I have a laptop and bought an extra AC adapter so that I don’t have to bring one from home to work, back home, ad nauseum. (well, not really nauseum, but you get the idea)
I leave both adapters plugged in all the time, mostly just because it’s convenient. I know at least some power is being drawn from the unused adapter, since the little green light is on – but how much energy am I wasting? Do modern AC adapters have intelligent circuitry to be able to tell when a laptop is plugged in, if the laptop’s battery is charging, etc? Or is it a simple matter of open/closed circuits, i.e. when the laptop is absent the adapter circuit is open and no electrons flow. I don’t think it’s quite that simple, given the concept of induction and that the AC portion is plugged in, but it’s been enough years since my E & M coursework that I thought I’d ask the Dopers.
For those of you who are more quantitative than the rest, the output specs on the adapter block are 19 V, 4.2 A, 80 W max, in case that information is useful to you in telling me how much energy I’m wasting.
Exactly how much depends entirely on the design of the adaptor. The switched mode power supplies commonly used with laptops tend to waste somewhat more than linear supplies, because they are actively running the chopping oscillator as long s they’re plugged in. Nevertheless, the amount wasted is probably still within the milliwatt range, and not significant to your energy bill. However, be warned that switch mode supplies can fail earlier if they are not connected to a load, unless they have additional circuitry to detect and handle this condition.
Correct. The adapter I bought for my digital camera warns that it is not to be left plugged into the AC line when not loaded, i.e. connected to the camera.
Thanks for the link LSLGuy (and you know the deal with searches here, ack!), and the details, Q.E.D.. Could you explain further what is meant by switched mode vs. linear adapters?
Follow-up:
Is it technologically feasible to add circuitry to detect loads and switch the transformer on and off? This obviously isn’t economically beneficial to the manufacturers now. How large would this circuitry be? How much power would the circuitry itself consume? How much more would these adapters cost if a similar number of them (compared to current adapters) were made? Has the device I have conceived of been built before and is just not in common usage?
LSLGuy’s link to howstuffworks contains the term ‘ghost load’, which I googled and found this page, which I found enlightening. The howstuffworks page also mentions the problem of transformers at sites powered by solar or wind power, so it makes sense to me that someone has looked at this problem before and come up with a solution.
A switched mode supply uses an oscillator, called a chopper to create high-frequency AC signal, in the area of about 20 kHz or so, typically. The regulating circuitry cuts off the waveform at the appropriate voltage level, and an error amplifier provides feedback to the regulating circuit to keep the voltage level constant under changing load conditions. Output conditioners smooth the resulting power into DC. That’s the basic idea. If it seems complex, it is, but switched mode supplies are very efficient compared to linear suppiles. They can also be much smaller, because the higher frequency of operation means transformers, inductors, and capacitors can be much smaller for the same power level.
A linear supply is simply a step-down or -up transformer, the output of which is rectified, filtered, and possibly regulated to provide the DC output. Electrically, they are much simpler than switched mode supplies, however, linear regulators are inefficient and the transformers and filter capacitors must be large to operate at a given power level, since they run at 60 Hz AC, rather than tens of kHz.
Basically, a linear regulator just uses a relatively simple semiconductor arrangement which keeps the output voltage at the right level. Everything is pretty much constant, although if more current is required and the voltage starts to drop the regulator will change to raise the voltage back up where it belongs.
A switching regulator, in contrast, basically just turns the output on and off really fast so that the average is the desired output level. In addition to the things QED mentioned, semiconductors tend to be most efficient when they are either switched all the way on or all the way off. When they are somewhere in the middle (like they are constantly for a linear regulator) then they tend to generate a lot more heat. Also, with all of that switching on and off going on, they tend to be electrically very noisy.
Something like a stereo system will almost always use a linear regulator just because a switcher makes too much electrical noise, which would show up as noise at the speakers. Car stereos will sometimes use switchers just because they need the size and heat efficiency. They have to add a bunch of filtering to the output to deal with the noise issue. A PC almost always uses a switching supply (Tandy is about the only one I ever saw use a linear one), since digital circuits are fairly immune to noise, and size and heat are very much an issue in a PC.
Switching regulators also don’t tend to regulate very well at all unless they have a certain minimum load on them, typically about ten percent of their rated max. Using a PC as an example, if the power supply is rated for 300 watts, if you have less than 30 watts of load connected to it, it may not work right. This isn’t much of an issue with a modern PC since the processor draws so much power, but on the original IBM AT there was a large power resistor installed when you didn’t have a hard drive (it’s hard for people to imagine a computer without a hard drive these days). The purpose of the power resistor was just to suck up enough power to keep the power supply happy.
“Wall warts” (those little AC adapters like for a portable CD player) tend to be unregulated, or if they do have a regulator it tends to be a very cheap linear one that doesn’t work very well. More often, all they have inside is usually a rectifier and filter.
The easiest way for a novice to tell if a wallwart is switched or linear is by size and weight. Transformers are big and heavy. If you have one of each type with the same wattage rating, you can tell which is which in no time flat.
As to actual energy waste, you can be mathematical, take some volt and amp measurements off the AC plug end, do some trival math, etc.
Or…
Put your hand on it. Ignoring the tiny green light, all the wasted energy is being turned into heat. Not much, but still a waste.
They can also potentially fail in bad ways, such as starting a fire. Not likely, but given how many millions of them are out there, they probably take out a few houses a year. Why take a chance?
I think the easiest way to tell which is which is to look for the input specs on the adapter. On a linear one, it will say 120 V. On a switched, something like 100V-240V.
Interesting point. I just checked some switched ones nearby. The first 3 did indeed say something like 100-240V but the 4th one only says 120V. 100-240V means switched, but not 100% vice-versa I guess.