Fuse wire

fuse, also known as a current fuse, is defined as "fuse-link" by the IEC127 standard. Its main function is overload protection. Circuit in the correct placement of fuse, fuse will be in the current abnormal rise to a certain height and heat, its own fusible cut off the current, to protect the safe operation of the circuit. [1]

The fuse, invented by Edison more than a hundred years ago to protect the then expensive incandescent light bulbs, has evolved over time to protect electrical equipment from overheating and from serious damage caused by internal failures of electronic equipment.

When the circuit is faulty or abnormal, the current keeps rising, and the rising current may damage some important components in the circuit, burn the circuit or even cause a fire. If the circuit is correctly placed in the fuse, then the fuse will be in the current abnormal rise to a certain height and heat, its own fuse to cut off the current, so as to protect the safe operation of the circuit.

shape

1. Filamentous. An early primitive type of fuse, locked directly with screws, used in older switches and receptacles of various sizes.

2, flake (nude). Easier to use than the old silk.

3. Glass tube shape. They come in several different sizes and are commonly found in electronic products.

6.3 x 32 mm (diameter x length)

5 x 20 mm

4, ceramic tubular. Available in several different shapes and sizes to avoid glass cracking.

5. Plastic sheet with metal sheet pin: car fuse.

6. Surface following element (SMD) type.

7, cylindrical, plug-in type: directly welded to the circuit board, for the internal product.

mark

Logo Most fuses are marked on the body or end cap with markings indicating their rating. But "chip type" fuses feature little or no markings, making identification very difficult.

Fuses may appear similar to each other with significantly different characteristics that define their marking. The fuse mark will usually convey the following information:

Ampere fuse rating

Voltage grade fuse

Time-current characteristics, i.e., speed fuses

Approval by national and international standards bodies

Manufacturer/Product number/series

Interrupt capability

function

The fuse was invented by Edison more than 100 years ago to protect the then expensive incandescent light bulbs. As The Times went on, the fuse protected the electronic/electrical equipment from current/overheating and from serious damage caused by internal faults.

Working principle

When the current flows through the conductor, because the conductor has a certain resistance, so the conductor will be hot. The calorific value follows this formula: Q=0.24I2RT; Where Q is the calorific value, 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 the current flows through the conductor. According to this formula, we can see the simple working principle of the fuse.

When the material and shape of the fuse are determined, its resistance R is relatively determined (regardless of its resistance temperature coefficient). When an electric current flows through it, it heats up, increasing its calorific value over time. The size of the current and resistance determines the speed of heat generation, and the structure of the fuse and its installation determines the speed of heat dissipation. If the speed of heat generation is less than the speed of heat dissipation, the fuse will not blow. If the rate at which heat is generated is equal to the rate at which heat is dissipated, it will not be fused for quite a long time. If the rate at which heat is produced is greater than the rate at which heat is dissipated, more and more heat is produced. And because it has a certain specific heat and mass, the increase of its heat is reflected in the rise of temperature, when the temperature rises above the melting point of the fuse fuse will be blown. That's how a fuse works. 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 ensure that they have consistent geometric dimensions. Because these factors play an important role in the normal operation of the fuse. Again, when you use it, be sure to install it correctly.

Basic composition

General fuse is composed of three parts: one is the melt part, it is the core of the fuse, fuse to cut off the current role, the same class, the same specification of fuse melt, the material should be the same, the geometric size should be the same, the resistance value as small as possible and should be consistent, the most important is the fuse characteristics should be consistent, household fuse commonly used lead antimony alloy made; The second is the electrode part, usually there are two, it is an important part of the melt and the circuit connection, it must have good conductivity, should not produce obvious installation contact resistance; Three is the bracket part, fuse fuse melt is generally fine and soft, the role of the bracket is to fix the melt and make the three parts into a rigid whole easy to install, use, it must have good mechanical strength, insulation, heat resistance and flame retardant, in use should not produce fracture, deformation, combustion and short circuit phenomenon.

Arc extinguishing device

Power circuits and high-power equipment used by the fuse, not only general fuse three parts, but also arc extinguishing device, because this kind of fuse protection circuit not only the working current is larger, and when the melt is fusible the voltage at both ends is also very high, often appear melt has been melted (fusible) or even vaporized, but the current has not been cut off, The reason is that at the moment of fusing under the action of voltage and current, the two electrodes of the fuse arc phenomenon. The arc extinguishing device must have strong insulation and good thermal conductivity, and be negative. Quartz sand is commonly used as an arc extinguishing material.

