MadSci Network: Engineering

Re: DC electric: Positive vs Negative?

Area: Engineering
Posted By: Tod Flak, Post-doc/Fellow Engineering
Date: Mon Jan 13 09:07:07 1997

MadSci Network: Engineering


First, if the AC-DC converter has a plug on it, you may be able to determine the polarity just by looking at the power supply. Many AC-DC converters have a symbol which indicates their polarity. Check for a icon that looks something like this:

If you have such a symbol, it is easy to determine the polarity. For example, the above symbol indicates that the plug is positive on the inner conductor, and negative on the outer conductor, as indicated in this picture. (The plug shown, called a "coaxial" plug, is often used for DC power supplies; if you instead have a connector which is like an audio plug, the tip is considered the "inner" conductor, while the large part closest to the plug body is considered the "outer" conductor.) On the AC-DC power converters I've encountered, the outer conductor is negative on probably about 75% of them. If there is no icon on your converter, you pretty much have to use a voltmeter as described below.

However, if you don't have a plug but just two wires coming out of the supply, determining the polarity is a little more difficult. Here are a couple of typical situations:

The one sure method is to use a voltmeter to determine the polarity. If you don't have a voltmeter, you can get a basic one at Radio Shack for less than $10, and you will probably find other uses for it in the future. Just set the voltmeter to read "DC volts", plug in the supply, and touch the wires (or the two conductors of the DC plug) with the two voltmeter probes. If you get a positive reading on the voltmeter, the conductor to which you touch the red probe is the positive. When doing this, be careful not to touch the two conductors of the power supply together, because a short circuit such as this can burn out the power supply.

Also note that the equipment into which you will be plugging the DC power supply may also have an icon indicating the polarity of the plug that it is designed to accept. Getting it backwards may damage the equipment, so be careful.

I'd be happy to give further help on a specific case if I can.

Best of luck,

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