LED Series Resistor Calculator
Enter supply voltage, LED colour, number of LEDs and current to get the resistor, its standard value and power rating.
Step-by-step
- Enter valid values to begin.
Vf values are typical datasheet figures at about 20 mA and vary with colour chemistry, current, temperature and unit-to-unit spread (often ±0.2 V or more); use the datasheet maximum Vf for worst-case current. The resistor is assumed to be a plain linear component fed by a steady DC supply with at least about 1 V of headroom. Resistor ratings use a 2× power headroom as practice, not a standard. For parallel strings give every string its own resistor.
LED Series Resistor: Setting the Current, Not the Voltage
Core Engineering Principles
An LED is a diode, and a diode’s voltage barely moves while its current changes enormously. A red LED sits near 2.0 V at 20 mA, but push the supply up by half a volt and the current can double. So we never connect an LED to a voltage source directly. We put a resistor in series, let it absorb the difference between supply and LED drop, and Ohm’s law sets the current: R = (Vs − N·Vf) / If. The resistor is the only thing in the loop that behaves predictably, which is why we budget for it first.
Pick the resistor from a preferred series, because that’s what is on the shelf. E24 is the usual bench choice and E12 is fine for indicator work. Rounding up costs almost no visible brightness; rounding down raises the current, so check the datasheet maximum first. Watch the headroom too. If the resistor drops only a volt or so, a small change in Vf from temperature or unit-to-unit spread moves the current a lot, and the string will flicker with supply ripple.
Rating ≥ 2 × PR • Efficiency = N × Vf / Vs
NEC & Standard References
IEC 60063 defines the E3 to E192 preferred number series, so E12 and E24 values come from there. IEC 60062 gives the colour-band and letter-code marking you read off the part. IEC 60115-1 is the generic specification for fixed resistors and sets rated dissipation and the derating curve versus ambient temperature. JEDEC publications cover the semiconductor side, but forward voltage and absolute maximum current always come from the manufacturer’s datasheet. The 2× power headroom used here is common practice, not a standard clause.1. Resistor drop = 12 − 2.0 = 10 V.
2. R = 10 / 0.020 = 500 Ω exact. The nearest E24 value is 510 Ω.
3. Actual current = 10 / 510 = 19.61 mA, which is −1.96% from target.
4. PR = 0.01961² × 510 = 196 mW. Doubling gives 392 mW, so use a 1/2 W part.
5. LED power = 2.0 × 19.61 mA = 39.2 mW; supply power = 235.3 mW; efficiency = 16.7%.
6. Three red LEDs in series need only (12 − 6) / 0.020 = 300 Ω and reach 50% efficiency. Maximum on 12 V with 1 V headroom is 5.
- Datasheet Vf is typical, not guaranteed. Design with the maximum Vf to see the worst-case current, and the minimum to see the worst-case dimming.
- Never share one resistor between parallel LEDs. The lowest-Vf device takes the most current, heats up, drops further, and runs away. Give each string its own resistor.
- Watch the hot-plug transient. Automotive supplies surge well above 12 V, and a resistor tuned for 20 mA can push an LED past its limit during load dump.
- Don’t run a resistor at its full rating. A part dissipating its whole rating reaches high surface temperatures that discolour the board; derate by half.