Author Topic: Does USB wall plug phone chargers waste a lot of power if the load is small?  (Read 374 times)

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Offline engineheat

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I read that if there is nothing connected, the wall plug (let's say it outputs 5V USB) will waste power in the form of vampire current, which is negligible usually.

Here's a typical example:

But what if there is a small load? I plan to use it to power an Arduino Pro Mini, which has a sleep mode that will draw a tiny current. I wonder if that will cause the vampire current to increase disproportionately and waste a lot of power? Or do those plugs require a minimum current draw to even function, in which case power gets cut off if the Arduino sleeps (and dies).

Please clarify how those plugs work. Thanks

Offline IanMacdonald

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There is a load of nonsense published on this subject, mostly by the Greens. The original concerns were over CRT TV sets which effectively were running when in standby, so as to give instant on. That wasted a fair amount of power.  Today, there are some devices that do waste a small amount of power (a few watts) with no load, but the majority do not.

The only sure way to find out is with a wattmeter. It needs to be a fairly good one too, because many cannot resolve power levels of only a few watts.

Online Benta

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It's extremely simple to test: does it get warm?
If yes, it's dissipating power.
If no, it's not dissipating power.

For "dissipating" read "waste" in your vocabulary.

Offline Yansi

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Modern phone chargers have absolutely minute no load consumption.

Out of curiosity, I have tested my NOKIA phone charger and with no load, it consumes just shy of 40uA @246V in the wall. Which happens to be just under 10mW.

//EDIT:  10mVA, not mW, but will be close. Whichever, is in check what the current integrated off-line switch-mode technology can do.
« Last Edit: February 11, 2019, 10:18:52 pm by Yansi »

Offline Doctorandus_P

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Some 20 odd years ago TV's tended to generate 10W or more in standby.
Then regularors started making laws for maximum standby currents.
Then the whole green stuff happened.
Combined with progress in electronics, HF smps circuits became cheaper then 50Hz clunks of metal transformers.

Result is that most of the power suppliesyou see  nowadays barely register on the 0.1W resolution power meters during idle.
They often charge a cap on the secundary side and then stop completely untill the capacitor gets discharged.

With very low secundary currents they just turn on more often.
Overall efficiency is probably almost constant, if you compensate for the divide multply by 0 on the output current.

Offline Rerouter

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Switchmode psu's can have almost 0 standby power. A good example is laptop chargers. There input capacitor can still be charged up to 300V many weeks since it was unplugged.

Its not to uncommon these days to find a single microcontroller acting as the soft power button running off a few uA that then switches on the main device power supply just to keep the standby power low

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