Light Emitting Diodes LED
They are available in all kinds of shapes, sizes, brightness and colours.
Its component outline and symbol are shown opposite.
When forward biased (anode positive with respect to the cathode) LEDs conduct and emits light.
The colour depends on what the LED is made from.
No light emission occurs in reverse bias. If the reverse voltage exceeds approximately 5V then the LED may be damaged.
Different coloured LEDs have different voltages across them, the voltage increasing with the frequency of the light.
E.g. An infra-red LED requires ≈1.5V, a red LED ≈2.0V, a blue LED ≈3.0V
An LED used as an indicator, will usually have a current of 5 - 10mA passing through it.
A resistor is usually connected in series with the LED to limit the current.
A suitable value for the series resistor is calculated by:-
• Subtracting the voltage across the LED from the supply voltage (if in doubt, use 2.5V)
• Then divide by the current (in amps) you want to pass through the LED.
• Select a prefered value resistor larger than what you have calculated.
E.g. A red LED is to operate from a 9V supply with a current passing of less than 10mA (0.01A).
Step 1: 9 - 2 = 7V
Step 2: 7/0.01 = 700Ω
Step 3: The next prefered value above 700Ω is 820Ω, but in practice a 1kΩ resistor will work fine.