Fuse breaker

In addition, there are some fuse indicating device, its role is when the fuse action (blown) after its appearance changes, easy to be found by maintenance personnel, such as: light, color, pop up solid indicator.

Breaking ability editing

When a current is applied to a fuse between the conventional unfused current and the rated breaking capacity specified in the relevant standard, the fuse shall operate satisfactorily and without endangering the surrounding environment. The expected fault current of the circuit where the fuse is placed must be less than the rated breaking capacity current specified in the standard; otherwise, when the fault occurs, the fuse will continue to fly, ignite, burn the fuse, melt together with the contact, and the fuse mark cannot be recognized. Of course, the breaking capacity of inferior fuse can not meet the requirements stipulated in the standard, and the above hazards will occur when used. [2]

classification

According to the protection form, can be divided into: over current protection and overheating protection. A fuse used for overcurrent protection is commonly referred to as a fuse (also called a current-limiting fuse). Fuses used for overheating protection are commonly referred to as "temperature fuses". Temperature fuse is divided into low melting point alloy shape and temperature trigger shape and memory alloy shape and so on (temperature fuse is to prevent heating appliances or easy heating appliances temperature is too high and protect, such as: hair dryer, electric iron, electric cooker, electric furnace, transformer, motor and so on; It responds to the temperature rise of electrical appliances, regardless of the working current of the circuit. It works differently from a "current-limiting fuse").

According to the scope of use, it can be divided into: power fuse, machine tool fuse, electrical instrument fuse (electronic fuse), automobile fuse.

According to the volume points, can be divided into: large, medium, small and micro.

Temperature fuse

Temperature fuse

According to the rated voltage, can be divided into: high voltage fuse, low voltage fuse and safety voltage fuse.

According to breaking capacity, can be divided into: high, low breaking capacity fuse.

By shape, they can be divided into: flat tubular fuse (which can also be divided into inner and outer welded fuse), pointed tubular fuse, guillotine fuse, spiral fuse, insert fuse, plate fuse, wrap fuse, and sticker fuse.

According to the fusing speed, it can be divided into: extra slow fuse (generally represented by TT), slow fuse (generally represented by T), medium speed fuse (generally represented by M), fast fuse (generally represented by F), extra fast fuse (generally represented by FF).

According to the standard points, can be divided into: European fuses, American fuses, Japanese fuses.

According to the type, it can be divided into: current fuse (patch fuse, micro fuse, insert fuse, tubular fuse), temperature fuse (RH[block type], RP[resistance type], RY[metal shell]), self-restoring fuse (plug-in, laminated, patch).

According to size, it can be divided into: patch type 0603,0805,1206,1210,1812 2016 2920; 2.4 x 7, non patch Φ Φ 3 x 7, 3.6 x 10 Φ, Φ 4.5 x 15, Φ 5.0 x 20, 5.16 x 20 Φ, Φ 6 x 25, Φ 6 x 30, Φ 6 x 32, Φ 8.5 x 8, Φ 8.5 x 8 x 4, Φ 10 x 38, Φ 14 x 51.

Self-restoring fuse

Low zero power resistance: self-resetting fuse self-impedance is low, low power loss during normal operation, low surface temperature.

Fast overcurrent protection: Because of its own material characteristics, the response speed of self-replicating fuse is much faster than that of other overcurrent protection devices.

Self-locking operation: The self-resetting fuse in the overcurrent protection state, with a minimal current locked in the high resistance state, only after the power supply is cut off or the overcurrent disappears, will be restored to the low resistance state.

Automatic reset: The self-resetting fuse will reset itself after overcurrent protection (troubleshooting) without disassembling.

High current resistance: self - fuses have excellent high current resistance, some specifications can withstand 100A current impact.

Application: PPTC has a wide range of applications, which can be used in various electronic products, communication products, power supplies, etc.

1. When the normal working current is operated at 25℃, the current rating of the fuse is usually reduced by 25% to avoid harmful fusing. Most conventional fuses are made of materials with low melting temperatures. Thus, the fuse is more sensitive to changes in ambient temperature. For example, A fuse with a current rating of 10A is not recommended to run at a current greater than 7.5A at 25℃.

