LED Series Resistor Calculator for 5V Supply with Red LED (20mA)
Typical setup using a 5V supply (e.g., USB) with a standard red LED rated at 20mA.
Calculate the required series resistor value for an LED circuit using Ohm’s Law. Enter your Supply Voltage (Vs), LED Forward Voltage (Vf), Desired LED Current (I) to get an instant required resistor. Formula: (vs - vf) / i.
Required Resistor
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How It Works
How It Works
This calculator finds the correct resistor value needed to safely power an LED. LEDs require a specific voltage and current to operate properly. Since most power supplies provide more voltage than the LED needs, a resistor is used to limit the current.
The calculator uses Ohm’s Law to determine how much resistance is required to drop the extra voltage. It subtracts the LED’s forward voltage from the supply voltage, then divides that result by the desired current.
- Step 1: Subtract LED forward voltage (Vf) from supply voltage (Vs)
- Step 2: This gives the voltage that must be dropped by the resistor
- Step 3: Divide that voltage by the desired current (I)
- Formula used: (Vs − Vf) / I
Understanding the Results
The result is the resistor value needed to safely run your LED at the chosen current. This value is given in ohms (Ω). Choosing a resistor close to this value helps prevent the LED from burning out.
If the exact resistor value is not available, select the next higher standard resistor value to ensure safe operation and slightly lower current.
- The result is measured in ohms (Ω)
- Higher resistance means lower current through the LED
- Using a slightly higher resistor increases LED lifespan
- Always double-check your voltage and current inputs
Frequently Asked Questions
What does this LED Series Resistor Calculator do?
This calculator determines the correct resistor value needed to safely power an LED using a given supply voltage. It uses Ohm’s Law to calculate how much resistance is required to limit the current to your desired level. This helps prevent burning out or damaging your LED.
When should I use this calculator?
Use this calculator whenever you are connecting an LED to a power source and need to determine the correct series resistor. It is especially helpful when working with microcontrollers, batteries, or power supplies where voltage and current must be controlled. Proper resistor selection ensures safe and reliable LED operation.
What values do I need to enter?
You need three values: the supply voltage (Vs), the LED’s forward voltage (Vf), and the desired LED current (I) in amps. The forward voltage is typically listed in the LED’s datasheet, and common LED currents are around 0.01 to 0.02 amps (10–20 mA). Make sure all values are entered in volts and amps.
What happens if I don’t use a series resistor?
Without a series resistor, the LED may draw too much current from the power source. This can cause the LED to overheat, fail permanently, or even damage other components in your circuit. A properly calculated resistor limits current to a safe level.
Can I use the exact resistor value calculated?
In practice, you may not find the exact calculated value in standard resistor series (like E12 or E24). Choose the nearest standard resistor value that is equal to or slightly higher than the calculated result. Using a slightly higher resistance will reduce current slightly and increase LED lifespan.
Can this calculator be used for multiple LEDs?
Yes, but you must account for how the LEDs are connected. For LEDs in series, add their forward voltages together and use the total as Vf. For LEDs in parallel, each LED should typically have its own resistor to ensure proper current sharing.
Disclaimer
This calculator provides estimates for informational purposes only. It is not professional advice. Verify results with a qualified professional. Disclaimer.