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Need help understanding V drops and Current

Discussion in 'General Electronics Discussion' started by jpeg666, Aug 17, 2012.

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  1. jpeg666


    Aug 12, 2012
    Okay Say I have multiple leds hooked up to a power supply that puts out 12v at 300mA

    So as soon as the current passes through an led the voltage on the other side drops because the led uses some of the voltage right? If the led need 1.5v to light, the voltage on the other side of the now lit led would be 10.5v am I understanding that correctly?

    If I am understanding that correctly, is it the same for the 300mA current? or does the current stay the same across all the leds and only the voltage drops?:confused:
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    The voltage across a LED does not vary a great deal in response to current.

    You might find this helpful.

    And yes, when things are in series, if each drop some voltage, the sum of voltages across the rest drops by that amount.

    If devices are in series, the current through them must be the same for all, so in this case ALL would be passing 300mA.

    Think of voltage as pressure, and current as flow.

    Each of the LEDs drop some amount of pressure as the water flows through them, but the same amount of water floes through each of them.
  3. john monks

    john monks

    Mar 9, 2012
    Your power supply put out 12 volts (think of pressure) when there is no load or that is no current is passing through the terminals.
    Now when you place an LED across the terminate current or electrons start to pass from the negative terminal to the positive terminal. And when the current reaches 300mA or 0.3 amperes of current the voltage or pressure of electrons between the terminals is reduced to 3 volts.

    Remember a volt is a unit of electromotive force or pressure.
    An ampere is a unit of current flow.
  4. gorgon


    Jun 6, 2011
    Thats a rotten power supply you have there. A normal 12V power supply rated for 300mA, will supply 12V up to a load of 300mA.
    What happens if you draw more than 300mA is another history.

    The output voltage may fall, so the max current is kept to something above 300mA. (There will normally be a safety margin for the current.)

    Or more likely for a decent power the output voltage will at some point collapse, and fold back to zero.

    For the cheap version, it will heat up and eventually break, either the fuse or let the magic smoke out.

    TOK ;)
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