A question on resistors

Discussion in 'Homework Help' started by Roen Hayden, Dec 3, 2007.

  1. Roen Hayden

    Thread Starter Member

    Sep 10, 2007
    22
    0
    What is the formula for figuring out how much resistance some needs will give an example

    Let’s say one has a LED which is rated for 1.5 volts and it is powered by 9 volt battier but it blows every time how does one figure how much resistant is needed to keep the led from blowing.
     
  2. Pootworm

    Member

    May 18, 2007
    29
    0
    This is assuming you're wiring your LED directly to the battery.

    If an LED is rated for 1.5V, you should assume a 1.5V drop across it. You'll need a resistor in series to limit the amount of current you're pulling - let's call that R. The amount of voltage dropped across R is 9V-1.5V (from the LED), so 7.5V. Dividing 7.5V by the amount of current you want to send through your LED gives you the value of the series resistor R. I usually run 10mA through mine, so 7.5V/10mA = 750 ohms.

    So, connecting a 750 ohm resistor in series with your LED to a 9v battery should light you up.
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
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