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Wall-Warts Power Usage and Power Supplies

Discussion in 'Electronic Basics' started by Dantanna, Mar 28, 2005.

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  1. Dantanna

    Dantanna Guest

    The main point of my question is to get a general idea on what kind of power
    usage AC power adapters consume when plugged into a household outlet. Why?
    Because even if you have nothing feeding off of the walwart it is still warm
    to the touch and that would indicate that it is wasting electricity and
    costing money.

    For example - I have a Sony Clie which is a rechargeable version of a Palm
    Pilot. I noticed that the adapter is rated to output 5.2V at 2000mA. My
    first thought was wow! What does a Palm Pilot need 2 Amps for??? So does
    the output rating with the Palm Pilot out of its cradle mean that the
    walwart is burning up my AC putting out a constant 5V/2000mA?

    Are they putting out a steady voltage and only providing current when
    current is called for? Or is the current also measurable even with the
    device absent?

    Bottom line is if these things are consuming power in idle mode - they need
    to be updated to be intelligent enough to turn off when the downstream
    device is off or removed.

  2. Indeed most wall cubes do waste a not totally negligible amount of energy
    when idleing. Exactly how much depends on the size and design of the wall
    cube, but an estimate of something around 2-4 watts while idleing isn't too
    unreasonable for normally sized wall cubes. If in your house you have 30 of
    these cubes all plugged in 24/7 each taking say 3W, and you pay 10 cents a
    killowatt hour that amounts to around $6.50 a month down the trapper. Most
    equipment you purchase will often specifically recommend you unplug the wall
    cube when you are not using the product (somewhere in the manual, but who
    reads or follows those?).

    Surely the Sony Clie doesn't need 10.4W during normal use, the high current
    is surely needed for recharging the battery. If you want to charge the
    battery in a reasonably short amout of time it takes a relatively high
    current. I don't have a Sony Clie, but it probably uses a switching power
    supply type of wall cube instead of the standard transformer + rectifier +
    capacitor arrangement. Switching power supply wall cubes can (although it
    doesn't mean they have to be) be designed to use much less power while
    sitting around at idle. These types of wall cubes usually do have higher
    efficiency while under load as well. Unfortunately they cost more, so low
    end cheap consumer products won't normally use them. Fortunately high tech
    products like cell phones normally include them these days.

    Indeed a 5.2V 2000mA rated wall cube won't likely waste 10.4W when at idle.
    They only supply 2000mA if the load wants that much. On the other hand most
    wall cubes will waste something while idleing, and the current flow is
    measureable. Although measureable, it isn't guaranteed that simple
    power=current*voltage formula can be used to accuratly measure the wasted
    power. An idleing transformer is an inductive device and therefore will not
    have unity power factor when at idle. Additionally capacitive loads fed
    through a rectifier don't draw current in a nice sinusoidal pattern, and
    they too lower the power factor (although this mainly only applies when the
    wall cube is loaded).

    Well that would cost more and make things more complicated. If you like you
    could go out and replace all of your standard mains frequency transformer
    based wall cubes with switching power supply ones, and assuming you get well
    designed ones this may make the idle power consumption negligible as well as
    improve the efficiency while under load.

    But if you really are concerned about waste make sure not to buy fuel
    wasting vehicles.
  3. Dantanna

    Dantanna Guest

    Lol - Thank you so much for that very detailed reply Fritz! This should be
    a FAQ or added to the faq.

    Thanks again - .
  4. I should add that it's wasted energy only if you don't /need/ the heat...

    If you have electric heating system in your house, then it's all the
    same which devices generates the heat. If you have some other system,
    then you're simply substituting part of it with electric heating (which
    may be chaper or more expensive depending on the price of kW for each).

    If you don't need the heat, then you just pay for the few watts and blow
    it out of your windows.

    If you don't need the heat AND have air conditioning unit removing the
    heat then you pay five times the energy. Once to generate it in the
    wallwart and then four times to move it away with the AC unit. (It costs
    about 4 times as much energy to move a unit of heat, as it does to
    create a unit of heat)

    And for prespective, having 10 wallwarts in the wall all times will take
    about as much energy as a single regular (40-60W) incandescent bulb
    running 24/7.
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