A 12V socket is a cylindrical receptacle that provides direct current (DC) at 12 volts to a device plugged into it. The most commonly known 12V sockets are found in automobiles, and were originally provided as part of a system that heated a metal coil in a heat-proof holder to light cigars and cigarettes. Cigarette lighters are still provided in automobiles as optional accessories, but the 12V sockets are still installed in cars as a power source. It’s not uncommon to find some vehicles with four or five 12V outlets to provide power to charge or operate a wide variety of consumer electronics.
When 12V sockets in cars were built primarily as cigarette lighters, a small hook in the sockets was engaged by a bimetallic strip in the plug that held it in place. When the the coils were hot enough to ignite a cigarette or cigar, the bimetallic strip would bend backward, disengaging the hook and activating a spring mechanism that popped the lighter’s handle out from the socket without the plug actually leaving the socket. This signaled that the lighter was ready for use. The 12V sockets installed in modern vehicles as power supplies, though, usually aren’t built to withstand the heat generated by automobile cigarette lighters, and users are warned against trying to use them for that purpose.
Like all electrical sockets, a 12V socket must be connected to a power source; this is usually the vehicle's electrical system. 12V sockets and their plugs, however, are markedly different from standard alternating current (AC) socket and plug combinations. The most significant difference is that the 12V socket and plug are each cylindrical, with the male plug fitting snugly into the female socket.
The insertion of a 12V plug in the socket completes the circuit. The positive contacts are in the centers of the cylinders — a smaller spring-loaded rod projects out from the tip of the plug. When the plug is securely seated in the socket, that rod is forced against the positive contact in the socket’s center.
The negative contact in the socket is its cylindrical inside surface, which is made of metal. Most modern 12V plugs have two springy negative contact strips protruding from their sides that make contact with the socket’s metal interior and maintain that contact as long as the plug remains inserted. The plug is built to fit snugly inside the socket; this, coupled with the springiness of the negative contact strips, keeps the plug securely inserted in the 12V socket.
Many portable electrical and electronic devices are built to operate on regular household current at 120V AC and on DC current as well, and can be connected to a car’s 12V socket. Another useful piece of equipment is a device that converts 12V DC to 120V AC. These devices have a 12V plug at one end and one or two standard 15-amp AC sockets on the other.