LED Series Resistor Calculator
Series resistor (Ω)
150.0
How it works
Connecting an LED directly to a power supply without a current-limiting resistor will destroy the LED within seconds — LEDs are current-controlled devices without intrinsic resistance limiting. The LED Series Resistor Calculator computes the correct resistor value to safely drive any LED at its rated current from any supply voltage.
**The formula** R = (Vsupply − Vforward) / Iforward
Where: Vsupply = power supply voltage (V); Vforward = LED forward voltage (typical values below); Iforward = LED forward current in amperes (convert mA to A: 20mA = 0.02A).
**Typical LED forward voltages** Red LED: 1.8–2.2V. Orange/Yellow: 2.0–2.4V. Green (standard): 2.0–2.4V. Green (high-brightness): 3.0–3.4V. Blue/White/UV: 3.0–3.6V. Infrared (850nm): 1.2–1.7V.
**Example calculation** Red LED (Vf = 2.0V) from 5V supply at 20mA: R = (5 − 2) / 0.02 = 150 Ω. Standard 150Ω resistor provides exactly 20mA. Round up to 180Ω for a conservative 16.7mA — slightly dimmer but longer LED lifespan.
**Resistor power rating** Also check that the resistor power dissipation is within its rating. P = I² × R = (0.02)² × 150 = 0.06 W. A standard 1/4 W (250mW) resistor handles this easily. For high-current applications (100mA+ LEDs), use at least a 0.5W rated resistor.
**Multiple LEDs in series** For n LEDs in series: R = (Vsupply − n × Vforward) / Iforward. Ensure Vsupply > n × Vforward + 1V headroom. Maximum series LEDs from 5V supply with blue LEDs (Vf=3.3V): (5 − 3.3) / 0.02 = 85Ω; only one LED fits with a small margin.
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Frequently Asked Questions
- Without a current-limiting resistor, the LED will draw current limited only by its extremely low internal resistance (typically 5–20Ω at rated forward voltage). From a 5V supply with a 2V red LED: I = (5−2)/10Ω ≈ 300mA — approximately 15× the rated 20mA. At this current the LED dissipates 300mA × 3V ≈ 900mW of heat in a device rated for 60mW — it burns out within seconds. Even from a 3.3V supply with a 3.3V blue LED (Vf ≈ supply), small temperature variations make current unpredictable. Always use a resistor or constant-current LED driver.
- Technically yes, but not recommended. LEDs have slightly different forward voltages (manufacturing variation of ±0.1–0.2V). The LED with the lowest Vf draws more current and runs hotter, which further reduces its Vf (creating a thermal runaway tendency). The safer approach: individual resistors for each parallel LED. This adds minimal cost (a 150Ω resistor is pennies) but ensures each LED runs at the intended current regardless of manufacturing variation. For series LED strings, one resistor per string is fine since all LEDs in series carry the same current.
- Resistor power dissipation: P = I² × R = (V_resistor / R) × V_resistor = V_resistor² / R = I × V_resistor. For a 150Ω resistor at 20mA: P = (0.02)² × 150 = 0.06 W. A standard 1/4W (0.25W) resistor handles this with 4× headroom (running a resistor at ≤50% of rated power is good practice for reliability and temperature). For high-current LEDs (100mA+): 100mA through 33Ω = (0.1)² × 33 = 0.33W → use at minimum a 1/2W resistor, preferably 1W for thermal headroom.
- A constant-current driver maintains a fixed output current regardless of supply voltage variation or LED forward voltage variation. For simple indicator LEDs (3mm, 5mm standard LEDs): a resistor is always sufficient. Use a constant-current driver when: (1) Driving high-power LEDs (1W, 3W, 5W+) where resistor power loss is significant and efficiency matters. (2) Long LED strips where supply voltage drops over length, causing dimmer LEDs at the far end. (3) Applications requiring precise colour consistency (colour-matching, photography, medical) — constant current ensures consistent junction temperature and colour point. (4) Battery-powered devices where supply voltage varies as the battery discharges.