Wall Adapter Question: How is 1A Output Possible?

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Discussion Overview

The discussion revolves around the operation of wall adapters, specifically addressing how they can output 1 amp despite being connected to a higher-rated outlet. Participants explore concepts related to voltage, current, and power in the context of AC to DC conversion and device power requirements.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant questions how wall adapters can output only 1 amp when the outlet is rated for 20 amps, suggesting a misunderstanding of current and voltage relationships.
  • Another participant clarifies that the outlet's rating indicates maximum capacity, and devices connected to the adapter will only draw the current they require, which may be less than the adapter's maximum rating.
  • A further explanation emphasizes that the wall adapter's output current is determined by the load it serves, not solely by the voltage drop, referencing the power equation P=IV.
  • An example is provided where a mobile phone charger rated at 5V 2A typically only draws 1A, illustrating that current varies based on the device's needs.

Areas of Agreement / Disagreement

Participants generally agree on the principle that the current drawn by a device depends on its requirements rather than solely on the voltage supplied. However, there is some initial confusion regarding the relationship between voltage and current, which is clarified through discussion.

Contextual Notes

The discussion touches on the assumptions regarding power supply behavior and device load characteristics, but does not delve into specific mathematical derivations or detailed electrical principles.

Who May Find This Useful

This discussion may be useful for individuals interested in electronics, particularly those seeking to understand the functionality of power adapters and the relationship between voltage, current, and power in circuits.

Sir Physics
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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?

Thanks!
 
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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.


Sir Physics said:
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?
Thanks!

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.
 
So if current MUST increase when voltage decreases..

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).
 
Ahhhhh - that makes sense - Thanks for all the help!
 

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