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What Is Fusible Resistor?Fusing Resistors Principles Of Operation

What Is Fusible Resistor?Fusing Resistors Principles Of Operation

September 27 , 2023 0

A fusing resistor is a special component that serves the dual purpose of a resistor and a fuse. It is represented by the letter "RF" or "R" in the circuit. Fusing resistors are mainly used in the power supply circuits of colour televisions, VCRs and testing instruments, the line scan and field scan circuits of colour televisions.
Fusing resistors are passive components used to protect circuits. When the current exceeds a predetermined value, instantaneous heating occurs inside the fuse and melts, interrupting the circuit to protect the equipment. It is commonly used at the input and output of high power devices.

What Is A Fusing Resistor

A fusing resistor is an electronic component that combines two functions: safety protection and current limitation. It works by connecting a fusible wire (usually copper or tinned copper wire) in series with the circuit. Under normal operating conditions, the resistance of the fusible wire is so small that it has no effect on the circuit. However, when the current in the circuit rises abnormally high enough to cause the fuse to melt spontaneously, the fusing resistor cuts off the current in the circuit.

Fusing Resistor Concept

Fusing resistors are power resistors designed to fail in a controlled manner. These resistors start as high power resistors and then have their lead ends soldered together and placed in a ceramic housing. If too much current flows through the resistor, it shorts out and the heat generated causes the fusible resistor to melt, thus encapsulating the melted material. Due to their special design, fusible resistors can only be used once, but they can be used in applications where they may need to be replaced because they may fail. Fusing Resistor Symbols

As shown below, this is a combination of a resistor and a fuse.

Structure of Fusing Resistors

Fusible resistors consist of a non-inductive ceramic core coated with a special glaze and a high resistance wire winding, with the entire assembly encapsulated in a rugged non-hygroscopic resin or epoxy housing.
Fusible resistors are constructed so that their resistance increases with temperature, a characteristic that allows the resistors to be used both as current limiting devices and as overcurrent fuses to protect circuits from transient over flow surges and fault conditions.
There are two most common methods of constructing fusing resistors.
The first consists of a series of high-resistance metal films sandwiched between two end caps, which are attached to the ends of the films.
The second consists of a non-metallic element wound into a coil and then coated with a thin layer of metal.

Fusing Resistor Parameters

The main parameters of fusing resistors include rated current, rated voltage, power dissipation and fusing time.

Rated current refers to the maximum current that the fusing resistor can withstand, exceeding this value will cause the fuse to melt and cut off the circuit. The rated voltage is the maximum voltage that the fusing resistor can withstand, exceeding this value will cause the fuse to be blown and burnt.

Power dissipation is the power consumed by the fusing resistor at the rated current. Do not exceed this value when operating at or below the rated current, otherwise the fusing resistor will be damaged due to overheating.

Fusing time is the time from the normal state of the fuse to complete melting. The fusing time must be less than the maximum permissible gate time of the protected equipment to ensure that the equipment is adequately protected.

Fusing Resistors Function

Fusing resistors are mainly used to protect circuits from short-circuiting or overloading due to overcurrent. When the current in the circuit rises abnormally, the fuse automatically melts and cuts off the power supply, thus preventing equipment from being excessively damaged or accidents.

In addition, the fusing resistor has the function of limiting the current. When the load is connected to the circuit, the current is reduced due to the presence of resistance, to protect the load and extend the life of the equipment.

Advantages Of Fusing Resistors

Advantages of Fusing Resistors
One of the main advantages of a fused resistor is that it allows current to flow through it until a certain maximum value is recruited, at which point the resistor will burn out, preventing damage to other components due to overload.
They are self-limiting, which means that if the resistance drops below a certain point, they will reduce the current to a safe level. They offer overload protection at an affordable price.
Affordable.
They can be used in both AC and DC circuits. This is particularly useful in applications where the power supply may change from AC to DC.
Fusing resistors require no monitoring or maintenance - they are passive components that require no attention at all.
Very low inductance.
Excellent high frequency characteristics make them an excellent choice for RF applications.
Wide range of resistance values from 0.1 ohms to 2 megohms.
Tolerances as low as 0.05 per cent
High stability.
Good frequency characteristics