What Is the Black Box on a Laptop Power Cord?

When you charge a laptop computer's battery from an electrical outlet, you plug it in to the socket using an electrical cord with a mysterious-looking sealed box in the middle of it. Some adapters include modular cords that make this "line lump" -- the box, sometimes black, sometimes other colors -- removable. Regardless of how they're designed, these devices perform essential functions that keep your computer running.

Your laptop and its battery can't use the full amount of electricity provided by a regular household or office outlet, which produces 120 volts of alternating current. Inside the adapter's "line lump," a stepdown transformer reduces the strength of the current to a level the system can use. The adapter also transforms AC power into direct current. A 60-watt laptop power supply may turn household current into 16.5 V of DC electricity.

Many laptop computers use switching power supplies that can accept the 220- or 240-volt electrical power used outside the United States as well as the 120-volt standard from regular U.S. outlets. When you travel abroad with devices that include this capability, you'll need a special converter to accommodate the shape of the plug the electrical outlets use, but you won't need a power converter for the current itself. Before you purchase adaptive devices for electronics, however, check the specifications for your devices and verify that they can run on various voltages.

Laptop adapters produce heat when they operate. The cord that connects the adapter to the electrical outlet on one end and to the computer on the other should stay cool. The adapter may become very warm, but it never should reach temperatures at which it becomes too hot to touch. For safe operation, set the adapter where airflow around it can help keep it cool. If you notice it becoming abnormally warm, verify that your battery charges correctly and replace the adapter if it continues to perform improperly.

If your laptop power adapter fails, you need a replacement of comparable wattage that provides the same amount of power as the original device. Choosing the wrong adapter can burn out your battery, your computer or both. You may find the adapter's electrical specifications listed on the device itself or in the documentation that accompanied your computer. Purchasing an exact replacement from the computer manufacturer offers your best alternative, unless you can find an aftermarket alternative with the same specifications as the original.