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How LEDs Work
Why are LEDs important?
• Most lamps emit light by using an electric current to heat a filament wire to a high temperature (white hot)
• This is very inefficient, so to generate the electricity needed a relatively large amount of carbon dioxide is released into the atmosphere from power stations
LEDs (Light Emitting Diodes)
“New Lamps for Old”
• An LED is a small lamp that uses electricity to generate light without getting hot
• LEDs operate with low voltages and currents, although some can have built in electronics to allow them to be used on the mains
• In an LED an electric current can generate light very efficiently, so that much less energy is consumed and less greenhouse gases are released
What is an LED?
Efficient conversion of electricity to light
• LEDs are available in many colours, including white• LEDs can be made that emit invisible Infrared and Ultra
Violet light• LEDs are now used in cars, traffic lights, domestic and
industrial lighting and many other applications• White LEDs are already more efficient than compact
fluorescent lamps• Research indicates that they can still improve by a factor of
5 beyond this
What do LEDs offer us?
Other characteristics
• The intensity of the light is proportional to the current flowing• An LED can continue to emit light for over 20 years with little
change• The light from an LED can follow changes in the current
very rapidly, up to millions of times a second (because they don’t need to heat up and cool down) – this means that their light can carry information
• As their numbers grow, then their price will fall substantially
Anything else?
How do LEDs Work?
Basic principles of electricity and LEDs
Electric currents
• An electric current is due to the flow of billions upon billions upon billions of tiny particles called electrons.
• Electrons flow from a region of low (or negative) voltage to one with a higher (or positive) voltage (this can be created by a battery).
electrons
electrons
Meet an electron.
It is part of a vast family where everyone looks identical.
Each electron is a tiny particle that causes electricity.
Electrons are found in every material.
Electrons
Like most family members electrons need their own space and repel other family members if they get too close.
Conductors and Insulators
• In an electrical conductor electrons are free to move about easily. Metals are electrical conductors.
• In an electrical insulator electrons are not able to move about easily. Materials such as gases, plastics, rubber and glass are common electrical insulators.
Electrical Materials
Electrical Materials
Semiconductors.
• A semiconductor is a funny material where some electrons may have enough energy to travel about freely, but only if they are warm enough, or in a bright light.
• As its name suggests, a semiconductor has properties in between those of an electrical conductor and an electrical insulator.
• When two semiconductors are joined, they can form a step. • For the electrons to flow around the circuit they must gain
energy to flow through the wire to the top of the step. • This energy can come from a battery or power supply
STEP
WIRE
WIRE
STEP
WIRE
WIRE
+
-BATTERY
+
-BATTERY
Electrons and semiconductors
LEDs are made of semiconductors
• An LED can be made from two semiconductors joined in this way and connected to a battery, or power supply.
• Light is emitted as each electron falls down the step
• More falling electrons means a brighter light
+
-
BASIC OPERATING PRINCIPLE OF A RED
LEDRED LIGHT NEEDS
A SMALL STEP
LIGHT
LED
+
-
BASIC OPERATING PRINCIPLE
OF A GREEN LED
GREEN LIGHT NEEDS A MEDIUM
STEP
LIGHT
LED
+
-
BASIC OPERATING PRINCIPLE OF A BLUE
LEDBLUE LIGHT
NEEDS A LARGE STEP
LED
LIGHT
Other types of LED
LEDs can emit many different colours of light
• The height of the step required increases as the colour of the light moves through the spectrum from Red to Blue and Violet
Red<Orange<Yellow<Green< Turquoise< Blue<Violet
LEDs can also emit invisible light
• Infrared light needs a smaller step than Red light• Ultra violet light needs a larger step than Blue light
White LEDs actually emit Blue light but include a material called a PHOSPHOR that converts some of this to Red and Green light
• Red + Green + Blue light is seen as White light
Polarity of Battery or DC Supply
For an LED to emit light • it must be correctly connected to the battery, or power supply• The voltage applied must be more than the minimum for the colour emitted
It will not emit light (and may be damaged) if the voltage is connected the wrong way round, as shown below
The circuit symbol for an LED isThe head of the “arrow” shape must always point to the negative terminal for light to be emittedThe long lead on an LED should be connected to the positive terminal of the supply +
In order to limit the current, a resistor is always used in series with an LED to prevent it overheating (+ -) although this may be built into some lamps
STEP
WIRE
WIRE
BATTERY
NO LIGHT
What are LEDs used for?
• White LEDs are used in torches, as backlights for LCDs , and as room lights, underwater lights and vehicle
headlights• Coloured LEDs are used in indicator
lights, in traffic lights, tail/signal lights in vehicles and decorative lights
• Coloured LEDs are also used in digital scanners and autofocuslights in digital cameras
• They are also used in computer mice and simple displays
• Infrared LEDs are used in remote controls, webcams, sensors & security cameras
• Ultraviolet LEDs are used to reveal secret signatures and security markings on property & banknotes
• Many other things
• An LED converts electrical energy to light energy.
• They can be made in many colours, including white
• They already have many useful applications
• They are much more efficient than filament lamps and “environmentally friendly” compact fluorescent lamps
• They generate much less heat and can last for >20 years
• They generate their light by electrons falling down a step.
• They will become much more affordable as more of them are sold
Summary