Fuse (fuse) is also known as current fuse, IEC127 standard defines it as "fuse (fuse-link)". Its main role is to play the role of overload protection. Properly placed in the circuit fuse, the fuse will be in the current abnormally high to a certain height and heat, their own fuse cut off the current, to protect the safe operation of the circuit.
More than a hundred years ago by Edison invented the fuse for the protection of expensive incandescent lamps, with the development of the times, the fuse to protect electrical equipment from overcurrent overheating, to avoid serious injury caused by internal malfunctions of electronic equipment.
Basic introduction Chinese name ：fuse Foreign name ：fuse IEC127 definition ：fuse-link Shape ：bar wire, sheet shape Fusing mark ：luminous, color changing, pop-up solid indicator Arc extinguishing material ：quartz sand Definition, introduction, shape, mark, role, composition, basic components, arc extinguishing device, fusing device, breaking capacity, classification, self-repeating fuse, relevant Description, Intelligent, High-voltage Fuses, Other Types, Definition When a circuit malfunction or abnormality occurs, it is accompanied by a rising current, and the rising current may damage some important devices in the circuit, or it may burn the circuit or even cause a fire. If a fuse is properly placed in a circuit, it will blow when the current is abnormally high and hot, thus protecting the circuit. VICFUSE Introduction Shape 1, strip wire. Early primitive type of fuse, directly locked with screws, used in various sizes of old switches and sockets. VICFUSE 2. Chip (Bare Chip). Easier to use than the old filament type. 3, Glass tube. Available in several different sizes, commonly used in electronics. 6.3 x 32 mm (Diameter x Length) 5 x 20 mm 4. Ceramic Tubular. There are several different shapes and sizes, can avoid the glass burst. 5、Plastic sheet with metal tabs: automotive fuses. 6、Surface mounted device (SMD) type. 7、Cylinder shape, external program type: directly soldered to the circuit board for use inside the product. Markings Most fuses are marked on the body or end cap with markings that indicate their rating. However, "chip type" fuses feature little or no markings, making identification very difficult. Fuses may appear similar with significantly different characteristics that define their markings. Fuse markings typically convey the following information: Ampere rating of the fuse Voltage rating of the fuse Time-current characteristics, i.e., speed fuse Approved by national and international standards bodies Manufacturer/product number/series Interrupting capacity Function Invented more than a hundred years ago by Thomas Edison to protect the expensive incandescent lamps of the day, fuses have evolved over the years to protect electronic/electrical equipment from overcurrent/overheating and to prevent electronic equipment from being damaged by overcurrent/overheating. Fuses protect electronic/electrical equipment from overcurrent/overheating and prevent serious injury caused by internal malfunctions in electronic equipment. Principle of operation When current flows through a conductor, it heats up due to the resistance of the conductor. The amount of heat generated follows this formula: Q=0.24I2RT; where Q is the amount of heat generated, 0.24 is a constant, I is the current flowing through the conductor, R is the resistance of the conductor, and T is the time it takes for the current to flow through the conductor; according to this formula it is not difficult for us to see the simple principle of the fuse's operation. When the material used to make the fuse and its shape are determined, its resistance R is relatively certain (if you do not take into account its resistance temperature coefficient). When current flows through it, it generates heat, which increases with time. The current and the size of the resistance to determine the rate of heat generation, the construction of the fuse and its installation to determine the rate of heat dissipation, if the rate of heat generation is less than the rate of heat dissipation, the fuse will not blow. If the rate of heat generation is equal to the rate of heat dissipation, the fuse will not blow for a long period of time. If the rate of heat generation is greater than the rate of heat dissipation, more and more heat will be generated. And because it has a certain specific heat and mass, the increase in heat is expressed in the increase in temperature, when the temperature rises above the melting point of the fuse when the fuse has blown. This is the working principle of the fuse. We should know from this principle that you must carefully study the physical properties of the materials you choose when designing and manufacturing fuses, and make sure they have consistent geometry. These factors are critical to the proper functioning of the fuse. Likewise, when you use it, you must install it correctly. A fuse Composition of the basic components of the general fuse consists of three parts: one is the melt part, which is the core of the fuse, when the fuse to play the role of cutting off the current, the same class, the same specification fuse melt, the material should be the same, the same geometric dimensions, the resistance value of the smallest possible and to be the same, the most important is that the melting characteristics of the same, the household fuse is