Calculators
LED Series Resistor Calculator
Find the series resistor and power dissipation for an LED circuit.
Estimate the ideal current-limiting resistor for one LED or an identical series LED string from the supply voltage, forward voltage, and target current.
Ideal series resistance
150 Ω
LED string voltage2 V
Voltage across resistor3 V
Resistor dissipation0.06 W
LED string power0.04 W
R = (Vs − N × Vf) ÷ I, with current converted from mA to A.
Use the LED datasheet forward-voltage range and current rating rather than relying on color alone. Real supply variation, resistor tolerance, LED temperature, and forward-voltage variation change the actual current. High-power LEDs generally need a regulated current driver rather than this simple resistor model.
About This Tool
An LED normally needs a method of limiting current. For simple low-current DC indicator circuits, a resistor in series with the LED can drop the voltage left over after the LED forward-voltage drop and set an approximate operating current. This calculator applies that model to one LED or a string of identical LEDs connected in series. It returns the ideal resistance, voltage across the resistor, and estimated power dissipation so you can understand the electrical tradeoffs before selecting a real component.
How To Use It
- Enter the DC supply voltage.
- Enter the forward voltage of one LED at the intended operating current, preferably from its datasheet.
- Enter the target LED current in milliamps and the number of identical LEDs connected in series.
- Review the ideal resistance and resistor power. Select a practical resistor value and rating with appropriate engineering margin, then verify the resulting circuit against component limits.
Examples
5 V supply and one red LED
For a 5 V supply, a 2.0 V LED, and 20 mA target current, the resistor drops 3 V. R = 3 / 0.02 = 150 Ω and ideal resistor dissipation is 0.06 W.
Three LEDs from 12 V
Three 2.2 V LEDs in series drop about 6.6 V. At 20 mA from 12 V, the resistor drops 5.4 V, giving 270 Ω and about 0.108 W dissipation.
Low-current indicator
A 3.3 V supply, 1.85 V forward drop, and 5 mA target gives an ideal resistance of 290 Ω. A real design should recalculate current for the standard resistor value actually selected.
Useful Notes
LED series resistor formula
For N identical series LEDs, their assumed total forward drop is N × Vf. The resistor sees the remaining voltage: VR = Vs − N × Vf. Applying Ohm's law gives R = VR / I, where I is in amperes.
Resistor power
The resistor converts its voltage drop into heat. Ideal dissipation is P = VR × I, equivalent to I²R. A physical resistor should not be selected solely by matching the calculated dissipation; use suitable margin and consider ambient temperature, enclosure, pulse conditions, and manufacturer derating guidance.
Forward voltage is not fixed
LED forward voltage varies between devices and changes with current and temperature. Color can suggest a broad range but is not a reliable design specification. Use the datasheet values for the actual LED and evaluate the expected minimum and maximum current when tolerances matter.
Why supply voltage must be higher
The simple resistor model needs positive voltage left across the resistor. If the LED string's assumed total forward voltage equals or exceeds the supply, the requested current cannot be established by this calculation.
Series versus parallel LEDs
This calculator covers one series path. LEDs in series share the same current. Parallel LED branches can divide current unevenly because forward voltages differ, so independent current limiting or a suitable driver is generally needed rather than treating parallel LEDs as one ideal series string.
When a resistor is not enough
High-power LEDs, wide supply variation, precision brightness control, and thermally demanding designs often require a regulated constant-current driver. This calculator is intended for the common low-current series-resistor model, not as a replacement for driver design or component datasheet requirements.
FAQ
What forward voltage should I enter?
Use the LED datasheet forward voltage at or near your intended current. If the datasheet provides a range, checking both ends helps show how much actual current could vary.
Can I use the exact calculated resistor value?
The result is an ideal value. Real resistors come in preferred values and have tolerance. Choose an appropriate available value, then recalculate the resulting current and verify it remains within the LED's ratings.
Does each LED in a series string need its own resistor?
A single series string can commonly use one current-limiting resistor because the same current flows through every component in that path. Separate parallel branches are a different case and should not be assumed to share current equally.
Is this suitable for high-power LEDs?
Generally no. High-power LEDs are commonly driven with regulated constant-current electronics and require thermal design. Use the LED and driver manufacturers' specifications.
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