Embedded Systems Design Resources

What are fuses and when do I need them?

A fuse is an electronic component that protects a circuit by creating an open circuit if it draws too much current. Fuses do not protect against voltages that are too high. 

Fuses should be inline to the power source and come before all components. Anything receiving power before the fuse is at risk to overcurrent. 

Fuses are most useful around power supplies, both when power comes into a system and on individual power rails. For example, if you have a 12V rail that should only draw 750 mA and a 5V rail that should only draw 400 mA, you might put a 750 mA fuse in series with the 12V supply output and a 500 mA fuse in series with the 5V supply output.

A common mistake that fuses protect against is accidentally shorting the power rail to ground (e.g., with a DMM probe as you are trying to touch the correct pins). This causes an infinite amount of power to be drawn from the power supply, and can sometimes damage or destroy voltage regulators or power supplies.

What are the most common types of fuses?

• Standard fuses contain a small piece of wire made of an alloy that melts readily. If the current gets too high, it burns out and opens the circuit. Standard fuses come in many different shapes and sizes.

• Advantages: Inexpensive, replaceable

• Disadvantages: Must be replaced when blown, sometimes no visual indication that they are blown

• Examples from Littelfuse

There are two types of standard fuses. 

• Slow Acting

• Advantages: Longer delay before fuse is tripped. This ensures that the device will not blow its fuse on start up, decreasing false tripping 

• Disadvantages: Can allow high current to reach sensitive components potentially damaging them before tripping. If not specified slow acting, it's probably a fast acting fuse. 

• Examples from Littelfuse

• Fast acting 

• Advantages: Very short delay until circuit is tripped from overcurrent. Protects components more efficiently from overcurrent. 

• Disadvantages: Can false trip. 

• Examples from Littelfuse

• Resettable PTC fuses are tripped by heat caused by current above the rating of the fuse. If the current flowing through the fuse goes above its rating, the fuse changes from having very low resistance to having a high resistance (thereby stopping the flow of current through the circuit). It is self-resetting after power is removed and the fuse cools.

• Advantages: Reusable, useful when cause of overcurrent is a user failure (e.g., plugging a connector into the wrong pins on a PCB)

• Disadvantages: More expensive than standard fuses, small leakage current after breaking, significantly lower voltage ratings (60V) compared with non-resettable fuses (600V)

• Examples from Littelfuse

• Circuit breakers are typically used in AC power applications like homes. Circuit breakers act as resettable fuses by using magnetic forces and solenoids to move a metal strip breaking the circuit. The circuit breaker can be reset by manually switching the lever, resetting the metal strip. 

• Advantages: Reusable and easy to reset by hand. This reduces downtime and repair costs. Easy-to-see indication of whether the circuit breaker is tripped. Useable as an on-off switch.

• Disadvantages: Very expensive, not well suited for low-voltage DC applications. Slow to react when compared to fuses. 

• Examples from Littelfuse