A banana connector (commonly banana plug or banana jack for the male, banana 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 4mm parts. The plugs are frequently used to terminate patch cords for electronic test equipment. They are also often used as the plugs on the cables connecting the amplifier to the loudspeakers in hi-fi sound systems.
The plug was invented in 1924 by Richard Hirschmann (former Richard Hirschmann GmbH & Co.).
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The plug consists of a cylindrical metal pin about 25 mm (one inch) long, with a diameter of 4 mm, which can be inserted into a matching 4 mm socket to make an electrical contact. The pin has one or more lengthwise springs that bulge outwards slightly. These press against the sides of the socket, improving the electrical contact and preventing the pin from falling out. The curved profile of these springs is probably the origin of the name "banana plug". 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. An insulating plastic cover is usually fitted over this rear end of the connector.
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" 4 mm plug.
For high voltage use, a special sheathed version of the banana plug and socket is used. This version has 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 usually black with some physical feature such as a molded ridge marked "Gnd" indicating the relative polarity of the two plugs.
Besides plugging into specific banana jacks, banana plugs may plug into five-way or universal binding posts on audio equipment.
A number of widely used plugs are based on combining 2 or more banana plugs with a plastic handle 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 consisting simply of two banana plugs spaced 3/4 inch (about 19mm) apart.
US-style double banana (pictured): A plastic housing containing two banana plugs, allowing simultaneous connection of a signal line and a ground (earth) line; see the photo. The housing may allow the connection of individual wires, a permanently attached coaxial cable providing both signal and ground, or a coaxial connector such as the BNC connector shown in the photo. By convention, multiple full-sized banana connectors are spaced on ¾ inch centers.
Older European audio equipment used double banana plugs with a 3rd center pin (round 4mm banana for speakers, 4 mm banana or flat pin for turntable to amplifier connection) for audio signals. The center pin prevents accidental insertion in mains sockets, except the Italian "type L" socket.
Some specialized multi-pin plugs and sockets consist of 5 or more banana plugs arranged in a circle.
A miniaturized version of the banana connector was also produced. About 1/3 the size of the standard connector, these were useful in high-density applications but never achieved the same sort of popularity as the larger banana connectors. They are substantially more fragile than the larger connectors. Multiple miniature banana connectors are usually spaced on ½ inch centers.
An exposed banana plug can obviously present electrical hazards if the wire to which it is attached is energized. A plug that is only partially inserted into a jack can also present a risk of accidental contact as the conductive surface of the plug will not be completely covered. The hazards include electric shock, electrocution, burns from accidental short circuits, and damage to the attached equipment.
Where electrical safety is an issue, various kinds of protected plugs and sockets are available. These have sliding covers on plugs and/or other devices to protect the user from accidental contact with live conductors, but are still largely compatible with the original design.
A typical design is now required (IEC 61010) on digital voltmeter test leads and several other measurement and laboratory equipment. In this design, the metal banana plug is entirely sheathed in plastic and presses into a deep recess in the DVM. Alternatively, the DVM has the male part of the banana plug and it is deeply recessed; the test lead contains a sheathed banana jack.
In most European countries the standard mains power receptacle will physically accept banana and even US-style "double banana" plugs (the standard US pin spacing of 3/4 inch (19.05 mm) is close enough to the mains plug spacing of about 19 mm., and the pin diameter is also compatible), leading to a risk of electrical shock. For safety reasons, it can be difficult to purchase US-style laboratory "double banana" plugs in these countries.
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