Values for SMD Resistors for LED Lighting

Some Simple Arithmetic

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The first thing to take into account is the voltage dropped across your LED. This depends on its colour - white and blue LEDs drop around 3 volts while green and yellow drop about 2 volts, and red only around 1.6 volts. Hence, when connecting to an HO or N decoder output which will supply nominally 13 volts, we will have to drop the remaining volts across the series resistor. Assuming, for this example, you are using white LEDs, there will be 10 volts to be dropped across the resistor.

Now, most LEDs will be sufficiently bright with 10 mA current flowing through them (1000 mA = 1 Amp). The value of the series resistor is calculated by dividing the voltage across it (10 volts) by the current (10 mA = 0.01 A), which gives a value of 1000 ohm = 1 Kohm. The values can be easily scaled so, if you find you need twice the current to get a sufficiently bright light, then halve the value of the resistor (500 ohm - or use the nearest preferred value of 470 ohm) - or double the resistor value to get half the current (5 mA).

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The power dissipated by the resistor is calculated by dividing the square of the voltage (volts times volts) by the value of the resistor. For our example, with 10 volts across the resistor, the power will be (10 x 10)/1000 = 0.1 watt - so a 1/8 watt resistor will be fine. With a 470 ohm resistor the power will be approximately twice as much, so needing a 1/4 watt resistor.

Hope this helps.

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