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# Light Emitting Diodes

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead.

• LEDs emit light, only when they are forward biased.
• LEDs are easily destroyed if too much current flows (20mA is a typical maximum).
• LEDs are easily destroyed if the reverse bias voltage ever becomes too big. (As little as 5 Volts might cause damage.)
• LEDs should be protected with a polarity protection diode if reverse bias voltages are likely.
• Typical LEDs have about 2 Volts across them when lit but this varies from 1.6 to 3.5 Volts (depending on the colour and type of LED).
a

## LED Circuit

In the circuit below, the resistor limits the current through the LED to a safe level. A typical LED current is 10mA.

b

## LED Properties

• A typical absolute maximum forward current is 20 mA. They are more often run at 10mA to give a wide safety margin. Some 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.
• Contraflow road works lights now use LEDs. These can be battery powered. The older bulb lights needed a small diesel generator.
c

## Resistance Calculation

This circuit 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 = ( 5 - 2 ) / 10mA
• R = 3 / 0.01
• R = 300R

Allowing a small safety margin, the E24 330R was chosen allowing just under 10mA to flow.

d

## Resistor Power Dissipation Calculation

This circuit has a 5 Volt supply. There are 2 Volts across the LED. The remaining 3 Volts are across the resistor.

• Power = V I

• Power = ( 5 - 2 ) 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.
e

## LED Power Dissipation Calculation

This circuit has a 5 Volt supply. There are 2 Volts across the LED.

• Power = V I

• Power = 2 x 10mA

• Power = 2 x 0.01

• Power = 0.02 Watts (20mW)
f

## Whole Circuit Power Dissipation Calculation

This circuit has a 5 Volt supply.

• Power = V I

• Power = 5 x 10mA

• Power = 5 x 0.01

• Power = 0.05 Watts (50mW)
g

## LED Characteristic Curve

The graph below shows the approximate characteristics of an LED.
The reverse leakage current has been exaggerated to make it show on the graph.
The reverse breakdown voltage is quite low (5 volts approx).
It's easy to destroy LEDs. Too much forward current or too much reverse voltage will do the job.

h

## Infra Red LED

This uses Gallium arsenide instead of Silicon as the semiconductor material.

i

## LED Power Indicator - Practical Task

Circuit diagram and Layout for an LED Power Indicator ...

For the Falstad Circuit Simulation, CTRL+Click LED Circuit
In options, check European Resistors and uncheck Conventional Current.

Alternatively view LED.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

Measure ...

1. The voltage across the LED
2. The voltage across the resistor
3. The voltage across the polarity protection diode
4. The power supply voltage
5. The current flowing through the circuit

Measuring Current

j

## LED Colours and Voltages

Infra-Red photons have a low energy. Ultra-Violet photons have a much higher energy. The photon energy is proportional to the LED voltage.

 Colour Approximate Voltage Drop Photon Energy Photon Wavelength Infrared 1.6 V Lowest Longest Red 1.8 V to 2.1 V 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 Ultraviolet 3.5 V Highest Shortest
k

## LEDs in Parallel - Bad Idea

For the Falstad Circuit Simulation, CTRL+Click LEDs in Parallel - Bad Idea
In options, check European Resistors and uncheck Conventional Current.
Turn on between one and five LEDs and watch how bright they are.

Alternatively view LED_Parallel.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

This is a bad idea. The current limiting resistor sets the total current which is shared between the LEDs. The same fixed current is split between how ever many LEDs are turned on. One LED might hog the current because the warmest LED will carry the most current so it also stays the warmest. The voltage across LEDs is about two volts. This stays the same if several LEDs are connected in parallel. Connecting a fourth LED in parallel does not increase the total current. It just makes all the LEDs dimmer because the same current is shared between more LEDs.

l

## LEDs in Series - OK

LEDs in series work just fine unless there are too many and the power supply voltage is no longer sufficient.

R = V / I = (12 - 2 - 2 - 2) / 0.01 = 600Ω        Using 560Ω the LED current is 10.7mA. This is safe.

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