On a 9V power supply how can I tell if the inside of the barrel is positive or negative?

Question

I have a US Robotics p/n 1.015.1286 transformer. It does not indicate if the center is + or -.

Answer 1

Have you checked the plug above the metal carefully? Some manufacturers also make the notation on the transformer body itself. It has been my experience that the outside is usually negative, but I have also seen some the other way around! Almost all products of any worth have a diode right after the input jack to prevent current flowing in the wrong direction. Check with a voltmeter if you have one, or I'd be brave enough to actually connect it. It will either work or not and should not damage either the transformer or the device....

Answer 2

Use a meter, of course, set to a DC range higher than you expect the supply to deliver. If you have no meter, you might try opening the modem (I assume a modem since US Roborics makes them) and examine the point where the power connects. Look for an electrolytic capacitor. A lot of devices will have a filter capacitor at the power input point to filter electrical noise from the power supply. These are tubular shaped and usually stand up from the circuit board. These are polarized and have an indicator, usually showing which pin in negative. Trace the foil lead back to the connection point and it may show which is positive on the power socket. I have used this way when I had an electronic device and no power supply and I was looking for what power I should use.

Answer 3

The US Robotics power supply 1.015.1286 is a 9v AC, 1000ma power supply. AC power supply connectors are not polarized postitive and negative.

Answer 4

US Robotics transformers are often AC and not DC so positive/negative is irrelevant. They will be marked with a little sine wave symbol if that is the case.