LED Series Resistor Calculator
Calculate the series resistor value needed to limit current through an LED safely. Supports single LEDs and multiple LEDs in series.
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LED Resistor Calculation Guide
Why LEDs Need Resistors?
LEDs are current-driven devices. Unlike incandescent bulbs which self-regulate to some extent, an LED's resistance drops with rising temperature — without current limiting, current rises until the LED burns out. The series resistor solves this. The formula: R = (V_supply - V_LED) / I. Where: V_supply = your power source voltage. V_LED = LED's forward voltage drop (constant for a given LED). I = desired current. Example: 5V supply, red LED (V_f 2.0V), 20mA target. R = (5 - 2) / 0.020 = 150Ω. The resistor limits current to a safe level, since an LED's resistance alone would otherwise allow current to rise uncontrollably and destroy the LED almost instantly.
What's the key thing to understand about Standard LED Forward Voltages?
Forward voltage depends on LED chemistry: red (AlGaAs): 1.8-2.2V typically. Orange/yellow: 2.0-2.2V. Green (traditional InGaN): 2.1-2.4V. True green (different chemistry): 3.2-3.4V. Blue (InGaN): 3.0-3.4V. White (blue LED + phosphor): 3.0-3.4V. UV: 3.2-4.0V. Infrared: 1.1-1.5V. Variations within colour: brighter LEDs often higher Vf. Check datasheet for actual values. As current increases, Vf rises slightly (logarithmically). For 10-30mA operating range, treat as constant. Higher currents, above about 100mA, cause a more noticeable rise in forward voltage and generate significantly more heat, which is why high-power LEDs need proper heat-sinking.
What should I know about Multiple LEDs Configurations?
Series (same current through all): combined Vf = sum of individual Vfs. R = (V_supply - sum_Vf) / I. Example: 3 red LEDs in series on 12V at 20mA. Sum Vf = 6V. R = (12-6)/0.020 = 300Ω. Each LED has identical current. Good: efficient (one resistor, one current). Limitation: V_supply must exceed sum_Vf + headroom (need 2-3V minimum drop across resistor for stable current). Parallel (same voltage across each, currents add): NEVER directly parallel LEDs of same type. Minor forward-voltage variations between individual LEDs cause one to draw more current than the others when wired in parallel, potentially overloading and destroying it while the rest stay dim.
What should I know about Power Dissipation and Standard Values?
Resistor dissipates power: P = I² × R or P = V_dropped × I. Same example: 150Ω, 20mA. P = 0.020² × 150 = 0.060W = 60mW. Standard 1/4W (250mW) resistor: plenty of headroom. Use 1/8W only if known low power; 1/2W or 1W for higher currents (100mA+) or larger voltage drops. Standard E12 resistor values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. Multiply by 10, 100, 1000 for 100Ω, 1kΩ, 10kΩ ranges. Choosing between calculated value and standard: round UP to the nearest standard value — this gives a slightly lower current, which is safer for the LED and only slightly dims it, rather than rounding down and risking overcurrent.