What is the difference between slow blow and fast blow

The main difference between a slow blow fuse and a fast blow fuse is its ability to withstand the instantaneous pulse current, which means that it can resist the impact of inrush current when switching on and off without action, thereby ensuring the normal operation of the device, so the slow blow fuse It is often referred to as a surge-resistant fuse. From a technical point of view, slow-blow fuses have a large melting heat energy value I2t, and the energy required to blow the fuse is large, so for fuses with the same rated current, slow blow is much stronger than fast blow.

Because the I2t of the slow-blow fuse is larger than that of the fast-blow fuse of the same specification, the fusing time when an overcurrent occurs in the circuit will also be slower than that of the fast-blow fuse, so will it have poor protection performance as some people worry about? ? We said no! Because once the circuit fails, the overcurrent will not disappear by itself. The energy of continuous overcurrent will greatly exceed the I2t of the fuse. No matter what kind of fuse will be blown, the time difference between slow blow and fast blow is its protection requirements. It is not very important, only when sensitive devices in the protected circuit need to be protected, slow blow will affect the protection performance.

Due to these differences, slow blow fuses and fast blow fuses will be used in different circuits: pure resistance circuits (no or few surges) or circuits that need to protect sensitive and expensive devices such as ICs must use fast blow fuses; For capacitive or inductive circuits (there is a surge when switching on and off the machine), it is best to use slow blow fuses for the power input/output parts;

In addition to protecting the IC circuit, slow blow fuses can be used in most cases where fast blow fuses are used to improve their anti-interference ability; on the contrary, if fast blow fuses are used in places where slow blow fuses are used, it will often cause startup Phenomenon that the fuse cannot work normally.