Fuse

A fuse is a deliberate weak point. It holds a thin wire chosen to melt at a set current, so if a fault makes too much current flow, the fuse burns open and cuts the power before wires overheat or a fire starts.

A common type is a small glass cartridge. A tube of soda-lime glass is drawn from molten glass and cut to length. A carefully measured piece of copper wire, often plated with tin or silver, is stretched through it; its thickness sets the current at which it melts. Brass end caps, usually plated, are pushed over each end and soldered or crimped to the wire. The clear glass shows whether the wire has blown. Time-delay types use a coiled or specially shaped wire, so they ride out the brief surge when a TV is switched on but still blow on a lasting fault.

On the power supply board, the fuse sits right where mains power comes in. Its rating is printed on it, such as T3.15A 250V, where T means time-delay. A blown fuse usually means something else failed, so replacing it alone rarely fixes the TV. Many supplies use ceramic fuses instead of glass.