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Each electronic device consumes electric power when it is "idle" unless it has a mechanical switch. I can understand that for example a TV with a remote control needs to "be ready" to receive a command from the remote control. But even a cell phone charger consumes power when it is connected to the outlet and not connected to the phone.

For example, Nokia claims that one of its new chargers consume less that 30 milliwatts when not connected to the phone and they say it is very cool. I don't understand - the charger is a very simple device, what does it do with those 30 milliwatts?

Why can't this standby consumption me made lower when we already have microprocessors with gazillions of transistors fitting onto a plate size of fingernail? What's the fundamental problem here?

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    \$\begingroup\$ Short answer is AC to DC conversion is difficult to do efficiently. \$\endgroup\$ – Kellenjb Mar 23 '11 at 15:17
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    \$\begingroup\$ @Kellenjb: Okay, but when there's no phone connected the charger does nothing useful and still consumes power. \$\endgroup\$ – sharptooth Mar 23 '11 at 15:20
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    \$\begingroup\$ Why do you care? You'd have to run 1,000 such chargers for 1,000 hours to run up a bill of 1kW, which would cost less than 10 cents in most places. \$\endgroup\$ – Kevin Vermeer Mar 23 '11 at 15:29
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    \$\begingroup\$ sharptooth: The charger is doing something: waiting. You could say as well say that guards are not doing anything since they are just standing on their posts. \$\endgroup\$ – jpc Mar 23 '11 at 15:31
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    \$\begingroup\$ Also, to nitpick, nothing connected to AC, even if it contains a mechanical switch, truly achieves zero power consumption - Some energy is radiated by the antenna formed by the wiring, and there are other losses to capacitive loading. What I'm trying to say is, 30mW is close to zero standby power consumption. \$\endgroup\$ – Kevin Vermeer Mar 23 '11 at 15:32
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The mobile phone charger is a power conversion circuit which changes your power line voltage (110 or 220V) into something that is useful for your mobile phone (probably 5V). To do this it needs to have some electronic circuity inside which has to be powered and it has to function even if there is not phone around so it can detect one when you connect it.

The charger could be merely a mechanical device like the power socket itself but it would then require all the charging circuity to be inside your phone. Unfortunately it is quite big and relatively heavy so it would be inconvenient to carry it around all the time.

Regarding the actual 30mW figure: if instead of mW you consider the currents involved you arrive at around 300μA (30mW at 100V). This also means a resistance of \$330\,\mathrm{k\Omega}\$. It is quite difficult to work using resistances higher than and currents lower than this while still having to sense the moment when somebody plugs the actual load.

OTOH 30mW is really, really small. The vampire current draw problems are not as important as many believe. If you want a good review of many aspects of this then I suggest reading "Sustainable Energy – without the hot air", especially the chapter on this topic

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    \$\begingroup\$ Or to paraphrase grossly; Ohm's Law doesn't care if the electrons are being useful work or not, they will continue to they leak/ drain/ flow if there is a circuit between a voltage potential. \$\endgroup\$ – mctylr Mar 24 '11 at 22:28
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    \$\begingroup\$ Very interesting resource you have provided, however I would argue that the problem of standby current is not with devices that consume 1W... but with those that consume 20...50w on standby (which, as incredible as it may sound, there are many devices that do this) \$\endgroup\$ – payala Dec 16 '12 at 9:25
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It is very hard to make a PSU that can efficiently provide a couple of mW for standby as well as several Watts for actual use, so it's not too bad that Nokia managed to get standby consumption down to 30 mW for a charger.

The only way to be more efficient would be to have a separate PSU just to handle the standby consumption of the main PSU, but that could double the cost of a small charger, so it's unlikely to ever be done.

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Another point not yet mentioned is that energy-conversion devices (be they electronic, mechanical, chemical, or whatever) lose energy through a number of mechanisms. Some mechanisms waste energy proportional to the amount of energy being converted, while others waste energy largely independent of the energy being converted. A device which could convert 0-100W of power with 0.1W of waste would appear to be 99.9% efficient when used to convert 100 watts, but less than 1% efficient when being used to convert 1mW. In reality, most devices lose energy through a combination of mechanisms, some of which are proportional to the amount of energy converted, but there are design tradeoffs. For example, suppose the above device is used for a minute a day, and one could change the design to reduce the "constant" energy loss to 0.05W in exchange for accepting a 50% loss in conversion efficiency. Saving 0.05W continuously would make up for the loss of 50W during the minute of use, but dissipating 50 watts for a minute in a small device would cause it to get very hot, which might cause problems in and of itself.

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There are several problems. But the most obvious one is that every consumer product has some kind of standby mode. Don't forget, when your PC is off, it will easily draw about 100mA from the +5V. A ATX PSU has a special +5V Standby line, which can deliver up to 2A according to the specification. This is all just circuitry to monitor whether the PC needs to be turned on, wake up LAN, etc.

For a charger I could imagine that most of the power is wasted to some monitoring circuit to see if a phone connects. If so, it will probably active a 'bigger' supply to power the whole thing up.

Also, a switching power supply gets a peak effiency closer to it's maximum rating than to it's minimum rating. A controller needs current to operate as well. It needs to have a oscillator (generate a reference signal to PWM from), feedback, etc. Low duty cycles aren't helping neither, because little energy gets powered.

30mW is not much. If you assume they would use a perfect AC to DC transformation, you will still use only 2.5mA at 12V.

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  • \$\begingroup\$ I measured my PC's current consumption before and found it uses twice the current in standby versus when it's off. \$\endgroup\$ – Nick T Mar 23 '11 at 18:50
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    \$\begingroup\$ A standby situation will mean it turns of the CPU, but remains power on the RAM modules. Because of that, it will require more power. A 'complete off' (i.e. windows shutdown) situation still draws current, because an ATX system supports wake from LAN, etc. Some even power USB ports when they are off. \$\endgroup\$ – Hans Mar 23 '11 at 20:47
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    \$\begingroup\$ I know, I was just providing some anecdote to help shake off the misconception that "off" should mean zero. \$\endgroup\$ – Nick T Mar 23 '11 at 20:54
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Power Integrations have recently introduced the LinkZero-LP - a range of zero consumption integrated circuits specifically to eliminate AC-DC consumption when the phone is disconnected from the charger - whether the charger is connected to 115Vac or all the way up to 265Vac. http://www.powerint.com/en/products/linkzero-family/linkzero-lp

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    \$\begingroup\$ They are using "the IEC definition of zero no-load power consumption", where "zero" is defined as "less than 5 mW". Pretty impressive. \$\endgroup\$ – davidcary Mar 26 '11 at 13:13

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