LED Series Resistor Calculator

LED Series Resistor Calculator | Online Free Tool | Elysia Tools

Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category: Math & Numbers
Input types: number, select
Output type: html
Sample coverage: 4
API ready: Yes

Overview

The LED Series Resistor Calculator helps you quickly determine the correct current-limiting resistor for your LED circuits. By entering your supply voltage, LED forward voltage, forward current, and the number of LEDs in series, the calculator computes the ideal resistance, recommends the nearest standard E12 or E24 resistor value, and calculates the required resistor wattage rating to prevent overheating.

When to use

Designing custom LED indicator circuits on breadboards or printed circuit boards.
Selecting the correct standard resistor value from E12 or E24 series kits for prototyping.
Determining the minimum power rating (wattage) for a resistor to avoid thermal failure.

How it works

1Enter the supply voltage, the forward voltage of your specific LED, and the target forward current.
2Specify the number of LEDs connected in series and choose either the E12 or E24 standard resistor series.
3The calculator computes the ideal resistance using Ohm's law, snaps it to the nearest standard resistor value, and calculates the power dissipation.

Use cases

Calculating the resistor for a single red status LED running on a 5V microcontroller pin.
Configuring a series array of three white LEDs powered by a 12V automotive battery.
Verifying power dissipation to choose between 1/4 watt and 1/2 watt resistors for a custom PCB.

Examples

1. Single Red LED Indicator on 5V Supply

Hobbyist Maker

Background

Building a simple microcontroller project on a breadboard that requires a red LED status indicator powered by a 5V pin.

Problem

Needs to find the correct resistor value to limit the current to 20mA without damaging the LED or the microcontroller.

How to use

Set Supply Voltage to 5V, LED Forward Voltage to 2.1V, Forward Current to 0.02A (20mA), and select the E24 series.

Supply Voltage: 5V, Forward Voltage: 2.1V, Forward Current: 0.02A, Count: 1, Series: E24

Outcome

The calculator outputs an ideal resistance of 145 Ω, recommends a standard 150 Ω resistor, and shows a safe power dissipation of approximately 0.058 W.

2. Three White LEDs in Series on 12V Power

Electronics Technician

Background

Designing a custom 12V lighting strip using three white LEDs connected in series.

Problem

Needs to calculate the current-limiting resistor for the series chain to run safely at 20mA.

How to use

Set Supply Voltage to 12V, LED Forward Voltage to 3.2V, Forward Current to 0.02A, and Number of LEDs to 3.

FAQ

Why does an LED need a series resistor?

LEDs cannot limit their own current. Without a series resistor, they will draw excessive current and burn out.

How do I choose between E12 and E24 resistor series?

Choose E12 for standard 10% tolerance resistors, or E24 for more precise 5% tolerance resistors commonly found in modern kits.

What happens if my supply voltage is lower than the total LED forward voltage?

The LEDs will not turn on because the supply voltage must exceed the combined forward voltage drop of all series LEDs.

How do I determine the wattage rating for my resistor?

Select a resistor with a power rating at least double the calculated power dissipation to ensure safe operation.

Can I use this calculator for LEDs connected in parallel?

No, this calculator is designed specifically for single LEDs or multiple LEDs connected in a series configuration.

Supply Voltage: 12V, Forward Voltage: 3.2V, Forward Current: 0.02A, Count: 3, Series: E24

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Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Overview

When to use

How it works

Use cases

Examples

1. Single Red LED Indicator on 5V Supply

2. Three White LEDs in Series on 12V Power

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