commonly made of lead-antimony alloy. Commonly used lead-antimony alloy made of; two is the electrode part, usually two, it is an important part of the melt and circuit connection, it must have good electrical conductivity, should not produce obvious installation contact resistance; three is the bracket part of the fuse is generally slim and soft, the role of the bracket is to fix the melt and make the three parts of the rigid whole for easy installation, use, it must have good mechanical strength, It must have good mechanical strength, insulation, heat resistance and flame retardancy, and should not produce fracture, deformation, combustion and short-circuit phenomena in use. Tubular Fuse VICFUSE Arc extinguishing device Power circuits and high-power equipment used by the fuse, not only the three parts of the general fuse, but also the arc extinguishing device, because this type of fuse to protect the circuit is not only a large operating current, and when the melt occurs when the fuse is blown the voltage on both sides is also very high, and often appear to be the melt has been melted (melted), or even has been vaporized, but the current is not the same, but the current is not the same. Or even vaporized, but the current is not cut off, the reason is that in the melting of a moment in the voltage and current under the action of the two electrodes of the fuse arc phenomenon. The arc extinguishing device must have a strong insulation and good thermal conductivity, and is negatively charged. Quartz sand is commonly used as an arc extinguishing material. Fusing device In addition, there are some fuses have a fuse indication device, its role is when the fuse action (fuse) after its own appearance of certain changes, easy to be found by the maintenance staff, such as: luminous, color change, pop-up solid indicator, and so on. Breaking Capacity When a current between the conventional non-blowing current and the rated breaking capacity (of the current) specified in the relevant standard is applied to a fuse, the fuse shall operate satisfactorily and without endangering the surroundings. The expected fault current of the circuit in which the fuse is placed must be less than the rated breaking capacity current specified in the standard; otherwise, when a fault occurs the fuse will blow with sustained flying arcs, ignition, blowing of the fuse, melting of the fuse together with the contacts, and illegibility of the fuse markings. Of course, the breaking capacity of poor quality fuses does not meet the requirements of the standard, the use of the same will occur when the above hazards. Classification According to the form of protection, it can be divided into: overcurrent protection and overheat protection. Fuses for overcurrent protection are commonly known as fuses (also called current limiting fuses). Fuses for thermal protection are generally called "thermal fuses". Temperature fuse is divided into low melting point alloy form and temperature-sensitive trigger form and memory alloy form, etc. (Temperature fuse is to prevent heating appliances or easy to heat appliances overheating temperature and protection, such as: hairdryers, irons, rice cookers, electric stoves, transformers, electric motors and so on; it responds to the rise in the temperature of the electrical appliances, will not pay attention to the circuit of the size of the working current. (Its working principle is different from "current-limiting fuse"). According to the scope of use points, can be divided into: electric power fuses, tool machine fuses, electrical instrumentation fuses (electronic fuses), automotive fuses. According to size, can be divided into: large, medium, small and micro. According to rated voltage, can be divided into: high-voltage fuses, low-voltage fuses and safe voltage fuses. Temperature Fuses According to Breaking Capacity, can be divided into: high and low breaking capacity fuses. According to the shape , can be divided into: flat tube fuse (can be divided into internal welding fuse and external welding fuse), pointed tube fuse, guillotine fuse, spiral fuse, plug-in fuse, flat fuse, wrap-around fuse, patch fuse. According to fusing speed points, can be divided into: special slow fuse (generally expressed in TT), slow fuse (generally expressed in T), medium-speed fuse (generally expressed in M), fast fuse (generally expressed in F), special fast fuse (generally expressed in FF). According to the standard points, can be divided into: European standard fuses, U.S. standard fuses, Japanese standard fuses. According to type, it can be divided into: current fuse (chip fuse, miniature fuse, insert fuse, tube fuse), temperature fuse (RH [square type], RP [resistance type], RY [metal casing]), and self-recovery fuse (external program, laminated, chip). By size can be divided into: SMD 0603, 0805, 1206, 1210, 1812, 2016, 2920; Non-SMT Φ2.4×7, Φ3×7, Φ3.6×10, Φ4.5×15, Φ 5.0×20, Φ5.16×20, Φ6×25, Φ6×30, Φ6×32, Φ8.5×8, Φ8.5×8×4, Φ10×38, Φ14×51. Self-returning fuse Low zero-power resistance: Self-returning fuse has a low impedance of its own, which leads to a low power loss and a low surface temperature during normal operation. Fast overcurrent protection: Due to its own material characteristics, the overcurrent status of the self-repeating fuse reverberates much faster than other overcurrent protection devices. Self-locking operation: In the overcurrent protection state, the self-repeating