text: # 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?
  
  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 Distinguished Member
  
  Jan 8, 2009
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  5
  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.
  
  Apr 20, 2004
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  8. ### SgtWookie Expert
  
  Jul 17, 2007
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  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
  16
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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
  896
  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.