Light Emitting Diodes

Transcription

1 Light Emitting Diodes Topics covered in this presentation: LED operation LED Characteristics Display devices Protection and limiting 1 of 9

2 Light Emitting Diode - LED A special type of diode is the Light Emitting Diode (LED). This diode will emit light when it is forward biased. When reverse biased no current will flow, therefore no light will be emitted. The symbol for an LED is a diode symbol with arrows coming off it to indicate that it emits light. 2 of 9

3 Light Emitting Diode - LED When electrons jump across the junction between the n-type and p-type material to combine with a hole, their energy level changes. In a silicon diode, this energy is converted to heat, but some materials, such as Gallium Arsenide (GaAs), will convert this energy to heat and also to light. Depending on the material used, LEDs that emit red, green, yellow and blue light can be manufactured. 3 of 9

4 Light Emitting Diode Characteristics The I-V characteristic for an LED is similar to a standard diode. It can be seen that LEDs require a higher switch-on voltage than ordinary silicon diodes. From the I-V characteristic curve shown, it can be seen that red LEDs are the most efficient, requiring the lowest switch-on voltage. Typically the forward bias voltage required to switch on an LED is about 2V. This compares with about 0.6V for an ordinary silicon diode. 4 of 9

5 Light Emitting Diode LED The semiconductor material for an LED is encapsulated in a plastic case which also helps to colour the emitted light. Like a diode an LED has two connections - anode and cathode. The cathode is identified by a flat edge in the base of the LED and the connection wire for the anode is generally the longer wire, unless someone has cut it! 5 of 9

6 LED Displays Individual LEDs come in various shapes, sizes and brightnesses. LEDs are also available that can show two, or three colours depending upon how current passes through the LED. LEDs can also be grouped together in larger displays. A common display device is the 7-segment display. This contains individual LEDs for each segment that can be switched on and off to form the numbers zero to nine. A bar-graph display device has many LED bars. As the applied voltage increases, more bars illuminate. 6 of 9

7 LED Datasheet Some of the criteria, found on the product data sheet, to consider when selecting an LED. Typical forward voltage drop (V F typ) Maximum forward voltage (V F max) Maximum forward current (I F max) Maximum reverse voltage (V R max) Light intensity Light wavelength Viewing Angle 7 of 9

8 Current Limiting Resistor If an LED was connected directly across a power supply, there will be nothing to limit the current, which could cause the LED to overheat and be destroyed. A resistor is therefore placed in series with the LED to limit the current in the LED. The maximum forward voltage and current for an LED will be given on the product datasheet. The value of the current limiting resistor required is calculated using the formula: R V I F Where: V is the voltage drop across the resistor I F is the maximum forward current for the LED 8 of 9

9 Current Limiting Resistor Example Calculate the series resistor for an LED when: Supply voltage (V S ) = 5V Typ. forward voltage for LED (V F ) = 2V Max. forward current for LED (I F ) = 10mA. R V S I F V F 5V 2V 10mA 300 From the E24 resistor range, a 330 resistor would keep the current well within the maximum specified for the LED. Current through LED Current through Resistor Current through Resistor Voltage across Resistor Resistance of Resistor mA 9 of 9