To prevent damage to the LEDs due to the high voltage source, we must put in series a current limiter resistor. Resistance value appropriate to the LED current limiting resistor, can be calculated as follows.

The voltage source provided is VS (Volt). We want the LED current is I (Amperes). Assume that the LED forward voltage drop or VD to be produced is 2 V. Note the picture below

LED current limiting resistor
The voltage drop across the resistor R must be value VS - VD or equal to VS - 2. According to Ohm's Law, the value of this voltage drop should be equal to I x R, thus VS - 2 = I x R. Rearrange the equation to get the value of R will generate

VS - 2 = I x R
R = (VS - 2) / I

example question
An LED will be lit by the source voltage of 9 V and the estimated current draw of 15 mA, what is the value series resistors must be connected to the LED?

R = (VS - 2) / I
R = (9-2) V / 15 mA
R = 7 V / 0.015 A
R = 466.67 Ω
Obtained value of 466.67 Ω resistor or use a value of 470 Ω according to the reference value in the market.

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  1. What is the best resistor to use for the type of project?


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