Determine resistance value

Discussion in 'The Projects Forum' started by FND, Sep 7, 2009.

  1. FND

    Thread Starter Member

    Apr 2, 2009
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    Hi All,

    How do I determine the value of R to be used when I'm using a 9v battery to power an LED with voltage rating of 3v?

    Help please.
     
  2. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    Go to aac books,check ohms law.
     
  3. FND

    Thread Starter Member

    Apr 2, 2009
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    Sorry, this thing some what makes me really confused.

    I don't know the current, so how would i know the resistance? The only info I have is the LED's Voltage rating is 3V.

    How? I know it's the most basic thing and it's a silly question. But please help understand.

    Thank you.
     
  4. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    Where there a current meter,there a way.
     
  5. FND

    Thread Starter Member

    Apr 2, 2009
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    ok. Thank you. silly me
     
  6. jj_alukkas

    Well-Known Member

    Jan 8, 2009
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    Resistance value for current limiting for an LED = (Vcc-Vf)/I

    Vcc = supply voltage = 9v
    Vf = LED forward voltage (anywhere between 1.8 -4.3V for different types)
    I = Led current (Normally 15 or 20mA) , use as 0.02 in formula.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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  8. SgtWookie

    Expert

    Jul 17, 2007
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    Bad advice, loosewire! :(

    I hope our OP did not follow it, as they probably fried their LED or meter by now.

    jj_alukkas gave the proper reply. I usually spell it out a little bit more.

    The basic formula is:
    Rlimit >= (Vsupply - Vf_LED) / DesiredCurrent
    Vsupply needs to be higher than Vf_LED, of course.

    In your case, assuming your LED's Vf was measured at 20mA current:
    Rlimit >= (9v - 3v) / 20mA
    Rlimit >= 6v/0.02A
    Rlimit >= 300 Ohms
    300 Ohms is a standard value of resistance. You can use a higher value of resistance, but you should not go lower, or you risk burning out your LED.

    There is a table of standard resistance values on this site:
    http://www.logwell.com/tech/components/resistor_values.html
     
  9. loosewire

    AAC Fanatic!

    Apr 25, 2008
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    Thanks,Sgt. Will be more careful.
     
  10. FND

    Thread Starter Member

    Apr 2, 2009
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    Thank again to all of you for helping out.

    Appreciate it very much, really helped!!
     
  11. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    You cannot buy a 3.0V LED. They have a range of voltage like 2.5V to 3.5V.
    If you do not measure its actual voltage then your circuit must limit the current enough that a low voltage LED does not burn out and must allow enough current for a high voltage LED to be bright enough.
     
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