How can you tell if the AC adapter is supplying enough juice to your laptop? Well, You can test your adapter with a voltmeter. Here, I’ll show you how to test a HP 18.5v AC adapter with a digital voltmeter.

 Most AC adapter has a label on them with their input and output specs. This one shows 18.5v(Voltage) and 3.5A(Amperage) and the inside is positive.  

 adapter-label

This is a basic auto range digital voltmeter. You can get on for about $15 to $25 form most electronic store.

multimeter

Now set the meter to voltage testing position.

 meter-dial

Connect the Negative(Black) probe to the outside of the connector and the Positive(Red) probe to the inside of the connector.

 adapter-test

As you can see the measurement on this is a little higher then what’s labeled. This is normal and there’s nothing wrong with the adapter.  However, if the reading is much lower or 0v the adapter is defective. If you need to buy a replacement adapter you need to get one that has the same voltage and the amperage can be same or higher.