LED Resistor Calculator

The following are the simple steps to get the power dissipation of LED's and resistor easily. Follow these guiding principles and obtain the result in a fraction of seconds.

• Get how LED's are connected in a circuit, supply voltage, current passing through LED and voltage drop.
• Check the formulas to compute the power dissipation of single LED, all LEDs.
• Substitute the values in the formula and perform the required operations to get the output.

LED (Light Emitting Diode) is a small electronic device. When current passes through an LED, it emitts light in different colors like red, green or blue. If the high current passes through the doide, then it will damage LED. So, to limit the current flow through a diode, we add a resistor to the circuit.

A circuit can have multiple number of LED's. Those LED's will be connected either in series or in parallel.

LEDs in Series

The formulas to find the resistance and power dissipated for LEDs in series are along the lines:

• Resistance R = (V - n * VΓéÆ) / IΓéÆ
• Power dissipated in a single LED PΓéÆ = VΓéÆ * IΓéÆ
• Power dissipated in all LEDs (total) P = n * VΓéÆ * IΓéÆ
• Power dissipated in the resistor Pr = (IΓéÆ)² * R

LEDs in Parallel

The formulas to get the resistance and power dissipated for LEDs connected in parallel are here.

• Resistance R = (V - VΓéÆ) / (n * IΓéÆ)
• Power dissipated in a single LED PΓéÆ = VΓéÆ * IΓéÆ
• Power dissipated in all LEDs (total) P = n * VΓéÆ * IΓéÆ
• Power dissipated in the resistor Pr = (n * IΓéÆ)² * R

Where,

n is the number of LED's connected

Vₒ is the voltage drop across LED

V is the supply voltage

Iₒ is current across one LED.

Example

Question: If 5 red colour LEDs are connected in series. The current passing through LED is 20 mA and supply voltage is 15 V. Find the resistance, power dissipation of single LED and all LEDs?

Solution:

Given that

LEDs are connected in series.

Number of connected LEDs n = 5

Voltage drop across LED Vo = 2 V

Supply Voltage V = 15 V

Current through LED Io = 20 mA

Resistance R = (V - n * Vₒ) / Iₒ

R = (15 - 5 * 2)/20

= (15 - 10)/20

= 5/20 = 0.25

= 250 Ω

Power dissipated in a single LED Pₒ = Vₒ * Iₒ

= 2 * 20

= 40 = 0.04 W

Power dissipated in all LEDs (total) P = n * Vₒ * Iₒ

P = 5 * 2 * 20

= 200 = 0.2 W

Power dissipated in the resistor Pr = (IΓéÆ)² * R

= 20 * 20 * 250

= 100,000 = 0.1 W

The resistance is 250 Ω, power dissipated in a single LED is 0.04 W, power dissipated in all LEDs is 0.2 W, and power dissipated in the resistor is 0.1 W.

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1. Do I need a resistor for every LED?

An LED is a light-emitting diode that emits light when an electric current passes through it. In an electric circuit, if the voltage drop of the LED is equal to the voltage source, then there is no resistor is needed.

2. How to calculate the resistor need for an LED?

The formula to calculate the resistor is Resistor = (Battery Voltage - LED Voltage)/desired LED current. Use the formula and find the resistor requirement for an LED.

3. Write the difference between parallel and series circuits?

For series circuits, all the LED's are arranged in a line and all LED's are arranged parallel to each other for parallel circuits. If resistors are kept in a series circuit, the voltage across each resistor is different even though the current flow is the same through all of them. When resistors are put in a parallel circuit, the voltage across each resistor is the same.

4. What are the formulas to calculate the power dissipated if LEDs are connected in series?

The formulas to find the power dissipated for LEDs in series are along the lines:

Resistance = (V - n * Vo)/Io

Power dissipated in one LED Po = Vo * Io

Power dissipated in all LEDs P = n * Vo * Io