Fuse Resistors
Fused resistors (model: RF) are special components that have the dual functions of resistors and fuses. Under normal circumstances, they have the electrical characteristics of ordinary resistors and function as ordinary resistors. When the circuit fails, such as the power supply voltage changes or the components are short-circuited, the fuse resistor will be overloaded and blown quickly, acting as a fuse to protect the circuit and prevent the fault from expanding. Fused resistors are available in cylindrical shell and square shell structures.
Fuse Resistance
Classification of fuse resistors
Fuse resistors can be divided into resettable and one-time fuse resistors according to their working methods:
(1) The resettable fusing resistor is to weld an ordinary resistor (or resistance wire) with a low-melting point solder to an elastic metal sheet and then seal it in a cylindrical or square casing. When the load current is too large and the temperature is too high, the solder joint of the low-melting point solder will melt, and the elastic metal sheet will be separated from the resistor to disconnect the circuit. After the circuit fault is eliminated, the resistor and the metal sheet are welded in series, and the normal operation can be resumed.
(2) Disposable fuse resistor (non-recoverable fuse resistor), which acts as a fixed resistor when the circuit is working normally. When the working current exceeds the rated current, it will fuse quickly to protect the circuit. Fused resistors are used in color, black and white TV sets, tape recorders, amplifiers and other electrical appliances, and their appearance and electrical graphic symbols are shown in Figure 1
Figure 1: Symbol for the fuse resistance
The fuse resistor has the following characteristics
1. Fast current response. The resistance wire of the fuse resistor is usually very thin, and when the current passes, it will heat up rapidly and fuse, so its response speed is very fast, and it can cut off the current in time in the circuit.
2. Small current capacity. Because the resistance wire of the fuse resistor is very thin, its current capacity is relatively small, usually between hundreds of milliamperes and several amperes. Therefore, when choosing a fuse resistor, it is necessary to make a reasonable choice according to the current in the circuit.
3. Need to replace after fusing. Unlike ordinary resistors, fuse resistors need to be replaced after protecting the circuit to keep the circuit functioning properly.
Selection of fuse resistors
Because the fuse resistor is a resistor with a protective function, its dual performance should be considered when selecting it. According to the specific requirements of the circuit, select its parameters such as resistance value and power, not only to ensure that it can quickly fuse when over-current, but also to ensure that it can work stably for a long time under normal conditions. When replacing this resistor, it must be absolutely the same as the original resistance value and rated power (wattage). If the resistance value and power are too large or too small, there will be no protection.
The resistor in the circuit shown in Figure 2 is a fuse resistor, and its function is: from the power distribution to the connection resistance of each part of the circuit, as a resistor for short-circuit protection (acting as a fuse). When burning out a resistor and replacing it, choose a resistor with the same resistance and power as the original one, and do not increase or decrease the resistance and power at will. If the resistance value of the replacement resistor is equal to the original one and its power is increased, when a short-circuit fault occurs in the circuit, the resistance will continue to burn, and it will not work as it should.
Figure 2: Fused Resistors in circuits
Detection of blown resistor
In the circuit, if the surface of the fuse resistor is found to be black or burnt, it can be concluded that its load is too heavy and the current passing through it exceeds the rated value many times. If there is no trace on the surface and it is open, it means that the current flowing is just equal to or slightly greater than its rated fusing value.
To judge the quality of the fuse resistor without any traces on the surface, it can be measured with a multimeter R×10 Ω file. For accurate measurement, the fuse resistor should be soldered off the circuit. If the measured resistance value is infinite, it means that the fuse resistor has been opened. If the measured resistance value is too far from the nominal value, it means that the resistance value has changed and can no longer be used. In the maintenance practice, it is found that there are also very few fuse resistors that are broken down and short-circuited in the circuit, so pay attention when testing!
The difference between a fuse and a fuse resistor
1. The principle is different:
Fuses work using thermal effects. When the current passes through the fuse, because the current will cause the fuse to generate heat, when the current is too large, the heat will cause the fuse to blow, thus cutting off the current. The fuse resistor works by using metal fusing. When the current passes through the fuse resistor, due to the limited conductive area of the resistance wire, high temperature will be generated at the resistance wire. When the current is too large, the resistance wire will be fused to cut off the current.
2. Different appearance:
A fuse is typically a glass or ceramic tube with a thin wire wrapped around it, while a fuse is a small metal rectangle or cylinder much smaller in size than a fuse.
3. Different reusability:
Fuses are usually one-time use, and even if the fault of the circuit has been resolved, it needs to be replaced with a new fuse before it can continue to be used. And some fuse resistors can be reused, just need to replace the fuse.
The difference between a fuse resistor and a normal resistor
The difference between a fuse resistor and a normal resistorDifferences | Fusible Resistors | Ordinary Resistors |
|---|
Function | Overcurrent Protection | Circuit Stabilization, Adjustment Resistor |
Features | Once the current exceeds the rated value, the fuse resistor acts as a fuse to protect the circuit | The resistor simply absorbs energy to reduce the voltage and dissipates it like heat |
Resistor value | Generally small | Can have a large range |
Uses | Commonly used in electronic product protection circuits | Commonly used in electronic products and various circuits |
Price | Relatively expensive | Relatively cheap |
Materials | Metal Alloys/Ceramics | Metal Films/Carbon Films |
Appearance | Generally small, covered | Covered or exposed |
Pressure resistance | generally low | can have a larger range |
Advantages of Fused Resistors
The advantages of fusible resistors include:
Automatic protection: The fuse resistor can automatically disconnect the circuit to prevent damage caused by circuit overload or short circuit.
High reliability: The fuse resistor is made of metal material, which has high reliability and long life.
Small size and light weight: Compared with fuses, fuse resistors are small in size and light in weight, which is convenient for installation and wiring.
Reusable: Some fuse resistors can be reused by replacing the fuse after they are fused.
Good high temperature resistance: the fuse resistor can work in a high temperature environment and has good high temperature resistance.
High precision: The resistance value of the fuse resistor is stable and has high precision.
Low inductance: Fused resistors have low inductance and are suitable for high frequency circuits.
Low noise: The noise of the fuse resistor is low and will not interfere with the circuit.
Disadvantages of fuse resistors
The main disadvantages of fuse resistors are as follows:
The fuse resistor is burned out in a short time and needs to be replaced. Therefore, the blown resistor cannot be reused, nor can it be automatically restored, and the resistor needs to be replaced manually.
The thermal effect of the fuse resistor is large, which may affect the temperature of the circuit. In a high temperature environment, the fuse resistor may appear "false melting", that is, under normal working conditions, the fuse resistor may automatically fuse, affecting the normal operation of the circuit.
The capacitance of the fuse resistor is large, which may affect the high frequency response of the circuit.
The price of fuse resistors is relatively high, which may not be suitable for some low-cost electronic products.
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