2T based on the actual surge waveform, and then we can calculate the I2T value required for the selected fuse based on the number of uses (10,000 times in this design). For example, the actual measured I2T is Ir2t. Based on 10,000 uses, If2t=Ir2t / 22%
. 3. In addition to I2T, you can also refer to the withstand current time curve as shown below:


4. The difference between fast-acting and slow-acting fuses

The biggest difference between slow-blow fuses and fast- blow fuses is their ability to withstand instantaneous pulse currents.

That is to say, both can resist the impact of surge current when turning on and off the machine without moving, thus ensuring the normal operation of the equipment.

Therefore, slow-blow fuses are often called surge-resistant fuses.

　　From a technical perspective, slow-blow fuses have a larger melting heat energy value It, and the energy required for the fuse to blow is larger. Therefore, for fuses with the same rated current, slow-blow fuses have much stronger pulse resistance than fast-blow fuses.

　　Since the It of a slow-blow fuse is larger than that of a fast-blow fuse of the same specification, its fusing time when an overcurrent occurs in the circuit will also be slower than that of a fast-blow fuse. Will the protection performance be poor as some people worry?

　　Verified by experiments: No!

　　Because once a circuit fault occurs, the overcurrent will not disappear on its own. The energy of the continuous overcurrent will greatly exceed the It of the fuse, and any type of fuse will be blown.

　　The time difference between slow-blow and fast-blow is not very important for the protection requirements. Slow-blow will only affect the protection performance when there are sensitive devices in the protected circuit that need to be protected.

　　Due to the above differences, slow-blow fuses and fast-blow fuses are used in different circuits:

　　Fast-blow fuses must be used in purely resistive circuits (no or little surge) or circuits that need to protect sensitive and valuable devices such as ICs;

　　It is best to use slow-blow fuses for capacitive or inductive circuits (surges when turning the machine on and off) and power input/output parts.

　　In addition to protecting IC circuits, most situations where fast-blow fuses are used can be replaced with slow-blow fuses to improve their anti-interference capabilities.

　　On the contrary, if a fast-blow fuse is used instead of a slow-blow fuse, the fuse will often break when the power is turned on and will not work properly.