Fusible Resistor

A fusible resistor

What is a Fusible Resistor?

A fusible resistor is a type of resistor that also acts as a fuse, opening the circuit when exposed to high currents. Like a fuse, the fusible resistor is designed to provide protection of the circuit.

Fusible resistors combine the functions of both resistors and fuses while reducing PCB space and manufacturing cost. However, they also function differently from fuses and are not easily replaceable like most fuses are.

Fusible resistors exist in both metal film and wire-wound varieties. Fusible film resistors are generally used in lower power applications. Wire-wound resistors are more often used in higher power applications and are more popular overall. Note that these applications are still relatively lower in power than circuits that need the greater protections afforded by fuses. High power applications will generally require fuses for adequate protection to be achieved.

Fusible resistors are related to both fuses and thermistors in being made of thermally sensitive resistive materials.

How Do Fusible Resistors Work?

Fusible resistors are temperature sensitive devices that are designed to fail when the current exceeds safe levels. Often made of nichrome, fusible resistors will fail thermally when exposed to a sufficiently high current.

A fusible resistor is useful for highly sensitive circuits of lower power requirements and applications where the overload and surge handling requirements are not too severe.

In normal operation, fusible resistors operate like any other electrical resistor: they provide a specific amount of resistance for which they are rated, within a certain tolerance. If the circuit fails however, a fusible resistor will prevent a short circuit through it by opening when exposed to a sufficiently high current. This fuse-like quality is why it is known as a fusible resistor.

Unlike fuses, fusible resistors are not designed as a point of repeated failure or to be replaceable. Fuses are placed within circuits at strategic points and are often designed to fail well before a fusible resistor would fail; they will typically provide better protection than a fusible resistor.

Thus, the fuse is designed to do only one thing: provide adequate protection to a circuit, often repeatably (by replacing the fuse).

In contrast, a fusible resistor is designed to two things at once: 1) function as a normal resistor during normal operation, and 2) to open under short circuit conditions and thereby prevent fire or damage to other components or systems.

Since fusible resistors aren’t designed to be easily replaced, their protection is often used as a destructive failsafe within devices seen as replaceable. The device they protect will either be replaced with a new one, or maintenance will be required to rebuild the circuit.

Fusible resistors often have a rough texture appearance when compared with other resistors:

Advantages of Fusible Resistors

Fusible resistors bring several advantages that lead to their popularity in specific applications. By combining the features of a resistor and a fuse, circuit board space, volume, and weight can be saved. This means reduced complexity, greater reliability, lower manufacturing cost and lower shipping costs.

Fusible resistors can be used to add protection to circuits that don’t otherwise have any protection. For a small increase in cost, a device can be designed with a fusible resistor at a critical point instead of a traditional resistor. Although the fusible resistor would only open under failure conditions, this layer of protection is beneficial to the product and the consumer, and may also help in satisfying regulatory requirements.

The inclusion of a fusible resistor can help protect more than just the device in which it is installed. In the case of a power supply, a fusible resistor can also help protect the devices that are receiving the power.

Fusible resistors can be used in both AC and DC circuits. They have low inductance and feature excellent properties at high frequencies. They are extremely stable and can be made with a wide variety of resistance values as well as tight tolerances.

Fusible Resistor Symbol

The symbol for fusible resistors in a circuit diagram is a resistor with a box around it:

Fusible resistor symbol

Fusible Resistor vs. Fuse

The primary difference between a fusible resistor and a fuse is the resistance value offered by the component. Fusible resistors can have high values of resistance whereas fuses offer little resistance. In addition, fuses are much more temperature sensitive than fusible resistors.

Another difference between a fusible resistor and fuse is temperature rating. Fuses are rated to 100C and will fail at this temperature as part of their protective features. This means that the fuse should fail if the environmental temperature exceeds 100C, helping to prevent fires. In contrast, fusible resistors are tested with cheesecloth, which will ignite at 400C. Thus there is a 300C difference between the temperatures allowed by the fuse and fusible resistor, and this should be taken into consideration.

FuseFusible Resistor
Two terminal passive deviceTwo terminal passive device
Protects circuitProtects circuit and provides resistance
Larger sizeSmaller size
Higher voltage systemsLower voltage systems
Swappable (easily replaceable)Not swappable

Fusible Resistor Calculation

Calculating the value for a fusible process takes several steps:

  1. Calculate the resistance needed by taking into account the supply voltage and desired current of the circuit in question.
  2. Find the appropriate power rating for your resistor. Choose a fusible resistor that has a power rating as close as possible to your intended power rating, without being higher than it.
  3. Determine the maximum current that will flow through the resistor (at the current at which you want the circuit to be protected). You can calculate this using Ohm’s law:

  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).