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 )
Voltage = Source Voltage – Voltage drop by LED
Suppose we need to calculate the value of resistance for green LED
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
Frequently asked questions
- 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.
- 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.