Calculate LED Series Resistor
Online calculator and formulas for calculating the resistor for an LED
LED Series Resistor Calculator
LED Parameters
LEDs require a series resistor for current limiting. Enter supply voltage, LED voltage and desired current.
LED Circuit
Circuit diagram: LED with series resistor
Basic Formulas
Series resistor:
\[R_s = \frac{V_S - V_D}{I_D}\]
Power dissipation:
\[P_s = (V_S - V_D) \times I_D\]
Symbol Legend
VS: Supply voltage (input voltage)
VD: LED forward voltage
ID: LED current (forward current)
Rs: Series resistor
Ps: Power dissipation in series resistor
Practical Calculation Examples
Example 1: Red LED at 5V
Given: VS = 5V, VD = 2.0V, ID = 20mA
Step-by-Step Calculation
1. Voltage drop across resistor:
\[U_R = V_S - V_D = 5V - 2.0V = 3.0V\]
2. Calculate series resistor:
\[R_s = \frac{U_R}{I_D} = \frac{3.0V}{20mA} = \frac{3.0V}{0.02A} = 150Ω\]
3. Power dissipation:
\[P_s = U_R \times I_D = 3.0V \times 20mA = 60mW\]
Result: Rs = 150Ω, Ps = 60mW
Practice: Next E12 value: 150Ω, resistor ≥ 1/8W
Practice: Next E12 value: 150Ω, resistor ≥ 1/8W
Example 2: Blue LED at 12V
Given: VS = 12V, VD = 3.2V, ID = 20mA
Step-by-Step Calculation
1. Voltage drop across resistor:
\[U_R = V_S - V_D = 12V - 3.2V = 8.8V\]
2. Calculate series resistor:
\[R_s = \frac{U_R}{I_D} = \frac{8.8V}{20mA} = \frac{8.8V}{0.02A} = 440Ω\]
3. Power dissipation:
\[P_s = U_R \times I_D = 8.8V \times 20mA = 176mW\]
Result: Rs = 440Ω, Ps = 176mW
Practice: Next E12 value: 470Ω, resistor ≥ 1/4W
Practice: Next E12 value: 470Ω, resistor ≥ 1/4W
Example 3: White LED at 24V (Industrial Application)
Given: VS = 24V, VD = 3.4V, ID = 30mA
Detailed Analysis
1. Voltage drop across resistor:
\[U_R = V_S - V_D = 24V - 3.4V = 20.6V\]
2. Calculate series resistor:
\[R_s = \frac{U_R}{I_D} = \frac{20.6V}{30mA} = \frac{20.6V}{0.03A} = 687Ω\]
3. Power dissipation:
\[P_s = U_R \times I_D = 20.6V \times 30mA = 618mW\]
4. Circuit efficiency:
\[η = \frac{P_{LED}}{P_{total}} = \frac{V_D \times I_D}{V_S \times I_D} = \frac{3.4V}{24V} = 14.2\%\]
5. E12 series resistor:
Next value: 680Ω
Actual current: \[I = \frac{20.6V}{680Ω} = 30.3mA\]
Actual current: \[I = \frac{20.6V}{680Ω} = 30.3mA\]
Important Note: At high voltages, efficiency is very low (14.2%).
For efficiency, a constant current source or switching regulator should be used.
Efficiency Comparison
5V → 2V LED: η = 40% (good)
12V → 3.2V LED: η = 27% (acceptable)
24V → 3.4V LED: η = 14% (poor)
The higher the supply voltage, the worse the efficiency
Resistor Selection (E12 Series)
Calculated: Calculate exact value
E12 series: Choose next higher value
Power: At least 2× calculated power
Tolerance: 5% or better for precise currents
LED Theory and Applications
Operating Principle
LEDs (Light Emitting Diodes) are semiconductor diodes that emit light when current flows through them. Unlike incandescent bulbs, LEDs have a non-linear current-voltage characteristic and require current limiting for safe operation.
Why a Series Resistor?
- Current limiting: LEDs have a very steep current increase with small voltage increases
- Overcurrent protection: Without limiting, the LED would be destroyed by excessive current
- Voltage adaptation: Supply voltage is usually higher than LED forward voltage
- Temperature compensation: The resistor stabilizes current with temperature variations
LED Characteristics by Color
| Color | Forward Voltage | Typical Current | Wavelength | Material |
|---|---|---|---|---|
| Red | 1.8-2.2V | 10-20mA | 620-750nm | AlGaAs, GaAsP |
| Orange | 2.0-2.2V | 10-20mA | 590-620nm | GaAsP, AlGaInP |
| Yellow | 2.0-2.2V | 10-20mA | 570-590nm | GaAsP, AlGaInP |
| Green | 2.0-2.4V | 10-20mA | 520-570nm | GaP, AlGaInP, InGaN |
| Blue | 3.0-3.4V | 10-20mA | 450-520nm | InGaN, SiC |
| White | 3.0-3.6V | 15-25mA | 400-700nm | InGaN + Phosphor |
| Infrared | 1.2-1.4V | 10-50mA | 750-1000nm | GaAs, AlGaAs |
Important Notes
- Never operate LEDs without current limiting
- Observe polarity (anode = longer leg)
- Observe maximum current per datasheet
- Consider heat dissipation at high currents
- Observe ESD protection during handling
Practical Tips
- Use E12 series resistors
- Choose resistor ≥ 2× calculated power
- For series connection: use LEDs of same type
- At high voltages: consider constant current source
- For dimming: PWM instead of analog control
Advanced Circuits
- Constant current source: Better efficiency
- Switching regulator: High efficiency
- PWM dimmer: Brightness control
- Series connection: Multiple LEDs
- Matrix control: Many LEDs
Operating Multiple LEDs
Series Connection (recommended)
Formula: \[R_s = \frac{V_S - n \times V_D}{I_D}\]
Advantages: Same current through all LEDs
Disadvantages: High voltage required
Application: LED strips, lighting
Parallel Connection (problematic)
Formula: \[R_s = \frac{V_S - V_D}{n \times I_D}\]
Advantages: Low voltage sufficient
Disadvantages: Unequal current distribution
Solution: Give each LED its own resistor
Typical Applications
Lighting
Lamps, strips, spots
Signals
Status, warnings, traffic lights
Displays
7-segment, matrix, backlight
Optics
Photography, sensors, IR
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