LED Resistor

R = (Vs − Vled) / Iled

Quick answer

This calculator solves R = (Vsource - VLED) / ILED for Resistance. With Source Voltage 120 V, LED Forward Voltage 56.4 V, LED Current 0.2 A, the result is 0.318 kΩ. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Resistance (Ω)
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Formula

R = (Vsource - VLED) / ILED

Description

LEDs are current-driven devices that require a series resistor to limit the current flowing through them. Without a resistor, the LED draws excessive current and burns out almost instantly because its forward voltage is nearly constant regardless of current. The resistor drops the difference between the supply voltage and the LED's forward voltage, converting excess energy to heat. Different LED colors have different forward voltages due to their semiconductor bandgap: red LEDs are typically 1.8-2.0V, green 2.0-2.2V, blue and white 3.0-3.4V.

Variables

  • Vsource — Supply voltage (V)
  • VLED — LED forward voltage drop (V)
  • ILED — Desired LED forward current (A)
  • R — Required series resistance (Ω)

Practical Notes

Standard indicator LEDs typically run at 20mA (0.02A) for full brightness, but 5-10mA is often sufficient and extends lifespan. High-power LEDs may need 350mA or more and require proper thermal management. Always round up to the next standard resistor value to stay within safe current limits.

Symbols and units

Symbol Meaning Default unit Accepted units
R Resistance Ω mΩ, Ω, kΩ, MΩ
Vs Source Voltage V pV, nV, µV, mV, V, kV
Vled LED Forward Voltage V pV, nV, µV, mV, V, kV
Iled LED Current mA pA, nA, µA, mA, A, kA

Step by step

  1. 1. What you know: Source Voltage 120 V, LED Forward Voltage 56.4 V, LED Current 0.2 A
  2. 2. The relation: R = (Vsource - VLED) / ILED
  3. 3. Substitute: Vs = 120 V, Vled = 56.4 V, Iled = 0.2 A
  4. 4. Result: Resistance = 0.318 kΩ

Related quantities from the same inputs

  • P = V × I = 24 W

Nearest standard component values

Series Nearest value Error
E12 0.33 kΩ 3.8%
E24 0.33 kΩ 3.8%

Worked examples

Minimum range

Given: Source Voltage 3.3 V, LED Forward Voltage 1.55 V, LED Current 1 mA

Result: Resistance = 1.75 kΩ

Low range

Given: Source Voltage 33 V, LED Forward Voltage 15.5 V, LED Current 10 mA

Result: Resistance = 1.75 kΩ

Typical

Given: Source Voltage 120 V, LED Forward Voltage 56.4 V, LED Current 0.2 A

Result: Resistance = 0.318 kΩ

High range

Given: Source Voltage 2.3 kV, LED Forward Voltage 1.08 kV, LED Current 5 A

Result: Resistance = 0.244 kΩ

Maximum range

Given: Source Voltage 23 kV, LED Forward Voltage 10.8 kV, LED Current 50 A

Result: Resistance = 0.244 kΩ

Solving for each variable

Solve for Resistance

Source Voltage 3.3 V, LED Forward Voltage 1.55 V, LED Current 1 mA → 1.75 kΩ

Solve for Source Voltage

Resistance 1 Ω, LED Forward Voltage 0.726 V, LED Current 100 µA → 0.7261 V

Solve for LED Forward Voltage

Resistance 1 Ω, Source Voltage 0.33 V, LED Current 100 µA → 0.3299 V

Solve for LED Current

Resistance 1 Ω, Source Voltage 0.33 V, LED Forward Voltage 0.726 V → -0.396 A

How Resistance changes with Source Voltage

Source Voltage (input) Resistance (result)
60 V 18 Ω
90 V 0.168 kΩ
120 V 0.318 kΩ
0.18 kV 0.618 kΩ
0.24 kV 0.918 kΩ
0.48 kV 2.118 kΩ
0.84 kV 3.918 kΩ
1.2 kV 5.718 kΩ

The same result in other units

Unit Value
mΩ 318000
Ω 318
kΩ 0.318
MΩ 0.000318

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Frequently asked questions

How do you calculate LED Resistor?

Use R = (Vsource - VLED) / ILED. With Source Voltage 120 V, LED Forward Voltage 56.4 V, LED Current 0.2 A, Resistance works out to 0.318 kΩ.

What is the formula for LED Resistor?

The formula is R = (Vsource - VLED) / ILED. LEDs are current-driven devices that require a series resistor to limit the current flowing through them.

How do you find Source Voltage when you know the other values?

Rearrange for Source Voltage: with Resistance 1 Ω, LED Forward Voltage 0.726 V, LED Current 100 µA, Source Voltage is 0.7261 V.

How do you find LED Forward Voltage when you know the other values?

Rearrange for LED Forward Voltage: with Resistance 1 Ω, Source Voltage 0.33 V, LED Current 100 µA, LED Forward Voltage is 0.3299 V.

What units does LED Resistor use?

Resistance is shown in mΩ, Ω, kΩ, MΩ. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Resistors formulas →
Series Resistance Parallel Resistance LED Resistor Resistor Power Derating Resistive Attenuator Resistor Tolerance Range Resistor Thermal Noise (Johnson–Nyquist) Potentiometer Output

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