# 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

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?

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2. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,063
1,099
you've got it correct..
so far
now try to calculate the required wattage for the resistor.

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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
5,063
1,099
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.

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5. ### abhimanyu143 Thread Starter Member

Aug 25, 2014
211
1
now next project
we have
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 ?

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6. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,166
1,188
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 Distinguished Member

Nov 4, 2008
504
90
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.

Feb 17, 2009
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