Standards
In an ideal world, specifying a voltage and a current for a power supply would also determine the connector, and it would be physically impossible to connect damaging voltages to a powered device. The world is far from that situation, but a few tentative steps have been taken in that direction.
At least two different national standards have been established, by EIAJ in Japan and DIN in Germany. In addition, several conventions have been adopted by specific manufacturers, notably by Sony, to indicate voltage by plug size.
Over time, there has been a trend to de facto standardize with negative DC voltage on the barrel (or sleeve) of a coaxial power connector. For example, Sony reversed its corporate standard during the 1990s, and adopted a barrel negative convention. One advantage of the barrel negative setup is its lower risk of short circuiting in automotive applications, since grounding the negative terminal of car batteries to the chassis has become an almost-universal standard. But barrel positive polarity is still occasionally seen, along with the continued usage of coaxial power connectors for low voltage AC power as well.
Some generic power supplies allow the polarity to be reversed by use of a switch, or by an unpolarized 2-pin plug and socket arrangement. There appear to be at least two not-quite-compatible standards in existence for these 2-pin connectors, and official specifications are hard to find.
One "standard" used by RadioShack for its Adaptaplug brand features pins measuring 1.5 mm in diameter, 6 mm long, spaced 4 mm apart (center-to-center). Another "standard" features asymmetrical pins measuring 1.3 mm and 1.9 mm in diameter, 5 mm long, spaced 4.4 mm apart (center-to-center). There are probably other "standards" in use as well; it is not known how many different 2-pin connector systems of this type exist.
RadioShack, Kobiconn, and Philmore sell these types of 2-pin adapter connectors, and they also sell DC power cables with mating 2-pin female connectors.
Read more about this topic: Coaxial Power Connector
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