Anti-fusible

An antifuse is an electrical device that performs the opposite function to a fuse. Whereas a fuse starts with a low resistance and is designed to permanently break an electrically conductive path (typically when the current through the path exceeds a specified limit), an antifuse starts with a high resistance, and programming it converts it into a permanent electrically conductive path (typically when the voltage across the antifuse exceeds a certain level). This technology has many applications. Antifuses are best known for their use in mini-light (or miniature) style low-voltage Christmas tree lights. Ordinarily (for operation from mains voltages), the lamps are wired in series. (The larger, traditional, C7 and C9 style lights are wired in parallel and are rated to operate directly at mains voltage.) Because the series string would be rendered inoperable by a single lamp failing, each bulb has an antifuse installed within it. When the bulb blows, the entire mains voltage is applied across the single blown lamp. This rapidly causes the antifuse to short out the blown bulb, allowing the series circuit to resume functioning, albeit with a larger proportion of the mains voltage now applied to each of the remaining lamps. The antifuse is made using wire with a high resistance coating and this wire is coiled over the two vertical filament support wires inside the bulb. The insulation of the antifuse wire withstands the ordinary low voltage imposed across a functioning lamp but rapidly breaks down under the full mains voltage, giving the antifuse action. Occasionally, the insulation fails to break down on its own, but tapping the blown lamp will usually cause it to make a connection. Often a special bulb with no antifuse and often a slightly different rating (so it blows first as the voltage gets too high) known as a "fuse bulb" is incorporated into the string of lights to protect against the possibility of severe overcurrent if too many bulbs fail. A much earlier application of antifuses was in the old series-connected streetlights.