How Multicolor RGB (Red Green Yellow) LEDs Work?

A Light Emitting Diode that emits one color when forward biased and another color when reverse biased is called a multicolor RGB LED.

Related Article: Working of Light Emitting Diode (LED)

One commonly used schematic symbol for these LEDs is shown below.

Multicolor RGB LED Schematic Symbol
Multicolor RGB LED Schematic Symbol

Working of Multicolor RGB LEDs

Multicolor RGB LEDs actually contain two PN junctions that are connected in reverse-parallel i.e. they are in parallel with the anode of one being connected to the cathode of the other.

If a positive potential is applied to the terminal as shown below, the PN junction on the left will light.

Working of RGB LED

Note that the device current passes through the left PN junction.

If the polarity of the voltage source is reversed as shown in the figure below, the PN junction on the right will light.

Note that the direction of device current has reversed and is now passing through the right PN junction.

How Colors are Formed?

Multicolour RGB LEDs are typically red when biased in one direction and green when biased in the other. 
If a multicolour RGB LED is switched fast enough between two polarities, the LED will produce a third color. 
A red/green LED will produce a yellow light when rapidly switched back and forth between biasing polarities.

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