Characteristics and functions of fuse resistors_Detection and replacement of fuse resistors

1: Characteristics and functions of fuse resistors

The fuse resistor has the dual functions of a resistor and a fuse. When the overcurrent causes its surface temperature to reach 500-600℃, the resistor layer will peel off and melt. Therefore, the fuse resistor can be used to protect other components in the circuit from damage to improve the safety and economy of the circuit. The appearance and graphic symbol of the fuse resistor are shown in Figure 4-36.

2: Detection and replacement of fuse resistors

① Measure. Use the multimeter R×1 or R×100 to measure. The measurement method is the same as that of ordinary resistors. If the resistance value is far beyond the range or does not work, it means that the fuse resistor is damaged.

② Repair and replacement. When replacing a fuse resistor, place it more than 10mm above the ground and do not place it close to the printed circuit board. If the fuse resistor is damaged and there is no original model to replace it, the following methods can be used as an emergency replacement according to the situation:

a. Use a resistor and a fuse in series. Connect a resistor and a fuse in series with the same current value. The specifications of the resistor can refer to the specifications of the fuse resistor. The current can be calculated by the formula I = square root P/R. If the specification of the original fuse resistor is 51Ω/2W, then the resistor can be 51Ω/2W, and the rated current of the fuse is 0.2A.

b. Use fuse as a substitute. When some fuse resistors with smaller resistance are damaged, fuses can be used as a substitute. The current capacity of the fuse can be calculated from the value of the original fuse resistor. The method is the same as above.

c. Use a resistor as a substitute. You can directly use a common resistor with the same power and resistance value as a substitute.

d. Use a resistor and a fuse resistor in series. If there is no suitable resistance value, use an ordinary resistor with similar resistance value and a small resistance fuse resistor in series.

e. Thermal cutoff resistors should be replaced with original models.

3: Application of fuse resistor

Figure 4-37 (a) is a power supply protection circuit. When a component in the circuit is damaged, the current increases and R fuses to protect it. Figure 4-37 (b) is an electric fan circuit. R is installed on the motor housing. When the motor temperature rises too high (usually 139°C), R fuses to protect it from burning out.