MINI LESSON 2 – LIGHT-EMITTING DIODES (LEDs)

 

 

LEDs

 

BASIC FACTS

 

  • LED = Light-emitting diode

  • Here is an image of an LED:


Image result for led"

 

  • All diodes are semi-conductor devices that allow for current flow in one direction only.  LEDs do the same but emit light when the current is actually going through the diode.

 

  • If a diode is connected backwards, it will stop current from going through.  For an LED, it would stop the current and also not go on.

 

  • A light-emitting diode is often used to give a visual result.  When the LED is on, then the line is considered to be 1. When the LED is off, then the line is 0.

FRAGILE

 

  • LEDs are fragile to currents that exceed their technical specification (about 30 mA). 

  • Even a brief exposure to a higher current will usually destroy the LED.

  • LEDs are fragile even if they are connected backwards.

  • It is important to always place a resistor in series with an LED in order to protect it.

 

ORIENTATION

 

·       The longer leg is positive or anode (A).

 

·       The shorter leg is negative or cathode (K).

 

·       You can also tell the negative side of the LED by finding the flat side of the plastic cover of the LED.

 

LED TECHNICAL DATA

 

The following shows the technical data for typical LEDs based on their colour.  The most important columns have been circled in red.

 

Notice how different coloured LEDs tend to have different specs.  That is because they have different semi-conductor material concentrations in them.

 

We usually go with 20mA for current for any LED.

 

CALCULATING NEEDED RESISTOR

 

We can simply apply Ohm’s Law over the resistor to find the needed resistor.

 

We know the current through the resistor is the same as the current through the LED.  So about 20mA.

 

We know that VR = VS - VL.

 

So, we use VR = IR x RR and sub in VR = VS - VL and, after rearranging, we get R = (VS – VL) / IR.