Most manufacturers use the center pin for positive voltage and the outer shield for negative voltage. Some manufacturers use the opposite convention. This manufacturing standard uses the center pin for the negative voltage. Connecting an adapter with the wrong polarity could cause damage to your electronic device. A simple test with a voltmeter will reveal the adapter's polarity.
Insert the voltmeter's red test lead into the red jack marked "V." Insert the black test lead into the black jack marked "COM."
Turn the voltmeter on. Set the range selector knob to the highest range marked "VDC."
Plug the AC/DC adapter into a wall outlet.
Place the red test lead into the center pin on the AC/DC's output connector.
Place the black test lead onto the outer shield of the AC/DC's output connector.
Read the voltage on the voltmeter's display. Do not let the two test leads touch each other and cause a short circuit. If the voltage read is positive, the center conductor is the positive voltage. If the voltage read is negative, the center conductor is the negative voltage.
Use a small piece of wire to measure the center conductor’s voltage, if the test lead is too large to insert.