Please help, Why is the led off

Discussion in 'Homework Help' started by sensor, Sep 8, 2009.

  1. sensor

    Thread Starter New Member

    Sep 8, 2009
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  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    There must be a voltage source at the input of the regulator.
    This must be at least 2 Volts higher than the output voltage.

    Greetings,
    Bertus
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    What is the input voltage to your 7805 voltage regulator?


    You need to have a voltage source connected to the input of the regulator.

    Your resistor is very large ... at 20 mA, it would drop 30 volts. Your LED is probably 1.5 Vf @ 20 mA. With a 5 volt supply, you would need a much smaller resistor.
     
  4. sensor

    Thread Starter New Member

    Sep 8, 2009
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    Hello,
    How did you calculate 30 volts?(V=IR)
     
  5. Mike2545

    Active Member

    Mar 26, 2009
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    Your resistor value is too big.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Ohm's law. E = IR , so .02(amps) X 1500(ohms) = 30(volts).
     
  7. sensor

    Thread Starter New Member

    Sep 8, 2009
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    How can i know this(about a led)?
    What's 1.5vf?
     
  8. steinar96

    Active Member

    Apr 18, 2009
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    Leds are pretty standard. It's easy to google up drops for different colors of diodes. Vf means the forward voltage drop which is the voltage measured accross it when it is forward biased (LED is actually a diode). . 1.5Vf means that when conducting 1.5 Volts are dropped accross it.
     
  9. sensor

    Thread Starter New Member

    Sep 8, 2009
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    But it was only 1K resistor.
    1k=1000(ohms) is it correct?
     
  10. steinar96

    Active Member

    Apr 18, 2009
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    sensor. You want 20mA trough your led correct ?

    The led will drop 1.5V. If you have a 5V supply it means you want the resistor to drop 3.5V accross it.

    Calculating (5V-1.5V) / 0.02A we get 175 ohms.

    So the resistor needs to be 175 ohms for this setup to work. With a 1k resistor the diode is deprived of current
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Your diagram showed a 1k5 .... that's 1.5k or 1500
     
  12. sensor

    Thread Starter New Member

    Sep 8, 2009
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    Thank you for your information.
    I did not know that 1k5 means 1.5K(kiloohms).
     
  13. sensor

    Thread Starter New Member

    Sep 8, 2009
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    http://img35.imageshack.us/i/35855855.png/
    You are right.
    But i can't find any resistor less than 1k in proteus/ISIS 7 pro.
    I got 0R1.
    What does it mean by 0R1?

    NB:This time the led is on.
     
  14. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    A resistor od 0R1 is a resistor of 0.1 Ohms.
    Can't you edit the properties of the resistors?
    I do not use the simulator but some have the option of a rightclick on the resistor,
    so a popup box shows up to edit the value.

    Here is a list of standard resistor values:
    http://www.educypedia.be/electronics/resistancevalues.htm

    These values can be muliplied by 0.01, 0.1, 1, 10, 100, 1K, 10K, 100K, 1M.

    More information on component coding can be found in the links of this page:
    http://www.educypedia.be/electronics/datacomponent.htm

    Greetings,
    Bertus
     
  15. sensor

    Thread Starter New Member

    Sep 8, 2009
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    Thank you.
     
  16. Wendy

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    Mar 24, 2008
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