centre pin negative question

Maybe not RULES perse, but I've found the bulk of the smallish electronic gear which uses those "wall-warts" to be center positive. Center positive is popular enough, that you can "guess" something is CP and be right probably 95 times out of 100. The most common wall power supplies are 9v and 12v, although you'll run into the occasional 5/6v models. 3Com, makers of networking equip, were famous for throwing everything away and using something goofy like 15 volts *AC* into their mini hubs and switches. Some small cisco equipment uses 5v and 12v, but uses a molex plastic connector with 4/5/6 pins instead of the DC barrel connector that most of the other devices use.

Keith

The center pin of plug packs can be positive or negative. You may have noticed some sort of pattern, but I wouldn't like to make any rules.

I think the reason for the apparent random assignment of polarity is that there never was a standard.

If the connector has switchover contacts which switch from battery to external power, then it may be important which polarity the switch breaks. I wouldn't think that would matter when using a "wall wart" supply which is isolated.

Roger

Probably goes back to those bitty Japanese AM radios that used PNP transistors, and grounded the positive side. I don't know what it is, but there is a sort of standard for the coaxial power connectors. You will find that for the same OD, there is a choice of center pin diameter. People have tried to correlate that to polarity.

Tam