LED current-limiting resistor.

How this works

R = (Vs − Vf) / If

Power dissipated in R:  P = (Vs − Vf) × If = (Vs − Vf)² / R

The resistor absorbs the voltage difference between supply and LED forward drop, while limiting current. The LED itself isn't ohmic — once it's above its forward voltage, current rises very rapidly with small voltage changes. The resistor's job is to convert this exponential current curve into a controlled, predictable current.

Why you can't drive an LED without a resistor (usually)

If you connect a 2 V LED directly to a 5 V supply, the LED will draw whatever current the supply can deliver — typically destroying the LED in milliseconds. The exception is when supply voltage equals LED forward voltage exactly (e.g., a 2 V LED on a 2 V battery), but this is fragile: battery voltage drifts down as it discharges, and small voltage changes cause large current swings.

Series and parallel LEDs

• Series LEDs share current. 3 red LEDs (Vf = 2.0 V each) in series = 6 V total drop. From a 12 V supply, R = (12 − 6) / 0.020 = 300 Ω. All LEDs see the same 20 mA.
• Parallel LEDs DON'T share current evenly. Manufacturing tolerance means one LED will hog more current than the others, get hotter, drop slightly in forward voltage, and hog even more current — leading to thermal runaway. Always use a separate resistor per LED branch, not one resistor for parallel LEDs.

When to use a constant-current driver instead

For LEDs above ~100 mA (high-brightness lighting, power LEDs, automotive applications), a series resistor wastes too much power as heat. A switching constant-current driver (LM3404, MAX16832, or any LED driver chip) is much more efficient and gives better thermal performance. For room lighting and most signal-indicator LEDs, a resistor is fine.

Common pitfalls

• Vf varies with current and temperature. An LED at 2.0 V forward at 20 mA might be 1.9 V at 5 mA or 2.2 V at 30 mA. The datasheet typically shows Vf at the nominal current.
• Standard resistor values aren't always available. Calculated R of 145 Ω rounds to the nearest E12 (150 Ω) or E24 (147 Ω). Slightly higher R means slightly less current — usually fine.
• Power rating matters. A 1/4 W resistor at 0.3 W will fail. For high-current LEDs, you may need 1/2 W or 1 W resistors.
• Supply voltage isn't always what you think. A "5 V" USB port is actually 4.75-5.25 V. A "12 V" car battery is 12.6 V at rest, 13.8 V running, dropping to 11 V at low charge. Design for worst case.
• Don't trust the LED's "absolute maximum" rating. Run LEDs at 50-70% of their max current for long life. An LED rated for 30 mA max should typically be run at 15-20 mA.

Sources

• Standard resistor values: IEC 60063 (E12, E24 series).
• LED forward voltage data: LED manufacturer datasheets (Cree, Lumileds, Nichia for high-end; generic 5 mm LED datasheets for hobbyist parts).
• Ohm's law: Foundational electronics — any introductory textbook.

Disclaimer. This calculator covers single-LED current limiting. For arrays, lighting, or anything above ~50 mA, use a proper LED driver.

See also