Amperage Question

Discussion in 'General Electronics Chat' started by mxabeles, May 23, 2009.

  1. mxabeles

    Thread Starter Senior Member

    Apr 25, 2009
    Hi there,
    A few years back I was in a electronics store looking at adapters. I saw one that had too much amperage for my project but the store clerk said "you can have more amps then what your project needs, just make sure the voltage is correct." Now, I know that Ohms law states that voltage, current, and resistance are all related. Thus, this leads me to believe he is wrong and that having too much current can hurt a device/component. Right?

    Thanks, i realize this is MAD basic stuff. :(

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    No, the clerk was correct. The load will require a certain power, which is the product of voltage and current. If the voltage term does not change, neither does the current. So a 60 watt bulb needs 500 ma current, and the ability of the circuit to supply 15 amps has not bearing on the power used by the bulb.

    The same is true of a DC load.
  3. bertus


    Apr 5, 2008

    Imagine you have a powersupply that CAN deliver 10 Ampere at 10 Volts
    You connect a resistor of 2 Ohms to the output.
    The current that will flow through the resistor is 10 Volts / 2 Ohms = 5 Ampere.
    This is OK for the powersupply. (Its maximum rating of 10 Ampere is not yet reached).
    So the powersupply will deliver the current that is needed (although there is stated 10 Ampere).

  4. mxabeles

    Thread Starter Senior Member

    Apr 25, 2009
    Thanks for the interesting responses.
    So say I have a 150mA LED that runs on 12v. I connect it to a 4 Amp 12v adapter, what happens to the extra 3.8 Amps?
    I always hear about limiting current or else an LED will burn out. Is that because most people use a 9 volt battery and thus are using a resistor to limit voltage?? argh, i realize you can't affect one and not the other.

  5. Audioguru


    Dec 20, 2007
    If you do not limit the current to an LED then the LED will burn out.
    You can use a 150mA 12V adapter, a 4A 12V adapter or a 100A 12V adapter and if the voltage for all of them is the same (they might be different) then the current-limiting resistor value will be the same.

    A 9V alkaline battery is too small to provide 150mA for much time. Its voltage will drop to less than 7V in less than 1 hour.