Series resistor calculation
For operation at higher voltages an appropriate series resistor must be used. In order to prevent that the LEDs do not shine or shine exactly once very brightly, one can calculate the series resistor by means of the following small formula and a pocket calculator.
Here are:
U - the DC voltage at which the diode(s) should be operated (in volts)
N - number of diodes to be connected in series
R - the series resistor (in ohms)
P - the power consumed in the series resistor (in watts)
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For light emitting diodes with approx. 3.2 V operating voltage and approx. 20mA current consumption
Calculation of the series resistor: R = (U - (N*3.2)) / 0.02
Calculation of the consumed power: P = (U-(N*3.2))* 0.02
where (N*3.2) must be <= U
Example: U= 12V, number N= 2 (white, blue, pink LEDs with approx. 3.2V voltage, and approx. 20 mA current)
series resistance R = (12V - 6,4 V) / 0.02A = 280 Ohm
Power P = (12V - 6.4V) * 0.02A = 0.112 Watt
So you would use a resistor 280 ohms with at least 0.1 watt. (270 Ohm / 1/8 Watt from the standard series E12, better 330 Ohm)
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For light emitting diodes with approx. 2 V operating voltage and approx. 20mA current consumption
Calculation of the series resistor: R = (U - (N*2.0)) / 0.02
Calculation of the consumed power: P = (U-(N*2.0))* 0.02
where (N*2.0) must be <= U
Example: U= 12V, number N= 4 (red, yellow, orange LEDs with approx. 2V voltage, and approx. 20 mA current)
series resistor R = (12V - 8 V) / 0.02A = 200 Ohm
Power P = (12V - 8V) * 0.02A = 0.08 Watt
So you would use a 200 ohm resistor with at least 0.08 watts. (220 Ohm / 1/8 Watt from the standard series E12).
Tips:
- If the diodes shine too dark or too bright, you can reduce or increase the resistor.
a series resistor of 10 - 20 Ohm should always be used