thumb|right|Various laboratory-style 4 mm banana plugs

thumb|Loudspeaker-style banana plugs connected to a loudspeaker

thumb|Double-stackable banana plugs for junction into two additional [[parallel circuits.]]

A banana connector (commonly banana plug for the male, banana jack (or socket) for the female) is a single-wire (one conductor) electrical connector used for joining wires to equipment. The term 4 mm connector is also used, especially in Europe, although not all banana connectors will mate with 4 mm parts, and 2 mm banana connectors exist. Various styles of banana plug contacts exist, all based on the concept of spring metal applying outward force into the unsprung cylindrical jack to produce a snug fit with good electrical conductivity. Common types include: a solid pin split lengthwise and splayed slightly, a tip of four leaf springs, a cylinder with a single leaf spring on one side, a bundle of stiff wire, a central pin surrounded by a multiple-slit cylinder with a central bulge, or simple sheet spring metal rolled into a nearly complete cylinder. The plugs are frequently used to terminate patch cords for electronic test equipment such as laboratory power supplies, while sheathed banana plugs are common on multimeter probe leads.

History

Invention of the plug is claimed by two entities:

  1. The Hirschmann company claims it was invented by Richard Hirschmann in 1924. In 1929 Richard Hirschmann was granted a patent in Germany for an improved banana plug. The patent describes a banana plug that consists of two parts only. Not needing a small screw to hold the wire in place, like in conventional Bananensteckern (banana plugs), is presented as an advantage of the invention.
  2. The General Radio Company claims it introduced the banana plug in the United States in 1924. Various banana plug (type 274) products are shown in a September 1928 catalog and prices were listed in a July 1928 newsletter.

Design

thumb|left|Banana plugs with the curved spring visible

The original plug consists of a cylindrical metal pin about long. This pin length is still common in Europe and other parts of the world. However, other sizes have emerged, such as pins, which can commonly be found in the US. Other lengths between and are less common.

The pin's diameter is nominally . The pin has one or more lengthwise springs that bulge outwards slightly, giving the appearance of a banana. Taking the springs into account, the actual diameter of a banana plug is typically a bit larger than 4 mm when not plugged in. When inserted into a matching 4 mm socket, the springs press against the sides of the socket, improving the electrical contact and preventing the pin from falling out. The other end of the plug has a lug connector to which a length of flexible insulated equipment wire can be attached, which is either screwed, soldered, or crimped into place. On banana plugs for laboratory use, an insulating plastic cover is usually fitted over this rear end of the connector, while banana plugs for speaker connections often use uninsulated metal bodies.

The rear end of a 4 mm plug often has a 4 mm hole drilled in it, either transversely or axially, or both, to accept the pin of another 4 mm plug. This type is called a stackable or stacking plug.

For high voltage use, sheathed versions of the banana plug and socket are used. These add an insulating sheath around both the male and female connectors to avoid accidental contact. The sheathed male plug will not work with an unsheathed female socket, but an unsheathed male plug will fit a sheathed female socket.

Individual banana plugs and jacks are commonly color-coded red and black, but are available in a wide variety of colors. Dual banana plugs are often black with some physical feature such as a molded ridge or thick tab, marked "Gnd" to indicate the relative polarity of the two plugs.

Besides plugging into specific banana jacks, banana plugs may plug into "five-way" or "universal" binding posts.

Standard spacing

thumb|right|Adapter between a female [[BNC connector and double banana plugs]]

A number of widely used plugs are based on combining two or more banana plugs in a common plastic housing and other features for ease of use and to prevent accidental insertion in other such plugs. Many of these plugs are derived from the "double banana" plug, which consists simply of two banana plugs spaced apart (measured from center to center of each individual plug). The  inch spacing originated on General Radio test equipment in 1920s, and their type 274-M dual-plug is a notable example from that period.