Fuses and breakers

A fuse is a piece of soft wire that melts, breaking a circuit if the current exceeds a certain level. Fuses are placed in series with the transformer primary (Fig.1). Any component failure, short circuit, or overload that might cause catastrophic damage (or fire!) will burn the fuse out. Fuses are easy to replace, although it’s aggravating if a fuse blows and you don’t have replacements on hand.

Fig.1: A fuse, F, in series with the ac input protects the transformer and diode, in case of overload.

If a fuse blows, it must be replaced with another of the same rating. If the replacement fuse is rated too low in current, it will probably blow out right away, or soon after it has been installed. If the replacement fuse is rated too high in current, it might not protect the equipment.
Fuses are available in two types: quick-break and slow-blow. You can usually recognize a slow-blow fuse by the spring inside. A quick-break fuse has only a wire or foil strip. When replacing a fuse, use the right kind. Quick-break fuses in slow-blow situations might burn out needlessly; slow-blow units in quick-break environments won’t provide the proper protection.
Circuit breakers do the same thing as fuses, except that a breaker can be reset by turning off the power supply, waiting a moment, and then pressing a button or flipping a switch. Some breakers reset automatically when the equipment has been shut off for a certain length of time.
If a fuse or breaker keeps blowing out often, or if it blows immediately after you’ve replaced or reset it, then something is wrong with the supply or with the equipment connected to it.