User:Lynnae/How to Test an AC Adapter

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AC Adapter

Work in progress- add troubleshooting steps section to testing procedure

  • Digital Multimeter
AC Adapter

An AC/DC adapter (commonly known as an AC adapter) is used to change alternating current (AC) from an outlet into the kind of electrical signal used by most consumer electronics, direct current (DC). It also adapts the incoming power to an appropriate level to send on to the device. Most consumer devices that plug directly into the wall without an adapter have an internal AC power supply built in, to achieve the same result. For a laptop, an external AC adapter reduces chassis weight, heat generation, and size.

Take this IBM AC adapter as an example:

INPUT: 100-240V~1.9A
OUTPUT: 16V  4.5A

The adapter can accept 100 to 240 volts (V) of AC power (~), and outputs 16 volts and 4.5 amps.

Voltage is the potential difference between two points, or how much work must be done to move a charge in a circuit. Amperage is a measure of the amount of the actual electrical current. Some laptop AC adapters also list the wattage of the supply; Wattage is voltage multiplied by amperage. This relationship is defined by Ohm's Law.

For our testing purposes, we are most concerned with the output section printed on the adapter. For an AC adapter to power the device correctly, it must

  • Match the input voltage (V) of the laptop exactly
  • Match or exceed the recommended amperage (A)
  • Fit into the laptop and provide power

On some occasions we can vary the voltage by half a volt or so, but check with your instructor first. It is easiest to start with adapters branded for the laptop you are working with (i.e. an IBM adapter for an IBM laptop), but it is fine to use an off brand supply, as long as you are following the requirements listed above. Polarity is another important factor, but is not something we run into with laptop adapters for the most part, and will not be covered here.

There are a few primary components that make up an AC adapter. If you find an old, clunky AC adapter and open it up (unplugged please!), you will see a metal square with two sides wrapped in wire. This is called a transformer. Depending on how the sides of the square are wound, a transformer is used to increase or decrease the incoming voltage, and send it on to the rest of the circuit. From there, the AC signal is converted to DC by a part called a rectifier, and additional circuitry components may be present to regulate and smooth out the signal. Modern adapters use much smaller transformers, and more complicated circuits. They are thus are lighter, and do not generate as much heat.

To test an AC adapter, you will need a Digital Multimeter. Make sure the red (positive) lead is connected to the jack marked VΩmA and the black (negative) lead is connected to COM, or common. Set the dial to 20 DCV. We are using this setting to measure direct current up to 20 Volts. Higher powered adapters should be tested at 200 DCV.

  • Plug in the adapter. Note whether or not there is an indicator light on the adapter, and whether or not it is on. **If there is a light and it is not on, try a different power cable or outlet to the supply.
    • If there is a light, and it is on, proceed to the next step.
  • Touch the tip of the red lead to the pin inside the barrel. Touch the black lead to the barrel, without letting your fingers touch any of the metal. You should see the voltage read on the meter.
    • If there is no reading, try repositioning your leads. It may be dead, or you may not be making good contact with the center pin.
    • If the voltage is lower than the amount the adapter is rated for, recycle it.
    • The most common reason for an AC adapter to fail is a short in the cable between the adapter and the barrel. Try jiggling the cable, and you may get an intermittent rating. If this is the case, or you see exposed wires or breaks in the cable shielding, recycle the adapter.