How come a power adapter will list the voltage and current?

Discussion in 'General Electronics Chat' started by speedster239, Nov 24, 2009.

  1. speedster239

    Thread Starter New Member

    Nov 24, 2009
    12
    0
    Hey guys! :D,
    I'm a bit confused here. I've been reading that current is determined based on the resistance in a circuit (Current = Voltage/Resistance), so I don't understand why a power adapter (such as the one you plug in to power a wah pedal on your guitar) lists the current although it hasn't encountered any resistance yet to even determine what the current is. I thought it would simply just list the voltage and the current would be determined by whatever I put in the path of the electrical flow such as a lightbulb, or a motor, etc based on how much resistance it has.

    My only good guess is that that the listed amperage is actually a listing of the maximum current that can be drawn from the adapter, which would make sense. You see, what's happening is I have a charger for my R/C planes but it only has a plug for the 12v cigarette lighter in your car. However, I found this power adapter that is labeled "12V and .42 amps". I tested the voltage, it came in at exactly 12 volts, and I spliced the end of this adapter and hooked it up to my charger. It appears to work fine, however it takes double the time to complete a charge. I assume, then, that the adapter is limiting the amount of current that the charger draws, and thus slowing the charging process.

    I can't see any other reason what the listed current "rating" printed on the adapter means, since the voltage is the force pushing the electrons, and the amount of electrons moving (current in amps) is being determined by the resistance in the circuit which the adapter has no idea it will be driving beforehand.

    Thanks,
    Vaughan
     
    Last edited: Nov 24, 2009
  2. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    You have guessed (or deduced?) correctly.

    The voltage is specified for a power unit, along with the maximum current draw.

    You might like to be aware that for cheap units the voltage at any load other than stated current will actually be somewhat higher. So a 12volt 0.5 amp unit may actually put out 16 volts on no load reducing to 12 volts at 0.5 amps.

    Keep up the good work.
     
  3. speedster239

    Thread Starter New Member

    Nov 24, 2009
    12
    0
    Thanks mate!, I really appreciate you getting back to me man. I'm also happy that I was able to get to the right answer (that's what it's all about), because my mind is like a puzzle and once I get stuck on one question (even as simple as this one), I can't stop thinking about it :).

    Thanks again,
    Vaughan
     
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