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I want to use my motorcycle battery to power a single led that has a maximum voltage of 3.6 volts. what resistance do i need to drop the voltage from 12 volt to 3?

Elijah Rydzewski
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1 Answers1

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Using your given forward voltage of 3.6 volts, I will assume it is the simple LED that uses only 20 mA of current, then:

$$V=IR$$

$$\frac{V}{I}=R$$

So voltage in this equation is voltage of the source minus the voltage lost or used by the LED = 12-3.6=8.4

SO:

$$\frac{12-3.6}{20mA} = 420 Ohms$$

Golaž
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Filek
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  • 420 ohms at 3.5 watts. A standard 1/8 watt resistor will quickly turn to smoke... – MarkU Feb 27 '15 at 07:35
  • @MarkU, how are you getting 3.5W for the resistor? At 20mA, I'm getting 0.02^2*420 = 0.168W. – Dan Laks Feb 27 '15 at 08:05
  • @mark 8.4 V × 0.02A is 0.168 Watts. A 1/4 Watt resistor is needed. How did you get 3.5 Watts? – Passerby Feb 27 '15 at 08:06
  • Whoops, must have gotten something mixed up... after re-checking I agree with 0.168 watt i.e. 1/4 watt resistor rating. Sorry for the false alarm. – MarkU Feb 27 '15 at 09:45