How to Distinguish Between Ordinary Resistors and Fuse Resistors? A Brief Discussion on the Inspection and Repair of Fuse Resistors

　　A fuse resistor functions like a regular resistor under normal conditions. However, once a circuit fault occurs and the resistor exceeds its rated power, it will disconnect the circuit within a specified time, thereby protecting other components. Fuse resistors are categorized into two types: non-repairable and repairable.

　　Function

　　In circuit diagrams, these components serve a dual role as both fuses and resistors, and are primarily used in the output circuits of power supplies and secondary power supplies. They typically have low resistance values (ranging from a few ohms to several tens of ohms) and low power ratings (from 1/8 W to 1 W). Their primary function is to promptly melt and disconnect when an overcurrent occurs, thereby protecting other components in the circuit from damage.

　　When a short-circuit fault occurs in the circuit load and overcurrent happens, the temperature of the fuse resistor rises rapidly within a very short time to between 500 and 600°C. At this point, the resistive layer heats up, flakes off, and eventually melts open, thereby acting as a fuse and enhancing the overall safety of the device.

　　Discrimination method

　　Although fuse resistors are widely used in power supply circuits, the methods of marking them in circuit diagrams vary from country to country and from manufacturer to manufacturer. Although there is currently no unified symbol for these markings, they do share certain common characteristics:

　　(1) They differ significantly from the markings typically used for ordinary resistors, making them easy to identify in circuit diagrams.

　　(2) It is generally used in power supply circuits with relatively large current capacities or in low-voltage or high-voltage circuits generated by secondary power supplies.

　　(3) The safety resistor has only one color band. As shown in the accompanying figure, the color of the band indicates the resistance value.

　　(4) In the circuit, the fuse resistor has its long lead soldered onto the circuit board (typically, the resistor is soldered closely against the circuit board), and it is positioned relatively far from the circuit board to facilitate heat dissipation and easy identification.

　　Specification standards

　　(1) RN1/4W, 10Ω fuse resistor with a black color band and a power rating of 1/4W; when an 8.5V DC voltage is applied across the fuse resistor, its resistance increases to more than 50 times its initial value within 60 seconds.

　　(2) RN1/4W, 2.2Ω fuse resistor with a red color band and a power rating of 1/4W; when a current of 3.5A flows through it, the resistance increases to more than 50 times its initial value within 2 seconds.

　　(3) RN1/4W, 1Ω fuse resistor with a white color band and a power rating of 1/4W; when an AC current of 2.8A flows through it, the resistance increases to more than 400 times its initial value within 10 seconds.

　　How to distinguish between a regular resistor and a fuse resistor

　　Both have resistance functions, but a resistor fuse is essentially a combination of a resistor and a fuse. When the applied power exceeds the resistor’s rated power, the fuse will blow—without the entire resistor itself catching fire or emitting smoke. In contrast, when the applied power exceeds a resistor’s rated power, the resistor itself will burn up and emit smoke.

　　How to check whether a fuse resistor is good or bad

　　A fuse resistor, also known as a fusible resistor, is widely used in electrical appliances such as televisions, video recorders, CD players, VCD players, and various instruments and meters. In circuits, the fuse resistor serves a dual purpose: it functions both as a resistor and as a fuse. Under normal operating conditions, it behaves like a conventional resistor. However, when a circuit fault occurs—for example, when the current exceeds the rated load of the protected circuit—the fusible material embedded in the resistor body reacts with the resistive film layer (the reaction time is measured in seconds), causing the resistive film to rupture. As a result, the resistance value of the fuse resistor increases dramatically or even becomes infinitely high, thereby protecting the sensitive components in the associated circuit from damage. Currently, there is no universally accepted standard for the circuit symbol and package form of fuse resistors.

　　Testing method

　　1. The appearance shall be inspected visually under normal lighting conditions.

　　2. The external dimensions of the resistor shall be inspected using a dial caliper with an accuracy of 0.02 mm under normal lighting conditions.

　　3. Electrical parameters are measured using the “YD2810D LCR Digital Bridge” instrument at a frequency of 1 kHz. For high-temperature testing, the component under test is placed in a thermostatic air-blowing drying oven set at 130℃, and measurements are taken after 5 minutes.

　　4. Electrical performance shall be tested by continuously subjecting the device to 30 consecutive impacts at room temperature using an “impact test bench (self-made, specifically for fuse resistors),” followed by a short-circuit melting test.

　　5. After wiping the resistor surface three times with an alcohol-soaked cotton swab, the markings should still be clear and legible.

　　6. Lead Bending Performance: Using tweezers, grasp either lead of the resistor at its midpoint, bend it at a 90° angle, and then straighten it out. This constitutes one cycle. The test requires three such bending cycles, all performed in the same plane and in the same direction, with no fractures allowed.

　　7. Solderability shall be tested in accordance with the solderability inspection specifications and shall comply with the requirements of 5.2.1.

　　What should I do if the fuse resistor is damaged?

　　What should you do if a fuse is damaged? Once you discover that a fuse resistor has burned out, first identify the root cause of the burnout. Do not replace it blindly, and definitely do not substitute it with a regular resistor. If there’s no fuse resistor of the same specification available, you can use the following method for an emergency replacement.

　　1. Using a resistor and a fuse in series as a substitute: Connect a resistor and a fuse (or fuse tube) in series to serve as a substitute. The resistance value and power rating of the resistor must match the specifications of the original fuse resistor. For example, if the original fuse resistor has specifications of 10Ω and 2W, you can select a resistor with 10Ω/2W. The rated current I of the fuse can be calculated using the formula I²R = 56%P, where R is the resistance value (in ohms, Ω), and P is the rated power (in watts, W). In this example, the rated current should be 0.3A.

　　2. When a fuse is used directly as a substitute for a low-resistance fuse resistor that has been damaged, this method is suitable for fuse resistors with resistance values of 1Ω or less. The fuse’s melting current rating can be calculated from the original fuse resistor using the formula mentioned above.

　　3. Small incandescent bulbs used as substitutes have a certain resistance value. When the current increases to a specific level, their filaments will either glow or burn out. Based on this characteristic, they can effectively serve as substitute fuse resistors. The cold-state resistance of commonly used flashlight bulbs typically ranges from a dozen ohms to several dozen ohms, and their rated current is usually several hundred milliamperes. During maintenance, the appropriate choice can be made according to the actual situation.

　　(Source: World of Electronic Products)