# resistor value calculation | resistor value for led

## Resistor Value Calculation for LED

In this blog I am going teach you calculation for the value of resistor that is used in the protection of led. Currently LED’s ( Light Emitting Diodes ) are used for wide range of purpose mainly as indicators.

Light emitting diodes are very sensitive. They easily get blown off with a slight higher voltage or current, to protect them we should add a resistance in series.

So before I show you the calculations, you must know that every colour of LED has a specific voltage drop across its terminal.

Now lets come to the calculation part

The value of resistance can be easily calculated by Ohm’s law

V ( Voltage ) = I ( Current ) x R ( Resistor )

Here,

Voltage = Source Voltage – Voltage drop by LED

Example

Suppose we need to calculate the value of resistance for green LED

Then,

Source voltage = 9v ( Applied voltage )

Voltage drop by Green LED  = 4 Volt

Current = 25mA = 0.025 Ampere ( This is the current consumed by LED )

R = ?

By Ohm’s law

Resistance = voltage / Current

Resistance = ( source voltage – Voltage drop by LED ) / Current consumed by LED

Resistance = (9 – 4) Volt / 0.025 Ampere

Resistance = 5 V / 0.025 A = 200 Ohm

### Power of resistor

Power of resistor can be easily calculated by

P ( Power ) = V ( Voltage ) x I ( Current )

P = ( Source voltage – Voltage drop by LED ) x I ( Current consumed by LED )

P = 5Volt x 0.025 Ampere

P = 0.125 W ( power of resistor must be greater than this value )

Every value of resistor is not available in market so we go for the nearest value available.

Resistor from market = 200 Ohm, 0.25 Watt

1. How to calculate resistance if two or more LED’s are connected in parallel ?
• When two or more LED’s are connected in parallel. Voltage across parallel combination of LED remains constant and current gets added.
• For example, if there are two led of 4v and 25 mA each. The parallel combination of two led will consume 4v and 50mA.
1. How to calculate resistance if two or more LED’s are connected in series ?
• When two or more LED’s are connected in series. Current across the parallel combination remains the same and voltage gets added.
• For example, if there are three led of 4v and 25mA each. Then the series combination of three led will consume 12v and 25mA.

Author

Akash Sharma