- LEDs have about 1.6 to 3.5 Volts across them when lit (depending on the colour)
- A typical absolute maximum forward current is 20 mA. They are more often run at 10mA to give a wide safety margin. New ultra bright LEDs need a lot more current.
- LEDs are much more efficient and reliable than light bulbs.
- LEDs are replacing bulbs in many applications like traffic lights and car brake lights.
- Bicycles use LED lighting for longer battery life.
- Efficient LED torches are for sale too.
- Infrared LED light pulses are used to carry data from remote controls to the TV, DVD player or other device being controlled.
- LEDs are used in fibre optic links.
- LEDs are even being used for domestic lighting because of their high efficiency and low heat generation.
- LEDs are replacing cold-cathode lights for computer and TV flat screen back-lighting
Resistance Calculation
The circuit above has a 5 Volt supply.There are 2 Volts across the LED. The remaining 3 Volts are across the resistor. Using Ohm's law ...
R = V / I
R = 3 / 10mA
R = 3 / 0.01
R = 300R
Allowing a small safety margin, 330R was chosen allowing just under 10mA to flow.
Resistor Power Dissipation Calculation
Power = V I
Power = 3 x 10mA
Power = 3 x 0.01
Power = 0.03 Watts (30mW)
This is a very low power so any small resistor would be suitable.
The graph below shows the approximate characteristics of an LED. The reverse leakage current has been exagerated to make it show on the graph. The reverse breakdown voltage is quite low (5 volts approx).
Infra Red Light Emitting Diode
This uses Gallium arsenide instead of Silicon as the semiconductor material.
LED Power Indicator
Circuit and Layout for an LED Power Indicator ...
Colour |
Approximate Voltage Drop |
Photon Energy |
Photon Wavelength |
Infrared |
1.6 V |
Lowest |
Longest |
Red |
1.8 V to 2.1 V |
Higher |
Shorter |
Orange |
2.2 V |
|
|
Yellow |
2.4 V |
|
|
Green |
2.6 V |
|
|
Blue |
3.0 V to 3.5 V |
|
|
White |
3.0 V to 3.5 V |
Lower |
Longer |
Ultraviolet |
3.5 V |
Highest |
Shortest |
LEDs used as a bar display
