Testing and Identifying Fuse Problems

What is a fuse? It’s this awesome little device that is designed to self-destruct in the event of a power surge or other over-current situation. Why would anyone create a device like that? Quite simply, to save the rest of the circuit it is placed in. Replacing a fuse is hugely inexpensive in comparison to replacing the entire circuit that would otherwise be fried.

Due to fuses being a very common component that needs replacing, we thought it beneficial to provide a little more insight to their purpose, how to test and diagnose them, and to display various styles as they come in a variety of shapes and sizes.

Step 1: Identify the blown fuse.

If the fuse can be removed from the circuit, then the easiest way to determine if it has blown is a continuity test. Grab a multimeter and select the continuity or resistance setting. Perform a quick test of the meter to ensure it’s functioning properly by touching the leads together until you hear a beep or see 0 ohms. Now, after the circuit has been powered off, place the leads on either side of the fuse and if you hear that same beep, and the meter has a very low resistance reading, the fuse is still good. If you do not hear the beep, and/or the multimeter reads OL, then the fuse is blown.

Another way to test the fuse is by measuring the voltage across it with a digital multimeter. This is helpful if the fuse is not able to be easily removed from the circuit. To do this, leave the circuit powered on and switch your meter to measure voltage. Make sure to select DC for DC circuits and AC for AC circuits. Take proper precautions to isolate yourself from dangerous voltages.  Place the meter leads on either side of the fuse. If there is little to no voltage indicated, the fuse is good. However, if there’s a difference in voltage (typically the full supply voltage), then the fuse is bad.

Step 2: Replace the fuse.

It’s best practice to replace the bad fuse with one that’s identically spec’d. One thing to be aware of is whether or not the fuse is fast or slow blow. A slow blow fuse can be replaced with either a fast or slow blow in most cases, but never the other way around. Slow blow fuses are used in inductive circuits. An example would be a motor starting. Though the motor is rated for lower amps, at startup it will draw a much higher current initially. Using a slow blow fuse will allow for a tolerance of the higher current for a short period, just not a prolonged amount of time. In this situation if there was a fast blow fuse in place, it would blow as soon as the current rating was surpassed. It’s best to stick with exactly what was there in the first place.

Still have questions or need help crossing a fuse? Watch the video below, then head over to our TechForum post on the matter.

About this author

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Ashley Awalt is a Technical Content Developer that has been with Digi-Key Electronics since 2011. She earned her Associate of Applied Science degree in Electronics Technology & Automated Systems from Northland Community & Technical College through the Digi-Key scholarship program. Her current role is to assist in creating unique technical projects, documenting the process and ultimately participating in the production of video media coverage for the projects. In her spare time, Ashley likes to – oh, wait, is there such a thing as spare time when you’re a mom?

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