DC plug

DC plug is the common name for a DC power supply plug, and particularly is used for one common type of cylindrical two-conductor plug available in a range of sizes and used to power small pieces of electronic equipment. It is also quite properly used to describe some older multi-pin plugs.

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Several competing standards exist, and in some cases incompatible plugs will fit or can be made to fit, possibly damaging equipment if:

• voltage is wrong.
• polarity is wrong.
• current ratings are exceeded.
• power supply filtering or stability is inadequate for the equipment being powered.
• non-matching connectors are forced together.

Contents

1 Cylindrical types 1.1 Generic types 1.2 EIAJ standard 2 Other DC connectors 3 Summary

Cylindrical types

Also known as barrel connectors or tip connectors, these come in an enormous variety of sizes.

The intended use of these plugs is on the cable connected to a power supply. The matching jack or socket is then mounted in the equipment to be powered. Some of these jacks contain a normally closed contact, which can be used to disconnect internal batteries whenever the power supply is connected, avoiding the risk of battery leakage or explosion posed by incorrect recharging of the batteries.

These plugs generally have an insulated tip constructed to accept insertion of a pin (but note also the internal pin in the larger size EIAJ plugs). The outer body of the plug is one contact, most often but not always the negative side of the supply. A pin mounted in the socket makes contact with a second internal contact. The outer plug contact is often called the sleeve, and the inner the tip, although the tip itself is actually non-conductive.

At least two different national standards exist, EIAJ in Japan and DIN in Germany. As well several conventions have been adopted by specific manufacturers, notably by Sony, to indicate voltage by plug size.

Some manufacturers use opposite polarity standards, and some generic power supplies cater for this by allowing the polarity to be reversed by use of a switch or an unpolarised two-pin plug and socket arrangement. Some power supplies allow the voltage to be switched. Four-way X connectors or six-way star connectors, also known as spider connectors, with multiple plug sizes and types are common on generic power supplies. On others there are arrangements for changing the connector, with from four to nine different connectors available.

It is quite possible that new sizes will continue to appear and disappear. The most likely reason for a new size is that a particular manufacturer wishes to discourage use of third-party power supplies, either for technical reasons or to promote use of their own products or both. At the same time there is a trend for newer equipment of a particular type to be more tolerant of its power supply than older designs, making previous standards and conventions unnecessary.

Contact ratings vary from unspecified (and probably less than 1A in practice) up to 5A with 2A typical. Voltage is again often unspecified, up to 48V with 12V typical, less for the smaller types both for current and voltage.

Generic types

The most common plugs are 5.5mm in outside diameter (OD) and 9.5mm in length. Two pin sizes are common in the jacks for this size plug body, 2.1mm and 2.5mm, and the plugs should ideally match. Generic plugs are often named for the pin diameter they are designed to take, so these types will be seen described as 2.1mm DC plugs and 2.5mm DC plugs respectively. These two sizes are easily confused unless seen together.

Slightly smaller 5.0mm OD plugs are sometimes used in both 2.1mm and 2.5mm pin sizes, as are longer or shorter 5.5mm OD plugs. All are typically 8mm to 14mm in length. There are also 6.5mm OD plugs taking a 3mm pin, 3.8mm OD plugs taking either a 1.35mm or a 1.05mm pin, 3.5mm plugs for a 1.3mm pin, and many less common sizes.

Use of a plug designed for a larger pin, for example a 2.5mm pin plug in a 2.1mm pin jack, may work perfectly depending on the design of the jack, but often the result is an unreliable contact or occasionally no contact at all. A pin that is too large for the plug will not allow insertion of the plug at all. Similarly, a larger plug body (OD) may not fit at all, a smaller one may fit but may not make a reliable contact. Length is less critical than the diameters but is sometimes a consideration, in general a longer plug is no great problem but a shorter one may fail to contact the pin reliably or at all.

After the two common 5.5mm OD plugs the next most common size is 3.5mm OD to take a 1.3mm pin, commonly 9.5mm in length but both longer and shorter versions also exist. These 3.5mm OD plugs are normally used for lower voltages and currents.

EIAJ standard

Five plug and matching socket or jack designs are defined by the EIAJ standard RC-5320A. Each of these plugs is used with a specified voltage range. Most manufacturers use a yellow insulating material to distinguish these plugs from other similar-looking DC plugs.

The outer body of the plug is the negative contact, and the internal contact is positive. The five specified voltage ranges and sizes are:

• up to 3.15V, plug diameter 2.35mm, interior diameter 0.7mm
• 3.15-6.3V, plug diameter 4mm, interior diameter 1.7mm
• 6.3-10.5V, plug diameter 4.75mm, interior diameter 1.7mm
• 10.5-13.5V, plug diameter 5.5mm, interior diameter 3.3mm, pin diameter 1mm
• 13.5-18V, plug diameter 6.5mm, interior diameter 4.3mm, pin diameter 1.4mm

Unlike most DC plugs, the larger two IEAJ sizes have an internal pin in the plug. The smaller three do not and are similar to the generic plugs in structure. The plug length is 9.5mm in all cases, and current rating 2A.

Other DC connectors

See also power connector.

Some plugs with three, four, five or more pins are also called DC plugs. These were common on vacuum tube equipment and continue to be used where several voltages are supplied. On vacuum tube equipment the pins are normally on the equipment side of the join for safety reasons.

Miniature and subminiature jack plugs and even phono plugs are used instead of DC plugs on some equipment, especially in the lower price brackets. Jack plugs typically short circuit while being plugged in. XLR connectors, especially the most readily available XLR3, continue to be used by some manufacturers as power supply plugs despite their being a well-accepted standard for other purposes.

Five-pin DIN plugs were used on many early personal computer power supplies. DIN and mini-DIN plugs of various pin numbers also continue to be used by some manufacturers for power supply purposes.

Summary

Modern equipment is often less sensitive to power supply mismatches than older equipment. However, the only safe courses are to use the manufacturer's recommended power supply, or to determine and match the voltage, polarity, current and filtering required. The fact that the plug does or does not fit the equipment is meaningless unless the equipment and power supply are from the same manufacturer, and unreliable in any case.