Choice is the choice of a traditional fuse or resettable fuse PTC, which should be selected, each fuse has its benefits depending on the specific product circuit.
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For example, many designs for computers, peripherals, and portable devices (such as smartphones, tablets, etc.) require the use of PTCs because PTC can reset itself. If a conventional fuse is used, it must be replaced every time an overcurrent occurs, which is unacceptable to the user.
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In other cases, it may be better to use a conventional fuse because the fuse can completely cut off the current under fault conditions. This may be a more desirable option if priority is given to safety or to avoid damage to downstream circuit equipment. The use of fuses is also useful for troubleshooting. It can help designers and users of equipment find the source of overcurrent faults.
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Overcurrent circuit protection
Faced with the task of providing over-current protection, circuit designers have the option of using the technology. Traditional fuses and polymer-based PTC (Positive Temperature Coefficient) devices are the most common solutions. If you can understand the difference between these two components, you can simplify the process of selecting the best application protection device.
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Fuses have always been considered as a "one-time" device, because the fuse will be blown once in the event of an overload condition, providing over-current protection and then having to replace it. The core of a conventional fuse is a piece of wire. When the current is too high, the wire is heated to the melting point. After the wire is blown, the current in the circuit drops to zero.
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The PTC can also respond to excessive currents, but it is a "self-healing" device. Polymer-based components can automatically reset after the overload disappears, enabling multiple overcurrent protection. When the conductive polymer is heated by an overcurrent, its resistance will increase, thereby limiting the circuit current.
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Select between PTC and fuse
Over-current circuit protection can be achieved using conventional fuses or using self-healing PTCs. Both are designed to respond to heat generated by excessive current in the circuit to achieve protection. Fuse breaks the current by melting, and PTC relies on limiting the current from low impedance to high impedance. Full understanding of the performance differences between the two devices will make it easier for you to choose the best circuit protection solution.
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The biggest difference between the two is that PTC can recover itself. The usual self-healing step after overloading is to shut off the power and cool the device. There are other operational differences between the two products. The terminology used by PTC is usually similar to the term used for the fuse, but it is not exactly the same. For example, two parameters of leakage current and breaking rating belong to this kind of situation.
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Leakage current:
When overloaded, the PTC changes from a low-impedance state to a high-impedance state, which is often referred to as "tripping." Limit the current to a leak level for protection purposes. The leakage current can increase from about one hundred milliamps at rated voltage to several hundred milliamperes at lower voltages. However, in the case of an overload fuse, the fuse blows to completely cut off the current, and the leakage current generated by the open circuit is "0".
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Breaking current:
The maximum short-circuit current specified by PTC at rated voltage. The fault current is the maximum current the device can withstand, and the PTC will not actually cut off the current (see “Leakage Current†above.) The standard PTC short-circuit rated current is 40A. In order to respond to overload, the fuse must actually cut off the current. The rated breaking current ranges from a few hundred amps to 10,000 amps at rated voltage.
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Circuit parameters can be tailored to the choice of components based on the standard device rating.
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Rated voltage:
The rated voltage of the conventional PTC does not exceed 60V, but the rated voltage of the fuse can reach 600V. Rated current: PTC rated operating current up to 11A, but the maximum rated operating current of the fuse may exceed 20A.
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Rated temperature:
The upper limit of the normal temperature of the PTC is generally 85°, and the maximum operating temperature of the fuse is 125°C. Both devices have their rated temperature down when operating at temperatures above 20°C. Circuit designers can also refer to other operating parameters when selecting overcurrent protection PTCs or fuses.
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resistance:
It can be found that the PTC has twice the resistance (sometimes higher) than the fuse at similar ratings. Time-current characteristics: By comparing the time-current curves of the PTC and the fuse, it can be found that the PTC response time is equivalent to the fuse's time delay.
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PTC Overcurrent Protection Applications
PTC materials are available in both radial leaded and surface mount options. The self-restoring PTC features are suitable for a variety of design applications:
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The PTC protects the hard disk drive from damage when the power supply device generates excessive voltage due to a fault and then causes overcurrent. Hard disk drive applications typically use surface mount PTCs.
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Power supplies are vulnerable to circuit failures. Without protective measures, the current supplied by the power supply device may cause a low resistance fault. When there are multiple loads or multiple circuits, separate PTCs can be used to protect each load. The device is usually placed in the output circuit and can be in a radial leaded or surface mount package.
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The high temperature generated by the over-current of the motor may damage the winding insulation. For a small motor, even a winding with a very small diameter may fail. The PTC does not normally trip at normal starting current. The motor is generally protected by a radial leaded PTC.
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Transformers can be damaged by overcurrents caused by circuit faults, and the PTC's current limiting function provides protection. The PTC can be placed on the load side of the transformer to minimize the effects of circuit faults. Many applications use radial leaded or surface mount devices. - Focus on "Qin Jin Electronics"
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