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Homework Help: Determine the resistance of Rl

  1. Mar 31, 2010 #1
    1. The problem statement, all variables and given/known data
    All LEDs in the following circuit are identical. Each LED has a switch on voltage of 2V and draws a current of 20mA for optimal light production.

    Determine the resistance of Rl that gives optimum operation for all LEDs.


    2. Relevant equations
    Ohms Law V=IR


    3. The attempt at a solution
    becuase the LEDs are connected in parallel I calculated the voltage for Rl to be 9-2 =7V then divided that by 20mA which gave me 350ohms, which is incorrect.
     

    Attached Files:

  2. jcsd
  3. Mar 31, 2010 #2

    rl.bhat

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    Re: Resistance

    Each LED draws 20 mA current. There are three LEDs. So the total current drawn from the battery is........?
     
  4. Mar 31, 2010 #3
    Re: Resistance

    OMG im an idiot, I didn't read the question properly, I thought the 20mA was divided amongst the 3 LEDs. Haha, anyway Thankyou!
     
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