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Need help for my question regarding circuit

  1. Jan 8, 2013 #1
    Hello ,
    I am having problem with one question.
    In the figure below which resistors values and voltage source values are sufficient so that LED can work properly.
    Is the circuit correct,do i need some modifications on that.
    If it is correct what will be the equivalent voltage.
    what does "@20 mA" means in this LED.

    Attached Files:

  2. jcsd
  3. Jan 8, 2013 #2
    Before going any further, what happens when you close S3?
  4. Jan 8, 2013 #3
    I dont know nothing at all ......... please guide me which you know of ...... if i am wrong in this circuit please guide me
    Last edited by a moderator: Jan 8, 2013
  5. Jan 8, 2013 #4


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    Staff: Mentor

    There may be problems with the circuit, but in order to help you find them we will need to know what the circuit should be doing. Is this your own design? If so, what are the design goals?

    The "1.83 V @20 mA" type labels indicate the expected voltage drop across the LED when the specified current of 20 milliamps is flowing through it. Note that 20 mA is a typical operating current for many LEDs.
  6. Jan 9, 2013 #5


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    Science Advisor
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    Gold Member

    S3 connects the positive of V2 to the negative of V2. What happens if you short out a voltage source like that?

    Ohms law... V=IR so I = V/R but R=0
  7. Jan 9, 2013 #6
    Besides being shorted out when S3 is closed, what purpose is V2 supposed to accomplish?
  8. Jan 9, 2013 #7
    As gneill says, we need to know what the circuit is supposed to do before we can help you. When will the switches be activated? Will they never be closed together or can it happen that all of them are closed at the same time? Are they supposed to light up together or one at a time? Why is there 2 voltages supplies?
  9. Jan 9, 2013 #8
    I was told in assignment to make a circuit and monitor each light blinking ........ this is why i made this circuit and want to monitor it with switches open and closed....... The major thing which i need to know is that
    1)With switch/switches closed which led/leds is going to blink
    2)The direction of current will go from where to where
    3)What should be the value of resistors,current in the circuit
    4)Which voltage source will dominate and what will be the total voltage of the circuit
    5)If there is any flaw in the circuit please let me know.I will change it.
  10. Jan 10, 2013 #9


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    Staff: Mentor

    Hi mremadahmed. I'm afraid that your explanation is still not clear. I think you are wrongly using the words "monitor" and "blink". Can you write your explanation again, but this time avoid those two words and choose other equivalent words that mean the same to you.

    Were you told that you must use two voltage sources? If not, why have you chosen two?

    Can you include a .jpg of the assignment question and its accompanying diagram? Post it even if it is not in English.
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