What is a Fusible Resistor : Working & Its Applications

A Fusible Resistor known by the names current-limiting resistors, fuse resistor, meltdown resistor is a type of resistor that has a low power rating and is designed to melt when a certain amount of current flows through it, thus breaking an electrical circuit. It is the only type of resistor designed to quickly burn up (and isolate itself) when overloaded with the current. Fusible resistors are used as safety devices to prevent electrical fires and other damage caused by excessive current being applied at once. They can be found in household appliances like washing machines and microwaves, as well as electronic devices like stereos. To learn more about fusible resistors and how they work, check out our below article.

What is a Fusible Resistor / Definition?

Fusible Resistors are power resistors that are designed to fail in a controlled manner. These resistors start as high-power resistors, then have their lead ends welded together and placed in a ceramic enclosure. If the resistor has too much current applied to it, it will short out and the resulting heat will cause the fusible resistor to melt, encapsulating the melted material.

Because of their design, they can only be used once, but they can be used in applications where they may need to be replaced because they are intended to fail. The fusible resistor symbol is shown below. This is the combination of a resistor and a fuse.

Fusible Resistor Construction

The resistor consists of a non-inductive ceramic core with a winding of high resistance wire. The core is coated with a special glaze and the entire assembly is enclosed in a tough non-hygroscopic resin or epoxy case.

Fusible resistors are constructed such that their resistance increases as the temperature rises. This characteristic allows the resistor to function as both a current limiting device and as an over-current fuse, protecting circuits from transient over-current surges and fault conditions.

There are two most commonly used ways to construct a Fusible Resistor which are as follows

The first is a series of high resistance metal films sandwiched between two end caps, which are connected 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.

Fusible Resistor Circuit

The Power supply or charger circuit using a fusible resistor is shown below. The main function of a fusible resistor is to restrict the inrush current flow. So this resistor is used as the overcurrent component within the charger or power supply circuit that can have 10ohms of resistance. This circuit is used to protect over current & restrict inrush current.

In contrast, a fuse and an NTC thermistor can also be used to attain inrush current limitation & equivalent over current protection. But by using Fusible resistors, we can save space instead of using separate thermistors & fuses. But, the heat produced through a fusible resistor could need spacing in between components. When this resistor is rated up to 10 then other parts need 0.5-inch spacing. Similarly, if this resistor is rated above 10 W then 1-inch space is necessary.

When power supplies, as well as chargers, need to be in small, then compact packages & the necessary spacing in the region of the fusible resistors could leave the designer of a circuit with insufficient space for the whole design.

Fusible Resistor Size Calculation

Calculating a fusible resistor value is a bit different from calculating other types of resistors. Here’s how:

Calculate the resistance you’ll need by taking into account the voltage and current of the circuit in question.

Find the appropriate power rating for your resistor, based on whether it will be in series or parallel with your circuit. This can be done by multiplying voltage by current and then dividing by 2 (for series) or 1 (for parallel).

Choose a fusible resistor that has a power rating as close as possible to your intended power rating, without being higher than it.

Determine the maximum current that will flow through it. You can calculate this using Ohm’s law which states that

  V = I x R, where V is voltage, I is current, and R is resistance.

For example, if your voltage is 12 volts and your desired resistance is 8 ohms, then the current through your fusible resistor would be 1.5 amps (V = 12 volts; R = 8 ohms; I = V/R = 1.5 amps).

Fusible Resistor Color Code

Queen Mao’s resistor color coding system was simply applied for Metal Film, Fusible, Carbon Film, Wirewound, Metal Oxide Film, etc. This color-coding system is used mainly for resistors once the exterior area is not enough to print the value resistance for the past time.

Presently, the Queen Mao resistor color-coding system is used for automatization. In a four-color band resistor, the first three color bands are used to decide the resistance. The fourth color band will represent the tolerance value.

In this color-coding system, the type of resistor can be determined based on the color band. In a resistor, if an additional 5th band color is black, then it is a wire wound resistor, if the fifth band color is white then it is a fusible resistor and If a resistor has one black band within the center, then it is known as a zero-ohm resistor.

Difference between a Fuse and a Fusible Resistor

The main difference between a fuse and a fusible resistor is discussed below.

Difference between a Fusible Resistor and a Resistor

The difference between a fusible resistor and a resistor includes the following.

Fusible Resistor Selection

There are many different types to choose from, depending on the needs of your system. Some come with special features, like automatic resetting after failure. Here is how to choose the fusible resistor that’s right for your system

What type of circuit will it be used in? This determines the maximum voltage you’ll need your resistor to handle.

What is your budget? You can spend a lot or a little on a fusible resistor; it all depends on what you need from it.

Is this an AC or DC circuit? Most fusible resistors are designed for DC circuits, but there are some that work with AC as well!

Advantages

The advantages of a fusible resistor are discussed below.

• One of the key advantages of a fusible resistor is that it allows current to flow through it until a certain maximum value is exceeded, at which point the resistor burns out, preventing damage to other components as a result of an overload.
• They are self-limiting, which means they’ll reduce current flow to a safe level if the resistance drops below a certain point. They provide overload protection and do so at an affordable price.
• They can be used in both AC and DC circuits. This is particularly beneficial in applications where the power source may vary from AC to DC.
• Fusible resistors don’t require monitoring or maintenance—they’re passive components that don’t need any attention at all.
• Extremely low inductance.
• Excellent high-frequency characteristics, making them a great choice for RF applications.
• A wide range of resistance values is available, ranging from 0.1 ohms to two megohms.
• Tolerances as tight as 0.05%
• High stability.
• Good frequency characteristics.

Disadvantages

The disadvantages of fusible resistors are discussed below.

• Fusible resistors have a limited lifespan and will eventually burn out under normal operating conditions, requiring replacement.
• The cost of replacing fusible resistors can become significant in applications where they are frequently subjected to overload conditions.
• Fusible resistors’ power ratings are low, which means they can’t take on high-voltage applications.
• Another disadvantage of fusible resistors is that they can only protect against overcurrents; they cannot protect against undercurrents.
• They also cannot protect against short circuits, which limits their use to just protection from overloads.
• They are also more apt to fail prematurely than other types of resistors because they are susceptible to thermal shock and vibration, which can cause them to crack or shatter.

Applications/How Are Fusible Resistors Used?

The applications of fusible resistors include the following.

• Fusible resistors are used in electrical circuits that may occasionally be exposed to short circuit conditions for a short period of time. They will blow out like a fuse in the event of a short circuit, protecting the circuit from damage and fire.
• Fusible resistors are used in motor controller systems, automobiles, power supplies, rectifiers and inverters, home appliances, power tools, and other equipment.
• The main benefit of using this resistor is that its resistance function controls inrush current.
• This resistor is used as the main above current component within a charger or a power supply.
• A fusible resistor performs dual functions. Once the power supply is exceeded, then it works like a resistor limiting current. Similarly, once the rating of power is exceeded, then it works as a fuse in the circuit to defend different components within the circuit from the surplus current.

Thus, this article covers an overview of a fusible resistor which is a special kind of resistor used to protect any circuit. This resistor is very helpful for extremely sensitive circuits which are used for lower power applications where the requirements of surge handling & overload are not too severe. Here is a question for you, what is a wire-wound resistor?