1. Light Emitting Diodes (LED)
AIM:
To know how to use LEDs in circuits and to be able to calculate the correct value of the series resistor
PRIOR KNOWLEDGE:
The resistor equation (V=IR) and standard values of resistors

2. Introduction
Light Emitting Diodes (LEDs) are very useful as indicators to show when something is on
LEDs work at low voltage and take very little current which makes them ideal for low power circuits
An LED usually needs a series resistor to stop it being damaged by too much current
LEDs come in different shapes, sizes and colours. The picture shows the standard round 5mm LEDs

3. The Flat side is the Cathode
LED Basics
An LED only conducts current in one direction
An LED only conducts when the voltage is above a certain minimum value that depends on the type of LED being used
The two connections are called the Anode and Cathode. Current only flows from the Anode to the Cathode
The Cathode can be identified by the flat edge on the LED
The Flat side is the Cathode

4. Forward and Reverse Bias
FORWARD BIAS
The anode is connected to positive and current can flow
The LED is illuminated
REVERSE BIAS
The cathode is connected to positive and no current flows
The LED is not illuminated
Note: The LED will only conduct when forward biased if the voltage is above the minimum value – usually about 2V. If the voltage is too low the LED does not conduct

5. LED Series Resistor
An LED needs a certain voltage (and no more) to work properly
AN LED takes a certain current at full power
To allow the LED to work properly a series resistor is required to limit the maximum current
EXAMPLE: An LED works at 2V and requires a maximum current of 10mA
The LED is connected to a 9V battery and a series resistor
Remember the LED needs 2V
What value of series resistor should be used?
Calculate the voltage across the resistor. In this case 9V – 2V = 7V for the resistor
The current through the resistor is the same as for the LED. In this case 10mA
Convert current to standard units. 10mA = 0.01A
Calculate the resistor value. R = V / I  R = 7 / 0.01 = 700Ω

6. Example 1 R = 270Ω An LED works at 2.2V and requires a current of 40mA
The LED is connected to a 12V battery
What series resistor should be used?
R = 270Ω
The voltage across the resistor is 12 – 2.2 = 9.8V
The current through the resistor is 40mA
40mA = 0.04A
The resistor equation gives R = 9.8 / 0.04 = 245Ω
A 245Ω resistor is not a standard value so use the next biggest (so that the maximum current is not exceeded)
Use R = 270Ω

7. Example 2
A high power LED works at 4V and requires a current of 200mA
Two LED are connected in series to a 24V battery
What series resistor should be used?
R = 82Ω
The voltage across the resistor is 24 – (2 x 4) = 16V
The current through the resistor is 200mA
200mA = 0.2A
The resistor equation gives R = 16 / 0.2 = 80Ω
An 80Ω resistor is not a standard value so use the next biggest (so that the maximum current is not exceeded)
Use R = 82Ω

8. Different types of LED LED Array 7 segment display High power LEDS
Infrared LEDS

9. Summary LEDs illuminate when current flows
An LED is forward biased when the anode is connected to positive and current can flow
An LED is reverse biased when the cathode is connected to positive and current can not flow
A minimum voltage is required for the LED to conduct
For normal LEDs, a working voltage of 2V and a current of 10mA or 20mA are reasonable assumptions
A series resistor is used to limit the maximum current
Some LEDs work in the infrared, other LEDs are high powered. White LEDs can be used to replace traditional lighting

10. Questions What does forward biased mean?
Why might an LED not conduct even when it is connected correctly (in forward bias)?
Why might an LED appear not to work even if it is in forward bias and current is flowing?
An LED works at 1.8V and requires 20mA. What series resistor should be used with a 6V battery?
Why is the series resistor necessary?

11. Answers
The anode is connected to positive, the cathode to negative or 0V of the supply
The supply voltage is less than the minimum required to allow the LED to conduct
It could be emitting in the Infrared or Ultraviolet parts of the spectrum hence can’t be seen with the naked eye
210Ω (but use 220Ω from the E24 series)
If the series resistor is not used or is too small then too much current flows and the LED will be damaged due to ohmic heating