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Wall Adapter Question

  1. Nov 17, 2013 #1
    Hey guys and gals - I was constructing a circuit yesterday and a question developed in my mind. Power out of an outlet is about 120 VAC at like 20 amps as most all of you probably know. So if current MUST increase when voltage decreases how are there wall adapters that only put out 1 amp when it should be a lot more?

  2. jcsd
  3. Nov 17, 2013 #2
    The outlet may be rated for 20 amps but that's not what will be drawn. The outlet can only handle 20 amps safely. The outlet must be connected to a breaker that is 20 amps or smaller if your house is wired correctly. That means that the breaker will trip before the outlet can draw more than what it is rated for.

    What you are talking about are maximums. The wall adapter is rated for 1 amp but whatever connects to it may only draw 200mA. You could short the adapter and it will draw much more than 1 amp before it burns. If the adapter can draw 2400 Watts before it burns then it will trip the breaker too (120VAC*20Amps =2400W).

    AC to DC converters typically lower the voltage in addition to rectifying AC but that does not mean that they must increase the current. It depends on what the connected device will draw.
  4. Nov 17, 2013 #3


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    That's only true if you want the same power delivered to the load. P=IV

    In most (but not all!) situations it's best to think of the power supply as the thing that determines the voltage and the load determines the current and power drawn from that supply.

    Example: My mobile phone charger is rated at 5V 2A but my phone typically only draws 1A from the charger. If I switch my phone on when it's charging the current goes up (some used to charge the battery and some to run the phone). When I disconnect my phone the current falls to zero (obviously).
  5. Nov 17, 2013 #4
    Ahhhhh - that makes sense - Thanks for all the help!
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