LED Resistor Sizing
Sizes the series resistor that sets the current through an LED. Enter the supply voltage, the LED’s forward voltage and the current you want. The page gives the exact resistance, the nearest standard resistor, the power the resistor burns and a power rating to buy. Hobbyists, panel builders and circuit designers use it for indicator and backlight LEDs.
Equations
R = (Vsupply − Vf)/If P = (Vsupply − Vf)·If
Vsupply is the voltage across the LED and resistor together. Vf is the LED’s forward voltage, the nearly fixed drop across it when it conducts. If is the forward current. The resistor takes the rest of the voltage, so Ohm’s law gives R, and P is the power it turns into heat [1]. The recommended rating is 2P, a common margin that keeps the resistor cool. Buy the next standard wattage at or above it.
The LED type list fills in the typical Vf at 20 mA of common 5 mm indicator LEDs [2]. Red, yellow and older green LEDs drop about 1.8 to 2.2 V; blue, white and true-green InGaN LEDs about 3.3 V. The page also picks the nearest value in the chosen IEC 60063 series [3] and says how far it is from the exact value.
Limits and common mistakes
- Vf varies from part to part and falls as the LED warms. Your LED’s data sheet governs. If the standard value is below the exact one, the current rises a little; choose the next value up when the LED is near its maximum current.
- A “12 V” car system runs at up to about 14.4 V. Size the resistor at the highest supply voltage so the LED is never overdriven.
- When Vsupply is barely above Vf, small changes in either make large changes in current. Leave at least a volt or two across the resistor, or use a constant-current driver.
- LEDs in series add their forward voltages; enter the sum as Vf. Don’t share one resistor between LEDs in parallel.
Worked example
A white indicator LED on a 12 V control panel should run at 20 mA. Which resistor goes in series, and how big must it be?
- Enter Supply voltage (Vsupply) 12 V.
- Choose LED type White (InGaN): 3.3 V typ., 4.0 V max (WP7113QWC/D). LED forward voltage (Vf) fills in as 3.3 V.
- Enter LED forward current (If) 20 mA.
- Choose Also show the nearest standard resistor from (IEC 60063) E24 (±5 %) and press Solve. The Solution shows:
Resistor value (R): 435 Ω
Nearest E24 value (±5 %): 430 Ω (1.1 % below)
Power dissipated: 0.174 W
Recommended resistor rating (2× margin): 0.348 W
Check by hand: 12 − 3.3 = 8.7 V, and 8.7 V / 0.020 A = 435 Ω. With the 430 Ω part the current is 8.7/430 = 20.2 mA, which is fine. The resistor burns 8.7 × 0.020 = 0.174 W, so fit a 1/2 W resistor, not a 1/4 W one.
