The question is, why do electrical machines consume power when they are not turned on (they are plugged into an outlet)? Also, suppose there is a seven-port USB hub with AC power. Is the power consumed by the hub always the same regardless of how I use the hub?
Asked
Active
Viewed 957 times
6
-
What makes you think they will consume power when off? They shouldn't. Can you give an example? – stevenvh Sep 27 '12 at 06:02
-
@stevenvh modified the question to avoid confusion. – AER Sep 27 '12 at 06:08
-
10You accepted an answer too quickly. It's better to wait until you have a few answers, or a day to give everybody the opportunity to answer. The answer you accepted is not a good one. You can unaccept and come back tomorrow for a better one. – Federico Russo Sep 27 '12 at 06:23
3 Answers
13
"Turned Off", can (unfortunately) mean different things, it may refer to the device still having power available but in a low power standby mode, where consumption is minimal. Many consumer devices do this for various reasons, such as waiting for a remote signal to turn on.
If power is completely cut off (e.g. the circuit is broken) then the device cannot use any power. Switching off a device by switching the wall socket or unplugging would be an example. Also many devices have a main power switch as well as a standby switch (e.g. a mechanical switch at the rear). This is what I understand as "turned off".
The USB hub will consume less power when not loaded (ideally almost none, probably a few mA) It's efficiency under load will typically be around 80% if using a switching converter, this will vary a bit with different loads.
Oli Glaser
- 54,990
- 3
- 76
- 147
10
If the machine is switched off it doesn't consume power. Final.
Sure, there are machines which consume power despite not being operating. Take my washing machine. It's being switched on and off with a push-button, not a switch. But that push-button is a small tact switch which isn't capable to interrupt the 230 V supply. Instead it will give a signal to the microcontroller, which in turn activates the machine: the display lights up, and commands are being accepted. When the machine is "off" the microcontroller remains active; it's cheaper to use that tact switch for on/off than a genuine mechanical switch, and also that mechanical switch is so much 1970, users won't accept it anymore in 2012.
So when I say my washing machine is switched off it actually means it's inactive: the motor, heater, pump and valve, etc are all off, but the logic remains active. That may consume 1 W or so.
The USB hub may have a switch to switch it on and off, and when off it will effectively be off: zero power. In the hub! It will be powered by a wall-wart, that remains plugged in to the wall socket, and that does not have a switch. So it will consume power, which you can feel when you touch it: it will be slightly warmer than the environment, even with the hub off.
The power used by the hub will vary a bit when you load the 5 V outputs, and that load of course also adds to the total power consumption (but only partially to the hub's itself). The hub's power consumption increase is due to the loss in the voltage regulator which provides the 5 V out.
1 W standby power doesn't seem much, but it adds: think of all the devices which you think are off, but have that one LED on. When I still had TV I had a VCR which consumed 11 W in standby (more than half of the 21 W it consumed when active!). Total standby power worldwide in households alone may be as much as a terawatt-hours a day.
stevenvh
- 145,145
- 21
- 455
- 667
-
2I like the last paragraph especially! Now the attention is increasing, and many people suggest using power plugs with mechanical switches, to turn off completely all the appliances – clabacchio Sep 27 '12 at 09:20
-
2@clabacchio - thanks. You can't ignore it: when I enter my apartment at night with the lights out it almost looks like a Christmas tree, with all those standby LEDs everywhere :-/ – stevenvh Sep 27 '12 at 09:32
-
I've read somewhere that power consumption of devices on standby accounts to 10-30% of household electricity use. Presumably this value is higher in the US where many people have A/C. – Christoph B Sep 27 '12 at 11:08
-
2
-
@stevenvh:- *with all those standby LEDs everywhere* , hope we can have next beer festival at your home ;) – perilbrain Sep 27 '12 at 13:52
-
1@perilbrain - nema problema! :-) Eight people in the living room, and three on the balcony, leaves just enough room for a couple of barrels :-) – stevenvh Sep 27 '12 at 13:56
5
AC/DC converters used to interface the grid with a low voltage devices consume some power even when there is no load (e.g. nothing connected to the hub) because they contain control electronics that need a little bit of power jest to keep the device ready.
If you take a look at typical efficiency curve you will see that if you draw very little power the efficiency goes towards zero. This is because you always need a small amount of power to keep the internals running and this is why an electronic appliance (containing SMPS or some standby features) will draw some power if connected to the socket and not running.
Szymon Bęczkowski
- 2,595
- 15
- 21
-
1It would be helpful if more devices included a 'source current vs. load current' curve. Efficiency numbers become meaningless at low currents, but absolute source-current numbers can be very useful. It's possible for a device to have very low quiescent current at zero load, and be very efficient at high loads, but be inefficient at low loads (e.g. a 9-volt to 3-volt switcher which operated as a linear regulator below 100uA might waste less than 10uW when supplying 2uW, and waste less than 1W when supplying 5W, but waste more than 600uW when supplying 300uW). – supercat Sep 27 '12 at 15:34