A fuse is a very thin wire, which either melts or vaporizes when too much current flows through it. The thin wire may be made of aluminum, tin-coated copper or nickel. The resulting open in the circuit stops current flow. In electronic equipment, most fuses are cylindrical glass or ceramic type with a metal cap at each end! The current rating also can be seen in one of the two metal end caps. There are two popular physical sizes: 1 – ¼ X ¼ - inch and 5X20mm. The 1 – ¼ X ¼ - inch size is used in many automobiles. You’ll find both sizes in many electronic equipment, but the smaller 5 X 20mm has become more common. Fuses are available with current ratings from 1/500 Ampere to hundreds of amperes.

Purpose of Fuses

The purpose of a fuse is to open an electronic circuit when current flow exceeds a certain amount, determined by the rating of the fuse. Opening a circuit under high current conditions can save electronic components from damaged and prevents overheating, which could cause a fire.

Types of Fuses

There is two basic types of fuses: fast acting and slow blow. The fast acting type will open very quickly when their particular current rating is exceeded. This is important for analog meter movements, which can quickly be destroyed when too much current flows through them, for even a very small amount of time. Slow blow fuse have a coiled construction inside. They are designed to open only on a continued overload, such as a short circuit. The purpose of coiled construction is to prevent the fuse from blowing on just a temporary current surge.

Do not use a slow blow fuse in place of a fast acting fuse. It may not open fast enough to prevent components damage under a high current condition. It’s not harmful to replace a slow blow fuse with a fast-acting fuse, but it will probably open up unnecessarily every now and then when the equipment is first switch on. A blown fuse can tell you something about your service problem. Often the glass case of the fuse appears clear, and you can still see the broken pieces of the fuse element. This means you have the kind of problem that causes a slow, gradual overload on the power supply. Some fuses even die of old age. But if the inside of the glass fuse is discolored, and there is no trace of the fuse element (the center connector), you know that the center connector was destroyed quickly and violently, using a lot of heat. The fault was a short circuit or other problem that caused a lot of current to flow very quickly.

Is some cases a fuse will open up fast enough when there is a surge so that other components aren’t damaged. If this is the case, replacing the fuse with one of the same type and rating restores operation. Unfortunately, a high probability exists that if a fuse blew, something in the circuit it was protecting shorted out. In this case, a replacement fuse blows right away. The blown fuse can easily be replaced by a new one, after the overload has been eliminated when come into replacement. Use only the same current and voltage ratings as the original. The common type and current rating of a fuse in a monitor are slow blow 2A to 5A. When you buy replacement fuses, get several – you can easily use all of them while you’re troubleshooting an elusive problem.

Testing Fuses

Turn the power off and remove the fuse from the circuit to check with a multimeter set to the lowest ohms range. Connect the probes to both end of the fuse. You may check the fuse while it still in circuit. A good fuse should showed continuity or read 0 ohm. A blown fuse is open which reads infinity on the meter.