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How to Use Fuses to Protect Electronic Line-Voltage Circuits

By Doug Lowe

A fuse is an inexpensive device that can carry only a certain amount of current. When a fuse is in an electronic circuit and the current exceeds the rated level, the fuse melts (blows), thus breaking the circuit and preventing the excessive current from flowing. Fuses are an essential component of any electrical system that uses line voltage and has the possibility of short-circuiting or overheating and causing a fire.

The most common type of fuse is the cartridge fuse, which consists of a cylindrical body that’s usually made of glass, plastic, or ceramic, with two metal ends. The metal ends are the two terminals of the fuse. Inside the body is a thin wire conductor that’s designed to melt away if the current exceeds the rated threshold.

As long as the current stays below the maximum level, the conductor passes the current from one metal end to the other. But when the current exceeds the rated maximum, the conductor melts, and the circuit is broken.

An AGC fuse, which is a small fuse made of glass, 1-1/4″ in length and 1/4″ in diameter. This particular fuse is rated at 2 A, but you can get AGC fuses in larger ratings, up to 15 A. (AGC stands for Automotive Glass Cartridge.)


Fuses should always be connected to the hot wire and should be placed before any other component in the circuit. In most projects, the fuse should be the first thing the hot wire connects to after it enters your project enclosure.


If you plan on using a fuse in your circuit, you’ll need to purchase a fuse holder to hold the fuse. For AGC cylindrical fuses, there are two distinct types of fuse holders to choose from. If the fuse will be mounted inside your project’s enclosure, you can use a chassis-type fuse holder. If you want the fuse to be accessible from outside the project’s enclosure, you should choose a panel-mount holder instead.