The car cigarette lighter socket or car adapter that most automobile drivers and passengers are familiar with is properly called a cigar lighter receptacle, since it was originally designed as a lighter for cigars — hence its large size and unheated center that make it less than ideal to light a cigarette.
These sockets were not originally designed to provide electrical power, and are not an ideal power connector for several reasons, notably the fact that three sizes exist (two for 12-volt DC and one for the older 6-volt DC systems) and the mating of the different sized 12 V DC plugs and jacks is problematic. Because of this, and the small-gauge wiring sometimes used, the power connections they provide are sometimes unreliable and not suitable for high-power devices.
Despite these limitations, they are often used for powering or recharging many devices; for example, electric razors, portable spotlights, laptop computers, mobile phones, PDAs, USB adapters, digital audio players, lamps, or even thermoelectric coolers. For devices that run on regular AC line voltage, a plug-in inverter is used to convert to alternating current, and transform it up to 120 or 240 volts AC. Inverters have become so popular that some larger vehicles (minivans, pickup trucks, SUVs, and especially recreational vehicles) come with them built-in, along with domestic AC sockets.
Low-power devices that run on even lower voltages can be plugged into these sockets by using a DC/DC converter. Universal ones have a switch that allows selection of 3, 4.5, 6, 9, and 12 volts. Some instead supply 5 volts to a USB "A" socket, allowing USB devices to be used or recharged.
The lighter is a cylinder containing a thin coil of nichrome wire, through which high current (15-20 amps) passes when the device is activated, usually by pushing its handle in. When pushed in, the lighter is held against the force of a spring by a hook attached to a bi-metallic strip. The heating element becomes glowing orange hot in seconds, causing the bimetallic strip to bend and unhook the mechanism, and the handle pops out. If the lighter is then promptly removed from its socket, it is capable of setting cigarettes, cigars and tinder (among other things) on fire.
In new cars, the socket might ship without the heating element (the "lighter") due to declining popularity of smoking in cars. However, the socket continues to exist to power consumer electronics in cars as the primary purpose. Often, a vehicle may come with several outlets for convenience, such as powering a tire pump or a vacuum cleaner from an outlet in the trunk (boot) or rear hatch. These usually have a plastic cap tethered to them, and are usually labeled as not for lighters, as they are not designed to take the heat generated by them.
For the 12-volt systems, the "contact point," which is the center part of the plug when viewed end-on, carries the positive voltage, whereas the "can" part, which is the outer part of the connector, carries the negative voltage (which is the "ground" connection for most American automobiles, which have a negative ground electrical system).6 volt cigar lighter receptacle and plug
An important note is to remember that most 12 volt systems charge at 13.8-14.2 volts, so one must be careful when modifying other electronics to plug in to the cigarette lighter.