LED Current-Limiting Resistors
Light Emitting Diodes (LEDs) require a controlled current to operate safely. Unlike ordinary lamps, LEDs cannot normally be connected directly to a power supply because even a small increase in voltage can cause a large increase in current, leading to permanent damage. The simplest and most common method of limiting LED current is to connect a resistor in series with the LED.
Why Do LEDs Need a Resistor?
An LED has a relatively fixed forward voltage but very little internal resistance. Without a current-limiting resistor, the LED may draw excessive current from the power supply and fail almost instantly.
The series resistor absorbs the excess voltage, allowing only the desired current to flow through the LED.
Basic Circuit
+V │ [R] │ |>| LED │ GND
The resistor and LED are connected in series, so the same current flows through both components.
Calculating the Resistor Value
Use Ohm's Law to calculate the required resistor value:
R = (VS − VF) ÷ IF
| Symbol | Meaning |
|---|---|
| R | Resistor value (Ω) |
| VS | Supply voltage (V) |
| VF | LED forward voltage (V) |
| IF | Desired LED current (A) |
Typical LED Forward Voltages
| LED Colour | Typical Forward Voltage |
|---|---|
| Infrared | 1.2–1.5 V |
| Red | 1.8–2.2 V |
| Orange | 2.0–2.2 V |
| Yellow | 2.0–2.3 V |
| Green | 2.0–3.3 V |
| Blue | 3.0–3.6 V |
| White | 3.0–3.6 V |
| UV | 3.2–4.0 V |
Worked Example 1
Calculate the resistor for a red LED powered from a 12 V supply.
- Supply voltage = 12 V
- LED forward voltage = 2.0 V
- Desired current = 20 mA
R = (12 − 2) ÷ 0.02 R = 500 Ω
Choose the nearest preferred value: 510 Ω
Worked Example 2
A white LED operates from a 5 V supply.
- Supply voltage = 5 V
- LED forward voltage = 3.2 V
- Current = 15 mA
R = (5 − 3.2) ÷ 0.015 R = 120 Ω
A standard 120 Ω resistor is suitable.
Calculating the Power Rating
The resistor must safely dissipate the generated heat.
P = I² × R
Example:
I = 20 mA R = 510 Ω P = 0.02² × 510 P = 0.204 W
Although a ¼ W resistor can handle this power, using a ½ W resistor provides a better safety margin and allows the resistor to operate at a lower temperature.
Common LED Resistor Values
| Supply Voltage | LED Colour | Typical Resistor |
|---|---|---|
| 3.3 V | Red | 68 Ω |
| 5 V | Red | 220 Ω |
| 5 V | White | 120 Ω |
| 9 V | Red | 390 Ω |
| 12 V | Red | 510 Ω |
| 12 V | White | 470 Ω |
| 24 V | Red | 1.2 kΩ |
Series and Parallel LEDs
When multiple LEDs are connected in series, their forward voltages are added together before calculating the resistor value.
When LEDs are connected in parallel, each LED (or each series string of LEDs) should normally have its own current-limiting resistor to help ensure proper current sharing.
Applications
- Power indicators.
- Status LEDs.
- Arduino projects.
- ESP32 projects.
- Control panels.
- Automotive lighting.
- Decorative lighting.
- Industrial equipment.
- Electronic instruments.
- Educational circuits.
Common Mistakes
| Mistake | Consequence |
|---|---|
| No series resistor. | LED may fail immediately. |
| Wrong supply voltage. | Incorrect LED current. |
| Ignoring resistor wattage. | Overheated resistor. |
| One resistor shared by multiple parallel LEDs. | Uneven current sharing and reduced reliability. |
| Incorrect LED polarity. | LED does not illuminate. |
Key Points
- Most LEDs require a current-limiting resistor.
- Calculate the resistor using Ohm's Law.
- Choose the nearest preferred resistor value.
- Select a resistor with an adequate power rating.
- For parallel LEDs, use a separate resistor for each LED or each series string.