Tools you might need next
Decode resistor color bands to resistance value in ohms or find the correct color bands for a target resistance and tolerance.
Calculate output voltage and resistor values for voltage divider circuits from input voltage, R1, R2, and load considerations.
Calculate RC circuit charge and discharge time constants, voltage curves, and time to reach a target voltage level in seconds.
Resistor drops the difference between supply and LED forward voltage at chosen current.
R = (V_supply − V_f) / IResistor wattage must exceed I²R or V_R × I to avoid overheating.
P = (V_supply − V_f) × IPick the next higher common E12/E24 resistor value so current stays at or below target.
Updated: July 2026
Find 220 Ω or 330 Ω for ~10 to 20 mA from a GPIO pin.
Size resistor for a white 3 V LED at 15 mA on a vehicle circuit.
See why each LED normally needs its own resistor unless using a matched constant-current design.
Run below absolute max for longevity and heat. Indicator duty rarely needs peak current.
Current splits unevenly if forward voltages differ. Use per-LED resistors in parallel strings.
Protect LEDs with the right series resistor. Enter supply voltage, LED forward drop, and target current to get ohms and minimum resistor wattage.