LED Current-Limiting Resistor

LED current-limiting resistor

An LED is a non-ohmic diode. Wire it straight to a supply and it pulls way too much current until it burns out. Putting a current-limiting resistor in series soaks up the extra voltage and pins the current where you want it. The formula is R = (Vsource − VLED) / ILED. The forward voltage V_LED shifts with color: red sits around 2 V, yellow 2.1–2.5 V, green 2.0–3.5 V, blue and white 3.0–3.5 V, UV 3.3–4.2 V. For indicator LEDs the current usually runs 5–20 mA. Check the dissipated power too, P = R · I², and grab a resistor rated above that figure (1/4 W almost always covers it). To work an example: a 5 V supply with a red LED at 2 V / 20 mA gives R = (5 − 2) / 0.02 = 150 Ω, dissipating 60 mW.

Applications

This shows up in Arduino blinky projects, panel indicators, flashlights, signage and holiday lights. Put LEDs in series and they share one current while their forward voltages add up. Branches in parallel are dicey unless each branch has its own resistor, since small manufacturing differences make the current split unevenly. Commercial LED strips ship with resistors built into each cluster, so they need no external one.

FAQ

Can I skip the resistor? Only when a constant-current driver does the job, or when the battery's own internal resistance already holds the current back, like a coin cell driving a small LED. On a regulated supply, leaving out the resistor means a dead LED.

What if my exact value doesn't exist? Round up to the next higher E12/E24 standard value. The LED ends up a touch dimmer, but it won't blow.

Why does V_LED vary by color? It traces back to the semiconductor bandgap (GaAs for red, GaN for blue), which fixes both the photon energy and the forward voltage drop.