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 is 32V, 125V, 250V, 600V.

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

4. Ambient temperature The current carrying capacity of the fuse is tested under the ambient temperature of 25℃, which is affected by the change of the ambient temperature. The higher the ambient temperature, the higher the operating temperature of the fuse and the shorter its life. Conversely, operating at lower temperatures will extend the life of the fuse.

5. Fusing rated capacity is also called breaking capacity. The fuse rating capacity is the maximum allowable current that the fuse can actually blow at the rated voltage. In short circuit, a transient overload current larger than the normal working current will pass through the fuse several times. Safe operation requires that fuses remain intact (no blowouts or breaks) and short circuits are eliminated.

intelligence

For most asynchronously rectified booster switching converters using inductors, there is a DC path between input and output, as shown in Figure 1. The existence of this path will cause two adverse consequences: first, once the output short circuit or serious overload time beyond a few hundred milliseconds will lead to the diode (usually Schottky diode) overheating damage; Second, when the switching oscillation circuit stops working due to some reason, such as artificial shutdown, there is still voltage at the load end, but the tube voltage drop is one diode lower than the input end, then the output will still consume energy. In addition, if the residual voltage is below the load steady-state operating voltage range, the circuit will be in an uncertain state.

For applications with relatively small output currents (less than 5A), the monolithic current mode controller and high-end current sampling technology can solve both of these problems well. In these circuits, the diode is replaced by a synchronous rectifier switch triode, so that the input/output path is truncated by turning off the internal switch, so that the load side is high resistance to the input side, which is the desired result. In normal operation, the high - end sampling resistance inside the circuit periodically samples the load current, thus avoiding catastrophic overcurrent consequences. Thus, the internal overheating protection circuit provides a safe working area (SAO) for the converter.

The MAX668 is an on-off controller that performs the booster function. A current feedback boost controller (MAX668) drives a low logic level N-channel enhanced MOSFET. The switch tube passes through a low current sampling resistance to ground. The high end switch is a Schottky diode, which was chosen mainly because it has a low positive guide pass voltage drop. As can be seen from the figure, the basic topology structure of the boost converter is not damaged. In this application, the MAX668 changes the 3.3V voltage to 5V and the load current can reach 3A.

Patch fuse VICFUSE

Patch fuse VICFUSE

Among them, P channel enhanced MOSFEt-Q1 is the key element to realize load break. When MAX668 is in off mode, diode D1 is still on, making the MAX810L's power supply voltage 3.3V minus the tube voltage drop of diode D1. Since MAX810L has a RESET threshold level of 4.65V, its reset output is high, forcing Q1 to shut off and thus disconnecting the load from the input power. The MAX668 sets a 5V output voltage via an external feedback resistor network. When the output voltage exceeds the reset threshold level of MAX810L, the internal monostable circuit starts to work and delays about 240ms. After that, the output of MAX810L is lowered to enable Q1 to pass.

After Q1 is turned on, MAX810L monitors the output voltage to determine if the output is overcurrent. Overload will cause the output voltage to drop. When it falls below the MAX810L threshold, the MAX810L output changes from high to low after a delay of 20μs, thus shutting off Q1 and disconnecting the load. Due to the boost effect of MAX668, the voltage of the MAX810 power supply terminal will be higher than its threshold level again. After the reset delay time of 240ms, the MAX810L output will change from high to low again, Q1 will be turned on and the load will be automatically connected again. This process is repeated periodically until excess load is removed or MAX668 is shut down to stop working. So MAX810L and switch Q1 together form a solid state switch (electronic fuse).

The MAX810L (micropower device) has a non-balanced push-pull output stage. When output current, it is equivalent to a 6kΩ resistance; When current is drawn from the outside, it is equivalent to a 125Ω resistance. When Q1 is switched on or off, the switching transients are slowed down because the resistance of MAX810L prevents the rapid charging and discharging of Q1's Miller capacitor and gate source capacitor. Assuming that the total equivalent capacitance of Q1 is 5000pF, the RC circuit time constant of MAX810 high current triode is about 0.6μs when it draws current (equivalent to 125Ω resistance). Whole conduction