how to calculate resistance for LED

Discussion in 'General Electronics Chat' started by abhimanyu143, Aug 28, 2014.

  1. abhimanyu143

    Thread Starter Member

    Aug 25, 2014
    211
    1
    I want to do some practical. I have started with LED and resistor
    I want to protect LED from high voltage and current

    I have following things

    bread board
    multimeter
    power supply
    red LED

    I need resistor ?

    I have attached datasheet for LED
    according to datasheet
    forward voltage of LED = 2 v
    forward current of LED = 20 mA
    I know supply voltage = 5.2 v

    formula R=V/I

    resistance= (supply voltage - voltage drop of LED)/ current throw LED

    R=(5.2-2)/ 20 mA
    R=3.2/0.020 A
    R= 160Ω

    can I use 160 Ω resistor?
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    you've got it correct..
    so far
    now try to calculate the required wattage for the resistor.
     
    abhimanyu143 likes this.
  3. abhimanyu143

    Thread Starter Member

    Aug 25, 2014
    211
    1
    formula for power
    P = (I*I)*R
    p=0.0004*160
    P=0.064 w
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,771
    971
    Correct..

    Also its common safety practice to multiply the required wattage by 2 or 3 to ensure that the resistor isn't running too hot.
    If I was to calculate a resistor using the power formula and the result was 1W then I would specify/buy a 2W or even 3W resistor.
     
    MrSmoofy, Metalmann and abhimanyu143 like this.
  5. abhimanyu143

    Thread Starter Member

    Aug 25, 2014
    211
    1
    now next project
    we have
    bread board
    4081B AND gate IC
    LED as indicator
    two switch

    LED is connected to output Of IC
    two switch are connected at Input of Ic

    I have upload datasheet of IC

    If we connect two switch directly to Ic without resistor, I think IC will be damaged
    so I think I need resistor to protect IC

    If we connect LED directly at the output of IC without resistor . I think LED will be damaged

    does we need 3 register to protect IC and LED

    how to calculate resistor value for LED and Ic ?
     
  6. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
    You need two 10K pull-down resistor (from gate input to GND) for the gate input .
    And one resistor for the LED. But you already know the equation for it.

    PS. and don't forget to connect all unused CMOS inputs to ground.
     
  7. abhimanyu143

    Thread Starter Member

    Aug 25, 2014
    211
    1
    how to calculate with formula
    actually I want to learn some real things according to formulas,

    I know here we can apply ohms law but I don't understand how to apply
     
  8. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    Just because a LED has a current rating of 20Ma does not mean you have to run it at that current. Many LED's are clearly visible and 1/4 to 1/2 the maximum current.
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,962
    1,097
Loading...