quick check

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gizmo13

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Ok.

I have an led project I want to install in a 12v dc operation and I want to see if I calculated the resistance properly.
I have 1 led with 3-3.6 forward voltage and 20-30mA current.
I used ohms law formula and came up with 1.3 ohm resistance.
Is this correct?
 

hi,
No
The resistance you need is 12V -3.6V = 8.4V/0.02 = 420 ohms

OK.?

Or the worst case with a 3V led

12-3= 9/0.02 =450R
 
Last edited:
Ok.
Here's the other one I had for 2 leds in series (same specs):

12-7.2= 4.8/0.02 = 240 ohms??

I see what I did wrong in the other but hopefully I did this one right.
 
I'll check the link but that's what confuses me a bit.
Do you use the minimum or the maximum values.
The range is 3.0v to 3.6 volts.
 
gizmo13 said:
I'll check the link but that's what confuses me a bit.
Do you use the minimum or the maximum values.
The range is 3.0v to 3.6 volts.
Click to expand...

The best way is measure the voltage drop across the LED, use say a 1K resistor from 12V, then measure the voltage across the LED.

If you cannot do that, the 'safe' way is to assume the lowest voltage drop, in your case 3.0V.
In that way the LED current will not exceed its rated value.. OK.?
 
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