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LEDs are everywhere – in our phones, in our cars, and even in our homes. Whenever an electronic device lights up, there is a good chance that an LED is behind it.
LEDs are like tiny lightbulbs. Low energy consumption, small size, rapid switching and long lifespan makes them ideal for mobile devices and other low-power applications.
LED stands for Light Emitting Diode. They are a special type of diode that convert electrical energy into light. They have very similar electrical characteristics to a normal PN junction diode. That’s why the symbol of LED is similar to the normal PN junction diode except that it contains arrows pointing away from the diode indicating that light is being emitted by the diode.
LEDs are so common, they come in a huge variety of shapes, sizes and colors. The LEDs you are most likely to use are the standard through hole LEDs with two legs. Following figure shows the parts of it.
The construction of an LED is very different from an ordinary diode. The PN junction of an LED is surrounded by a transparent, rigid plastic epoxy resin shell.
The shell is constructed in such a way that photons of light emitted by the junction are focused upward through the domed top of the LED, which itself acts like a lens. This is why the emitted light appears brightest on top of the LED.
Just as in an ordinary diode, the positive side of the LED is called the Anode, while the negative side of the LED is called the Cathode. The cathode is usually indicated by having a shorter lead than the anode. Not only this, the outside of the plastic case typically has a flat spot or notch which can also indicate the cathode side of the LED.
Not all LEDs are hemispherical in shape, some are rectangular while some are cylindrical, but they are mostly constructed in the same way.
Like an ordinary diode, the LED operates only in forward bias condition. When the LED is forward biased, the free electrons cross the PN junction and recombine with holes. Since these electrons fall from a higher to a lower energy level, they radiate energy in the form of photons (light).
In ordinary diodes, this energy is radiated as heat while in an LED, energy is radiated as light. This effect is called Electroluminescence.
Light emitting diodes are available in a wide range of colors with the most common being red, green, yellow, blue, orange, white and infrared (invisible) light.
Unlike ordinary diodes that are made of germanium or silicon, LEDs are made of elements such as gallium, arsenic, and phosphorus. By mixing these elements together in different proportions, a manufacturer can produce LEDs that radiate different colors as shown in the table below.
The actual color of an LED is determined by the wavelength of light emitted, which in turn is determined by the actual semiconductor material used to make the diode.
Therefore the color of the light emitted by an LED is NOT determined by the color of the body of the LED. It just enhances the light output and indicates its color when it is not illuminated.
LED Voltage and CurrentFor most low-power LEDs, the typical voltage drop is from 1.2V to 3.6V for currents between 10mA to 30mA. The exact voltage drop will of course depend upon the semiconductor material used, color, tolerance, along with other factors.
As the LED is basically a diode, its IV characteristics curves can be plotted for each color as shown below.
Unless otherwise specified, you should consider a nominal drop of 2V and forward current 20mA.
LED BrightnessThe brightness of an LED depends directly on how much current it draws. The more current it draws, the brighter the LED will be.
You can control the brightness of an LED by controlling the amount of current through it.
The Current Limiting resistorIf you connect an LED directly to a battery or power supply it will try to dissipate as much power as possible, and, it will destroy itself almost instantly.
Therefore it is important to limit the amount of current flowing through the LED. For this, we use resistors. The resistor limits the flow of electrons in the circuit and prevents the LED from trying to draw too much current.
The current-limiting resistor is placed between LED and voltage source in this way:
In above circuit, the resistor has a node voltage of VS on the left and a node voltage of VF on the right, the voltage across the resistor is the difference between the two voltages.
By applying Ohm’s law, the current-limiting resistor is calculated as:
Basic Example
Consider a red LED with a forward voltage drop of 1.8V is connected to a 5V DC power supply. Calculate the value of the current-limiting resistor required to limit the forward current to approximately 10mA.
Using the formula above, the current-limiting resistor is:
This suggests that we will need a 320Ω resistor to limit the current to 10mA. But 320Ω is not a standard preferred value, so we will need to choose the next highest value, which is 330Ω.
Let’s recalculate the forward current for the 330Ω current-limiting resistor:
We got a new forward current value of 9.6mA which is fine.
Multi-color LEDsMost LEDs produce only one output of colored light. However, multi-color LEDs are now available that can produce a range of different colors from within a single device. These actually have several LEDs fabricated in a single package.
RGB LEDsAt first glance, RGB (Red, Green, Blue) LEDs look just like regular LEDs, however, inside the usual LED package, there are actually three LEDs, one red, one green and yes, one blue. By controlling the intensity of each of the individual LEDs you can mix pretty much any color you want.
The RGB LED has four pins: one for each color, and a common pin. On some, the common pin is the anode, and on others, it’s the cathode.
Bi-Color LEDsUnlike RGB LED, Bi-color LED lacks blue LED inside the LED package. Typically, there are only two LEDs, one red and one green. By controlling the intensity of each of the individual LEDs you can mix only shades of Red and Green only.
The Bi-color LED has three pins: one for each color, and a common pin. Similar to RGB LED, On some, the common pin is the anode, and on others, it’s the cathode.
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