fuse is locked in the high-resistance state with a very small current, and only after cutting off the power supply or the disappearance of overcurrent, it will restore the low-resistance state. Automatic reset: the self-repeating fuse resets itself after playing the role of overcurrent protection (troubleshooting), without the need for removal and replacement. High-current resistance: the self-repeating fuse has excellent high-current resistance, some specifications can withstand 100A current impact. Sets: PPTC sets a wide range of applications, can be used in a variety of electronic products, communication products, power supplies, etc.. Related Note 1. Normal operating current is operated at 25℃, the current rating of the fuse is usually reduced by 25% to avoid harmful melting. Most conventional fuses are made of materials that have a low melting temperature. As a result, these fuses are sensitive to changes in ambient temperature. For example, a fuse with a current rating of 10A is usually not recommended for operation at ambient temperatures of 25°C at currents greater than 7.5A. VICFUSE Automotive Fuses 2. Voltage RatingsThe voltage rating of a 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. Resistance The resistance of a fuse is not important in the overall circuit. Since the resistance of fuses with amperage less than 1 is only a few ohms, this should be taken into account when employing fuses in low voltage circuits. Most fuses are manufactured from materials with a positive temperature coefficient, hence the distinction between cold and hot resistors. 4. Ambient temperature fuse current carrying capacity, the experiment is carried out in 25 ℃ ambient temperature conditions, such experiments are affected by changes in ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse, the shorter its life. Conversely, operation at lower temperatures will extend the life of the fuse. 5. Fusing rating capacity is also known as breaking capacity. The rated capacity is the maximum allowable current at which the fuse can actually blow at the rated voltage. In the event of a short-circuit, the fuse will pass several times an instantaneous overload current greater than the normal operating current. Safe operation requires that the fuse remain intact (no bursts or breaks) and that short circuits be eliminated. INTELLIGENCE For most non-synchronous rectifier boost switching converters that use inductors, there is a direct current path between the input and output, as shown in Figure 1. The existence of this path causes two undesirable consequences: one, once the output is shorted or severely overloaded for more than a few hundred milliseconds will result in overheating and damage to the diode (usually a Schottky diode); and two, when the switching oscillator circuit is stopped for some reason, such as an artificial shutdown, a voltage still exists at the load side, just one diode's tube voltage drop lower than the input, and the output will still consume energy at this time The output will still consume energy. In addition to this, if this residual voltage is below the load's steady-state operating voltage range, it will leave the circuit in an uncertain state. For suites with relatively small output currents (less than 5A), both of these problems can be well addressed by utilizing monolithic current mode controllers and high end current sampling techniques. In these circuits, the diode is replaced by a synchronous rectifier switching triode, so that the input and output paths can be truncated by turning off the internal switching triode, so that the load is highly resistive to the input, which is the desired result. In normal operation, the load current is periodically sampled by the internal high side sampling resistor, thus avoiding catastrophic consequences due to overcurrent. Therefore, an internal thermal protection circuit provides a safe operating area (SAO) for the converter. One of the MAX668 is a switching controller by which the boost function is accomplished. The current-feedback boost controller (MAX668) drives a low-side logic level N-channel enhancement MOSFET, which is passed through a low-side current sampling resistor to ground. The high side switch is a Schottky diode, chosen primarily for its low forward conduction voltage drop. As can be seen from the figure, the basic structure of the boost converter topology is left intact. In this setup, the MAX668 turns a 3.3V voltage into 5V with a load current of up to 3A. SMD FUSE VICFUSE One of the P-channel enhancement MOSFETs, Q1, is the key component to realize the load disconnect. When the MAX668 is in shutdown mode, the diode D1 is still on, making the voltage at the supply side of the MAX810L 3.3V minus the tube voltage drop of the diode D1. Since the reset threshold level of the MAX810L is 4.65V, the output of its RESET terminal goes high, forcing Q1 to turn off, thus disconnecting the load from the input power supply.The MAX668 sets the 5V output voltage through a network of external feedback resistors. When the output voltage exceeds the reset threshold level of the MAX810L, its internal monostable circuit starts to work and delays for about 240 ms. After that, the output of the MAX810L becomes low, making Q1 conductive. After Q1 turns on, the MAX810L keeps monitoring the output voltage to determine if the output is overcurrent. An overload will cause the output voltage to