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A banana connector (commonly banana plug for the male, banana socket or banana jack 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. The plug is typically a four-leafed spring tip that fits snugly into the jack. 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.
Invention of the plug is claimed by two entities. The Hirschmann company claims it was invented by Richard Hirschmann in 1924. A competing claim is made for the General Radio Company (Genrad), which stated "1924: GenRad developed banana plug - replaces pin plugs, this spring-loaded connector technology..." and that it was "introduced in this country [the US] by GR in 1924".
The original plug consists of a cylindrical metal pin about 20 millimetres (0.79 in) long. This pin length is still common in Europe and other parts of the world. However other sizes have emerged, such as 15 millimetres (0.59 in) pins, which can commonly be found in the US. Intermediate lengths of 11 millimetres (0.43 in) to 25 millimetres (0.98 in) are less common.
The pin's diameter is nominally 4 millimetres (0.16 in). 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 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 often black with some physical feature such as a molded ridge or thick tab, 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 a de facto standard, multiple full-sized banana connectors are often spaced on ¾ inch centers.
Older European audio equipment used double banana plugs with a third 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.
Miniature banana connectors
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.
A closely related but different design is the so-called pin plug and matching pin jack. The pin plug designs resemble banana plugs, but without the spring on the male pin. Instead, these designs rely on spring action in the female jack, or tightly machined tolerances, to ensure good contact. The pin plug design was used where maximum density of connectors was desired (such as in pin plugboards), or a very compact connector was needed.
Quality banana plugs and cables are typically rated for 30 V eff. or 60 V DC at 15 A. Cheap versions can have worse isolation, and might not withstand 15 A at all. Banana connectors are not usually rated for mains voltage.
One reason why banana connectors are not rated for mains usage is that an exposed banana plug or socket can present a shock hazard if connected to an energized source. A plug that is only partially inserted into a jack can also present a risk of accidental contact, because the conductive surface of the plug will not be completely covered. This can especially happen if a 20 mm long plug is inserted into a socket that is only intended for 15 mm long plugs. Further, some banana plugs have the screw used to fix the wire to the plug exposed. Also, when connecting plugs using transverse holes in the plug, large parts of the pin of the second plug are exposed. Another reason why banana connectors are not rated for mains usage is the lack of double isolation of wires and connectors. 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 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 equipment has the male part of the banana plug and it is deeply recessed; the test lead contains a sheathed banana jack. Also, the test lead wire is specified for higher voltage isolation.
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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