Calculate the current-limiting resistor value and power rating for automotive LED circuits with single or series LEDs.
Input Parameters
Results
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Resistor Value
ohms
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Nearest Standard
ohms
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Resistor Power
W
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Actual Current
mA
Note: Automotive supply varies 11-14.5V. Design for 13.8V nominal. Use constant-current drivers for high-power LEDs. Add reverse polarity protection diode.
Reference
V_resistor = V_supply − (V_forward × N_LEDs)
R = V_resistor / I_LED
P_resistor = V_resistor² / R
Actual I = V_resistor / R_standard
Use next higher E24 standard value