Knock the key! Fuse related knowledge
1, fuse definition and effect
A fuse is also known as a fuse, which is defined by the IEC127 specification as a "fuse-link". It is an electrical component that is placed in the circuit to ensure safe operation of the circuit. The effect of the fuse is: when the circuit is faulty or abnormal, the current is constantly rising, and the increased current may damage some important equipment or precious equipment in the circuit, and may also burn the circuit or even form a fire. If the fuse is properly settled in the circuit, then the fuse will blow up to a certain height and a certain moment, and the fuse will block itself and block the current, and then the effect of maintaining the safe operation of the circuit.
2, the operating principle of the fuse
When the current flows through the conductor, the conductor will have a certain resistance, so the conductor will heat up, and the heat is followed by this formula: Q=0.24I2RT. According to this formula, we can easily see the simple operation principle of the fuse. When the material of the fuse is fabricated and its shape is determined, its resistance R is relatively determined (if its temperature coefficient of resistance is not considered). When current flows through it, it heats up, and the heat added at the moment is also added. The current and the size of the resistor determine the rate of heat generation. The structure of the fuse and the condition of its device determine the rate of heat dissipation. If the speed of heat generation is less than the speed of heat dissipation, the fuse will not blow; if the speed of heat attack is equal to the speed of heat dissipation, it will not melt at a suitable length of time; When the speed is greater than the heat dissipation, the accumulated heat will increase. Because it has certain heat and mass, the heat is added to the temperature rise. When the temperature rises above the melting point of the fuse. The fuse is blown.
3, the voltage drop of the fuse is clear
The voltage drop of the fuse is the voltage drop across the fuse under additional current conditions. It reflects the internal resistance of the fuse and should not be too large. If a fuse with an excessive internal resistance (voltage drop) is placed in the circuit, it will affect the system parameters of the circuit, making the circuit unable to operate normally. The specification limits the voltage drop not to the upper limit of its value, and rules are also made for its consistency.
4. How to understand the extra voltage of the fuse
Whether the fuse is blown or not depends on the size of the current flowing through it, regardless of the operating voltage of the circuit. The extra voltage of the fuse is set from the point of view of the safe use fuse, which is the highest operating voltage of the circuit in which the fuse is safely operated. This means that the fuse can only be settled in a circuit where the operating voltage is less than or equal to the extra voltage of the fuse. As long as the fuse is safe and useful, it will continue to fly or be broken by voltage when the fuse is blown.
5. Is the extra current of the fuse the current that causes the fuse to blow?
No. It should only be regarded as a standard specification, and the current flowing through the fuse is so large and when it is blown. This has specific rules for it in the fuse product specification, and there are rules due to different specifications. Different. The fuse has a "fuse factor" that is greater than "1" (typically between 1.1 and 1.5) and is the ratio of "conventional non-blown current" to "extra current." It can be seen that even if the current flowing through the fuse is greater than its extra current and does not exceed the conventional non-blown current, the fuse should not be blown.
6. What is a slow fuse?
The slow fuse is also called a time-delay fuse. Its time-delay characteristic is that the circuit is in good condition when it presents a non-faulty pulse current and can provide maintenance for long-term overload. Some circuits have a current greater than a few times or even tens of times the normal operating current at the switching instant. Although this current peak is very high, it presents a very short time, which we call pulse current, surge current or surge current. A typical fuse can't withstand this kind of current. If such a circuit is used with a general fuse, it may not be able to start normally. If a larger specification fuse is used, then there is no useful maintenance when the circuit is overloaded. To solve the above problems, it is more appropriate to use a slow fuse. Since the melt of the slow fuse is processed by special materials, it has the effect of absorbing energy, and adjusting the energy absorption amount can make it both resist the inrush current and maintain the overload supply.
7. What is the significance of studying the temperature rise of the fuse?
The temperature rise of the fuse is the value of the fuse's temperature when the extra current flows through the fuse is 1.1 times, that is, the measured temperature minus the ambient temperature. The UL specification has an upper limit of 75 °C. Since the melt of the fuse is sensitive to temperature, it will have a long-term effect at a certain temperature, and its melting point and impedance will change, which will affect the accuracy of the fuse. This is what is commonly referred to as fuse aging. It is very dangerous to use an aging fuse in the circuit. Therefore, we should pay attention to the temperature rise of the fuse when using the fuse. In the same way, we should also note that even if the fuse used for a long time has not melted, it may be aging now, and it is best to replace it at this moment.
8, how to understand the breaking of the fuse
When the current between the conventional non-blown current and the additional breaking current of the relevant specification rules is applied to the fuse, the fuse should be able to operate satisfactorily without jeopardizing the surrounding environment. The expected fault current of the circuit in which the fuse is settled must be less than the additional breaking current of the specification. Otherwise, when the fuse is blown, it will continue to fly, ignite, fuse burn, and the fuse symbol is unrecognizable.