Light Emitting Diodes

Light Emitting Diodes

In this unit so far, the only light source used has been the lamp. A lamp is a useful component which converts Electrical Energy into Light Energy. The lamp does have a drawback, however, as to generate light it must also give out Heat Energy. This heat Energy is waste Energy, and so the lamp is a fairly inefficient light source.  

Another light source that can be used within a circuit is the Light Emitting Diode (L.E.D.). This component gives out very little heat Energy, and so is more efficient than a lamp. 

The diagram below shows the symbol for an L.E.D.:-

Note - In circuit diagrams, two arrows denote light:-

1. Two arrows leaving symbol - Light given out.

2. Two arrows entering symbol - Light taken in. 

Light Emitting Diodes also control the direction of current within a circuit. LEDs will only conduct when connected in a certain direction within the circuit (for an explanation of why this is, please see the Higher course).

The large triangle of the symbol can be used as a pointer arrow:-

1. Towards the negative terminal - LED lights.

2. Away from the negative terminal - LED does not light.

The diagram below shows an LED connected correctly within a circuit:-

Protecting LEDs

In the above circuit, the LED is connected in series with a Resistor.  LEDs require only very low currents to function, and are easily damaged, so the function of the Resistor is to reduce the current flow to a safe level. 

The size of the Resistor depends on the supply voltage and the requirements of the LED. The method below allows the calculation of the correct Resistor:-

Example 1 - 

The above diagram shows a simple LED circuit. If the LED functions at the given ratings, what is the required value of the Resistor? 

Vs  =  VR + VLED

VR  =  Vs - VLED

VR  =  12 - 2

VR  =  10 V

IR  =  ILED  =  10 mA  =  0.01 A

R  = VR / IR

R  = 10 / 0.01 

R  = 1000 Ω

R = 1 kΩ