LED resistor calculation

If the supply voltage is higher than LEDs voltage the resistor have to be included into the circuit in series. The resistor limits the current and produces the required voltage drop. To find the correct value for the resistor the following equation is derived from Ohm's law:

LED with series resistor

R = (U_in - U_f) / I_f
U_r = U_in - U_f
P_r = U_r ⋅ I_f

where:

• R is resistor.
• U_in is supply voltage.
• U_f is LED voltage.
• I_f is LED current.
• U_r is voltage drop on the resistor.
• P_r is power of the resistor that is transformed to the heat.

The current limiting resistor of a LED has to be not smaller than R and at least with the power of P_r.