Tech Files
The Powers that Be:
AC and DC Power in Notebook PCs
Let’s take a look inside the world of Carl. Carl is a highly motivated rookie sales executive, and he’s about to make a big presentation for what could be his first multi-million dollar contract. With only ten minutes to spare before his prospective client arrives, he reviews his work.
As he views his presentation, Carl suddenly becomes aware of a flashing battery icon in one corner of the screen. He’s seen this before and knows what it means: in five minutes, the laptop will run out of battery power, and he’ll be dead in the water. He could plug the laptop into a wall outlet, but in the true spirit of Murphy’s Law, the nearest one is in the back of the room—too far for the AC adapter to reach—and there’s not an extension cord available.
His first impulse is to reach for his spare battery, which is fully charged; then he realizes with a sinking feeling that this won’t solve his problem. Removing the dead battery would interrupt the power to his laptop, causing Windows to crash.
To avoid this, he’ll have to power down, change the battery, reboot, and then reload his presentation—but there’s not enough time to do all that before his client arrives.
Fortunately, Carl is cool under pressure, and with three minutes of power remaining, he realizes what he must do. He carries the notebook to the back of the room, connects it to the power outlet, and then changes the battery—all without having to shut down the computer. Then, with the fully charged battery in place, he pulls the plug from the power outlet and walks back to the conference table, just in time to greet his client with a confident handshake.
Wow! Carl sure got out of that situation smoothly, didn’t he? The key here was that he understood how his laptop’s power works, and he used that understanding to his advantage. Let’s take a closer look at how notebook PCs get the power they need.
Notebooks, like all computers, require direct current (DC) power to function. There are two ways to deliver that power to the notebook. The first method is through an Alternating Current (AC) power adapter, which plugs into an AC power outlet and sends that power to the computer.
Now, you may be thinking, “AC power?? You just said that notebooks need DC power!” Well, this is true. The secret of the power adapter is that it also houses a power converter.
The power converter is usually a rectangular box located about halfway along the adapter’s cable. Its purpose in life is to take AC power and convert it into DC power before it arrives at the computer. To use a power adapter, simply plug one end into the AC power outlet and the other end into the notebook computer. Be aware that the connector that attaches to the computer is usually proprietary, meaning it can only be used with a particular brand—or in some cases, only a particular model.
It’s important to note that even after the converter has turned the higher-voltage AC power into lower-voltage DC power, there’s still work to be done before the laptop can use that power. A device called the DC controller, usually built into the laptop’s motherboard, makes minute changes in the voltage to deliver the appropriate amount of power to the hard drive, LCD screen, and various other components. The DC controller is what enables these items with differing voltage requirements to work together happily in the same machine.
While working with an AC adapter is a viable option in most cases, it does tend to reduce portability, which is the main advantage of using a laptop. That’s why there’s another option for powering a portable PC: battery power.
In the early days of laptops and the nearly desktop-sized “luggables,” batteries were more of a nuisance than an asset. They usually offered at most about 30 minutes of power, and although they were rechargeable, the amount of time the battery could power the PC tended to decrease with each charge. Today, however, with Lithium-Ion batteries and power-saving features incorporated into notebooks, batteries can often last more than three hours, and can be recharged thousands of times with minimal deterioration.
Installing a battery into a notebook is easy: you just slide it into a slot on the notebook until it snaps into place. Removing the battery usually requires sliding a tab on the bottom of the notebook, which releases the battery and allows it to slide right out. Many batteries now even have a feature that enables you to check their charge without putting them into a computer. To check this kind of battery, look for a button you can press to light up a row of LEDs indicating the battery’s current charge.
There you have it—notebook PC power in a nutshell. Armed with the knowledge of how a laptop gets its power, and an understanding of your power options, you can keep your cool like Carl if your battery light ever takes you by surprise.