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Light emitting diodes (LED's) are electronic components that produce light without heat or high voltages. When using LED's in
a DC circuit there are some basic rules you will need to follow.
LED's must be operated in such a way that the current through the LED is kept within the limitations of the device. The
simplest is to place a current limiting resistor in series with the LED. The resistor value can be calculated using Ohm's law.
Each LED type has specification called "forward voltage drop", they also have a specified current limit. Both these
specifications are required in order to calculate the size of the resistor required to limit the current.
The following is a general guideline for LED's without specifications.
Forward Voltage Drop:
Regular red 2.0v
High brightness red 2.2v
Super bright red 3.0v
Orange or yellow 2.0v
Regular green 2.0v
High Brightness green 2.2v
Super bright green 3.3v
Blue 3.4-4v
White 3.4-4v
Forward Current
20mA is a common standard value for most LED's.
Calculating the value of the current limiting resistor
Using a standard green LED with 2.0 Volt forward voltage drop and a
forward current of 20mA and we want it to run in a
12 Volt DC
circuit. ( 20mA = .02 Amps )
Using Ohm's law:
Subtract the forward voltage drop
from the supply voltage.
12 Volts - 2 Volts = 10 Volts
Then:
10 Volts Divided By .02
Amps = 500 ohms
As resistors come in standard values, choose the next highest value,
this will reduce the current slightly.
To calculate the wattage rating of the required resistor:
W=A x V .02 x 10=.2 watts Therefore use a 1/4 watt resistor.
Most Led's have a long lead and a short lead, the longer lead is the anode ( positive ) and the shorter lead is the
cathode ( negative ).
Connecting
multiple LED's in series
Series LED's are connected cathode to anode to cathode to anode.. The limiting resistor is placed between the voltage source and one of the LED leads. In this type of circuit you need enough voltage to power the number of LED's you are using. To calculate the required resistor: Add the forward voltage drop for each LED together and subtract the total from the supply voltage.
For example if you have 4 LED's each with a 2 Volt voltage drop and a supply voltage of 12 Volts DC:
4 x 2.0V=8.0V 12V-8.0V=4.0V 4.0V will be the voltage at the resistor.
Then:
4.0V divided by .02A=200 Ohms - use the next highest standard resistor value
For the resistor wattage: .02A x 4.0V=.80W ( 1/4 watt resistor ).
With a series LED circuit you can use any combination of LED colors with different voltage drops.
Multiple LED's in Parallel
Series LED's are connected cathode to anode to cathode to anode.. The limiting resistor is placed between the voltage source and one of the LED leads. In this type of circuit you need enough voltage to power the number of LED's you are using. To calculate the required resistor: Add the forward voltage drop for each LED together and subtract the total from the supply voltage.
For example if you have 4 LED's each with a 2 Volt voltage drop and a supply voltage of 12 Volts DC:
4 x 2.0V=8.0V 12V-8.0V=4.0V 4.0V will be the voltage at the resistor.
Then:
4.0V divided by .02A=200 Ohms - use the next highest standard resistor value
For the resistor wattage: .02A x 4.0V=.80W ( 1/4 watt resistor ).
With a series LED circuit you can use any combination of LED colors with different voltage drops.
Multiple LED's in Parallel
LED's in parallel is not a good
idea. The LED's will not have exactly the same voltage drop and because
of these differences they will have different brightness levels and may
be destroyed by the larger current flowing through it.
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