LED Series Resistor Calculator

Find the right resistor to protect your LED.

Vs R
Required Resistor -- Ω
Nearest Standard (E24) -- Ω
Power Dissipation -- mW

What is an LED Series Resistor?

An LED series resistor is a critical component used in electronics to control current flow through a Light Emitting Diode (LED). Since LEDs are non-ohmic semiconductors, their resistance decreases rapidly as voltage increases.

Without a series resistor, even a small voltage fluctuation can cause the LED to draw excessive current, leading to immediate thermal failure. The resistor "soaks up" the extra voltage, ensuring the LED receives only its rated current (typically 20mA for standard LEDs).

How to Calculate Resistor Value (The Formula)

To find the correct resistor value, we use a specific application of Ohm's Law. The goal is to determine the resistance needed to drop the excess voltage at the desired current.

R = (Vsource - Vforward) / Iforward

Where:

  • Vsource: The power supply voltage (e.g., 5V from USB, 9V battery).
  • Vforward: The voltage drop across the LED (Forward Voltage). This varies by color (Red ≈ 2.0V, Blue ≈ 3.3V).
  • Iforward: The desired current in Amps (20mA = 0.02A).

Example Calculation

Connecting a Red LED (2.0V) to a 9V battery at 20mA:

R = (9V - 2.0V) / 0.02A = 7V / 0.02A = 350Ω

Practical Applications

This calculation is fundamental in countless device designs:

  • Indicator Lights: Power indicators on Wi-Fi routers, chargers, and PC cases all use series resistors.
  • Arduino Projects: When wiring LEDs to GPIO pins, a 220Ω or 330Ω resistor is standard to limit current to safe levels for the microcontroller.
  • Automotive Lighting: 12V car systems use resistor arrays to drive LED dashboards and tail lights.
  • Backlighting: LCD screens and keypads often use strings of LEDs with current-limiting resistors.

FAQ

Why do I need a resistor for an LED?

LEDs do not self-regulate current. Connected directly to a voltage source higher than their forward voltage, they operate effectively as a short circuit, drawing infinite current until they burn out. The resistor acts as a bottleneck to strictly limit this flow.

Can I wire LEDs in parallel?

Wiring LEDs in parallel with a single resistor is not recommended. Due to manufacturing tolerances, one LED usually has a slightly lower forward voltage than the others. It will "hog" the current, overheat, and fail, causing a cascading failure of the remaining LEDs. It is best to give each LED its own series resistor.

What happens if the resistor value is too high?

If the resistance is too high, the current flowing through the LED will be very low. The LED will not be damaged, but it will appear very dim or may not light up at all.