How to Test a Wall Wart


When portable electronic devices stop working, the problem may be in the power supply rather than the device itself. These power supplies, often called "wall warts," can be tested and replaced if defective.

Things You'll Need

  • Wall wart power supply
  • Multimeter with test leads
  • Electrical outlet
  • Check for physical damage. Unplug the wall wart. Check the outside of the case for cracks, corrosion, or anything leaking from the transformer. Inspect the prongs that go into the wall outlet, looking for bent, charred, corroded, or missing prongs. Inspect the wire for severe kinks, worn or chewed insulation, or exposed wire. Inspect the output plug that connects to the device, making sure the plug is not deformed and the metal contacts are clean and undamaged. If the wall wart is physically damaged, do not continue testing; replace the wall wart.

  • Test the cable for broken wires. According to Ray Carlsen of the University of Washington, Seattle, this is a common reason for wall wart failure, especially near the high-stress areas at the ends. These breaks may be too small to see in a visual inspection. Plug the wall wart into a working outlet and connect it to the portable device. Turn on the device. Wiggle, push, bend, and twist the cable along its length. If the device works momentarily while the wire is being moved, the wire inside the cable is broken. Replace the wall wart or cut and splice the cable to remove the broken portion, depending on your skill level.

  • Plug the black test lead into the negative (-) jack on the multimeter. Plug the red test lead into the positive (+) jack on the multimeter. Set the multimeter to measure resistance.

  • Touch one test lead to each prong of the plug on the wall wart. The multimeter reading should be greater than zero, but less than infinity. If the reading is zero or infinity, the input side of the wall wart is defective; replace the wall wart.

  • Touch one test lead to each contact on the wall wart's output plug. The multimeter reading should be greater than zero. If the reading is zero, the output side of the wall wart is defective; replace the wall wart.

  • On the label of the wall wart, find the output voltage. Set the multimeter to read DC voltage, in a range slightly higher than the output voltage. Find the polarity of the output plug, shown in a small diagram that looks like a letter "C" with a dot in the middle. The outside of the letter "C" represents the metal shield on the outer barrel of the plug. The dot in the center represents the metal core inside the barrel of the plug. In this diagram, the positive contact is labeled with a plus sign "+", and the negative contact is labeled with a minus sign "-".

  • Plug the wall wart into a working outlet. Hold the black test lead against the negative contact on the wall wart's output plug. Touch the red test lead to the positive contact. The reading on the multimeter should be equal to or greater than the output voltage on the label of the wall wart. If the output voltage is less than the voltage specified on the label, replace the wall wart.

Tips & Warnings

  • Make sure the metal tips of the test leads do not touch your fingers, each other, or any metal surface while using the multimeter.

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