How does an AC Adapator Work?

Factual Questions
1

I had a friendly disagreement with someone over the weekend about an AC adaptor for a laptop.

I said that when it’s plugged into the wall, but not plugged into the laptop, it’s not drawing much of a load and will last much longer than if it’s plugged into the laptop and the laptop is turned on.

He said that the adaptor draws the same load whether the laptop is plugged in or not and that he can prove it by the fact that the adaptor is very warm to the touch either way. He thinks it should be unplugged from the wall and not just the laptop or it will wear out.

So who’s right?

2

Well, the adaptor in question is a transformer, I believe, transforming AC outlet power to the DC power your laptop craves. The fact that it’s very warm indicates it’s drawing power, but I have no idea how much. As far as how long it will “last,” I have yet to own a transformer that didn’t outlive the device it was providing power to, so the wear and tear on the transformer may not be much of an issue. I’ve had transformers for things like my computer speakers and my DSL router running hot for months at a stretch with no ill effects except the unknown hit I’m taking on my power bill for being too lazy to shut off all the power strips they’re attached to.

3

Your friend is mostly right here.

Chances are good to excellent that you will either lose the adapter and have to replace it, or replace the laptop, thus getting a new adapter long before your existing adapter would “wear out” from being plugged in all the time.

More important than wearing the thing out is that if it’s getting warm, it’s consuming power. Most adapters now are “switching” power supplies (the technical details aren’t really important here) that will draw a relatively steady amount of power from the wall regardless of whether the laptop is connected or not.

Save yourself a couple bucks each year on the electric bill and unplug it if it’s not in use.

4

I don’t think that this is true at all. A switching power supply will definitely draw more current if it has a load.

5

Here’s a more informed opinion than mine:

How Stuff Works: How Much Power does a Wall Cube Transformer Draw?
Basically, if they’re just plugged in and not powering anything, they still draw about 5 watts.

6

Your basic el-cheap wall wart is just a transformer, a diode (or maybe a diode bridge) to do the rectification, and a capacitor to filter off what’s left of the AC components after rectification and make the output look more like DC. They are cheap, and have very few components. They don’t regulate the voltage very well, because they don’t really have a regulator in them, so if the input voltage varies a bit (which it usually does, the power company generally only guarantees that it will be within 10 percent) then the output voltage will vary as well. So, your 9 volt wall wart may be putting out 9.5 volts instead of 9.0. No biggie, as far as most devices are concerned.

Some devices though need that 9 volts to be always close to 9 volts. This is the job of a regulator. There are two types of regulators, called linear and switching. Linear regulators compare a known reference voltage (usually obtained by running current through a zener diode, or something else that provides a relatively stable reference voltage over a wide range of loads) and use this reference to adjust the output, which they adjust by how much they let current flow through certain transistors and such. This is hard to describe to someone without going into geeky electronics techie mode, but suffice it to say that if the output is too low it lets more current flow and if the output is too high it restricts the amount of current flowing. The problem here is that when you use transistors like this, the amount of restriction they put on the current all gets turned into heat.

This is where switching regulators come in. Switching regulators work by turning the output completely on or completely off really really fast, so that on average the output is right where you need it. If the output is too high, you spend slightly more time turned off. If the output is too low, you spend slightly more time turned on, until you get it adjusted just right. Transistors generate a lot less heat when they are completely on or completely off, so switching power supplies are much more efficient than linear supplies. However, they have two main drawbacks. First, all of that switching on and off makes for a lot of high frequency electronic noise. Second, switching regulators don’t tend to work very well when they don’t have a decent load connected to them. Typically a switcher needs about 10 percent of its rated max current, otherwise it’s not going to work properly (in other words, if a switching regulator has a maximum rating of 5 A, it has to have a load connected that draws at least 0.5 A or it’s not going to work right).

So, we have 3 basic cases to look at.

  1. Unregulated (wall wart type).

If you leave one of these plugged in with no load attached, there is going to be some heat generated by the windings in the transformer, and you will draw a slight amount of current, but you’re really not going to shorten the life of the wall wart or anything. It will however draw a lot less current just sitting there than it would with your CD player or whatever attached to it and running.

  1. Regulated, using a linear regulator

The nice thing about linear regulators is that they regulate properly even when they have little or no load attached to them. The regulator itself is going to draw some current, so in that respect it will draw more current than a wall wart would if nothing is attached to it, but still, it will draw more current when it is actually connected to something.

  1. Regulated, using a switching regulator

This could potentially be your problem case. Switching regulators don’t work well if they don’t have a minimum load, and if you unplug it from the laptop, then you don’t have any load at all on it. Under these conditions, some switching regulators will do very weird things, and you’ll have all kinds of oscillations and voltage fluctuations, and the regulator may actually self destruct. There are ways of handling this, but this adds components to the power supply. Adding components means the power supply is not only bigger, but it costs more too. Most laptop supplies probably can take being unplugged, but this always makes me nervous. A switching supply definately could have a much shorter life if you leave it plugged into the wall but not in the laptop.

Most laptops these days use switching power supplies.

7

It’s not the transformer will wear, it’s the Laptops Battery that can get screwed in the end. 99.9% of the “failures” in Notebook Computers, namely Apples iBooks and pretty much every Dell and HP Comcraps, are due to battery mischarge. Mischarging happens when you “fail” to unplug the adapter from the notebook for long periods of time. This is bad.

8

Are you sure about that with modern batteries? Our entire company uses perpetually docked laptops for days at a time. I’m pretty sure when the battery is 100% it will just shut off the charging circuit and run off the AC until the AC disappears.

9

engineer_comp_geek is correct.

Almost all power supplies dissipate power in proportion to the amount power they’re sourcing. In other words, if a power supply is not supplying much power to a load (or none at all), it will dissipate little power. Conversely, if a power supply is supplying a lot of power to a load, it will dissipate quite a bit of power.

As mentioned by engineer_comp_geek, power supplies come in two flavors: linear and switching. Though the previous paragraph is true for both types, switching power supplies are always more efficient than linear supplies (all else being equal). Power supplies for PCs and laptops are always of the switching type.