drop, and when it falls below the MAX810L threshold level, the output of the MAX810L goes from high to low after a 20-μs delay, turning off Q1 and disconnecting the load. Due to the MAX668 boosting effect, MAX810 power supply voltage will be higher than its threshold level, 240ms reset delay time, MAX810L output again from high to low, open Q1 and automatically connected to the load again. The above process will be repeated periodically unless the excess load is removed or the MAX668 is turned off to stop its operation. Thus the MAX810L and switch Q1 together form a solid-state switch (electronic fuse). Fuse The MAX810L (micro power device) has an unbalanced push-pull output stage. It is equivalent to a 6kΩ resistor when outputting current to the outside and a 125Ω resistor when drawing current from the outside. When turning Q1 on or off, the switching transients are slowed down because the MAX810L's resistance prevents the Miller capacitance and gate source capacitance of Q1 from charging and discharging rapidly. Assuming a total equivalent capacitance of Q1 of 5000pF, the time constant of the RC circuit of the high-current triode when the MAX810 draws current (equivalent to a 125Ω resistor) is about 0.6μs. The voltage transient reverberation time for the entire on-state process is about 10RC = 6μs. The time to completely turn off the same switch Q1 is about 48 times the time to fully turn it on. When the external load or C2 in the start-up moment to draw a large current, fast conduction Q1 may make the MAX810 input voltage below its reset threshold voltage, resulting in reset appears, so in Figure 2 based on the addition of a RC network to slow down the turn-on process, the appropriate choice of R, C can be made to continue the process of load connectivity to a few MAX668 switching cycle, so that the output voltage of the MAX668 has been higher than the MAX810 reset threshold voltage. Suppose R, C makes Q1 on-time prolonged, but also prolongs the off-time. Therefore a Schottky diode needs to be connected in parallel across the resistor to speed up the process of turning Q1 off when the load is overloaded. In order to obtain the enhanced channel and low on-resistance, the above circuits need to use logic level control P-channel MOSFETs, if the on-resistance value of Q1 is large and produces a large voltage drop across its terminals (especially in low output voltage sets or when the load is far away from the power supply), the output should be regulated by feeding back the voltage from the drain end of Q1. When designing the circuit, it is important to minimize parasitic parameters and carefully consider the circuit layout. The above remote regulation can be achieved using a low voltage analog switch (MAX4544) in a SOT23 package, which is controlled from the output of the MAX810L, as shown in Figure 4. According to the MAX4544 product parameters, its minimum operating voltage is 2.7 V. Since the input voltage is 3.3 V, and the Schottky's forward tube voltage drop is 0.3 V, the MAX4544 (and the MAX810) are in operation even if the boost converter is in off mode. At this time, MAX810 output high level, MAX4544 public **** terminal COM and its normally open terminal NO (Q1 source) connected. When the MAX668 enable, and MAX4544 male *** terminal connected to the resistor network to provide feedback voltage for the MAX668. Since the on-resistance of the MAX4544 can be up to 60Ω at 5V, the value of the feedback resistor should be large in order to get the minimum output voltage error. Since the on-resistance of the MAX4544 is only 120Ω at 3V operating voltage, the error voltage introduced by switching the MAX4544 is small, even at low output voltages. When the boost converter is enabled and its output voltage exceeds the MAX810's reset threshold level and after a reset delay, the MAX810's output will go from high to low, turning Q1 on and connecting the load. At the same time, the low level of the MAX810 output turns on the COM and NC terminals (normally closed terminals) of the MAX4544, causing the feedback resistor to be switched from the source of Q1 to the drain of Q1, thus allowing the output voltage to be regulated from the load side away from the converter. The switching process of the MAX4544 described above also switches the input of the MAX810 from the source of Q1 to the drain of Q1, so that the MAX810 can be used to monitor the load for overload. High Voltage Fuses High voltage fuses are often used in high voltage towers and poles as overcurrent or short circuit protection for transmission and distribution systems by using Fuse Cutout switches that also function as start/stop switches. Other types of Resettable Fuses Resettable Fuses or Overcurrent Protection Blocks. Resettable Fuses Thermal Fuses A thermal fuse is a fuse that deforms when the temperature exceeds a certain safe temperature, resulting in a loss of power, which can be restored when the temperature returns. Thermal cutoff A thermal cutoff is commonly found in today's electrical devices and will blow when the temperature exceeds a certain safe temperature. Examples of devices with thermal cutoffs are electric pots, heaters, coffee makers, and hair dryers. Although the specifications of thermal cutoffs also include an allowable current, it is important to note that thermal cutoffs do not provide the same level of current protection as current fuses and should not be confused with them.