Diodes
We have already learnt about some uses of diodes, from Light Emitting Diodes (LED’s) to diodes to protect
components from being damaged (see section on relays) and diodes used in rectifiers (see section on mains
power supplies). Before we look at the remaining uses of diodes, let us go back to the beginning and look at
a diode.
A diode is a one way conductor, that is current can pass through the diode in only one direction.
Current flows through the
diode in this direction only.
The symbol itself helps us to remember in which way current can flow as it consists of an arrow pointing in
the direction of current flow.
The diode has two legs, one is called the ANODE and the other is the CATHODE. Current flows from the
anode to the cathode, so the anode must be at a higher voltage than the cathode as current only flows from a
higher voltage to a lower voltage.
Anode
Cathode
Telling which leg is which is fine when we are looking at the circuit symbol, but how do we tell when looking
at the real component? Well, there are a number of ways.
Diodes
-
the band around the end of the component shows the Cathode leg
Anode
LED’s
-
Cathode
the Anode leg is longer than the Cathode leg
the Anode edge of the LED is round and the Cathode edge is flat
LED’s come in many different shapes, as the picture on
the left shows. The common LED, as you know, is
basically dome shaped, but you can also buy rectangular
LED’s and triangular LED’s. LED’s are also available in
different sizes; 2mm, 3mm, 5mm, 8mm and 10mm being
common.
They are also available in different colours; red, green,
yellow, orange, white, blue and clear.
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Applications of LED’s
This picture shows the rear brake lights on a Suzuki
motorbike. In this application one single LED would
not be bright enough, so a cluster of many LED’s is
used.
This picture shows a headtorch
manufactured by Petzl. It uses three
ultrabright white LED’s to give good
illumination in the dark.
LED Displays
When we need to display numbers we can make use of an LED display. These displays have 8 segments, 7
to display each part of the number and 1 for a decimal point. By using a control circuit the correct LED’s
behind each segment will be lit to display the number.
As you can see from the picture these LED displays come in many different sizes and are commonly red,
although there are some green displays available. The bottom left of the picture also shows what is called
a light bar. In this case LED’s light up successive rectangular bars as the voltage increases.
To control an LED display requires the use of a control circuit. This may be made up from dedicated
integrated circuits such as the 4026B which is specifically designed to drive LED displays, or a
microprocessor can be programmed to switch on the display.
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Liquid Crystal Displays (LCD’s)
Although they do not use LED’s, as we have looked at LED displays we will also look at the LCD. LCD
are found extensively in modern electronic products, whether it is the display on a hi-fi, the dashboard of a
motor car or the control panel on the microwave, LCD are widespread.
Basically, when a small current is passed through an LCD it changes its colour to black. When viewed
against the silver background it makes the display visible.
This picture shows an example
of an LCD used on the
dashboard of a car.
As well as numbers, letters and
pictures can be displayed. For
example there is a symbol for
the
cars
battery,
air
conditioning and an ice
warning.
It is also easy to see the black
segments against the silver
background.
The problem with LCD like the one above is that
in the dark the black segments cannot be seen.
To overcome this problem an LED backlight
panel, an example of which is shown in the
picture, is placed behind the LCD.
This
effectively changes the colour of the LCD so that
it is visible. Digital watch lights are a very good
example of LED backlight panels.
But LCD do not have to be just black. The
picture shows one of the latest LCD-TFT (Liquid
Crystal Display-Thin Film Technology)
televisions.
A very clear colour picture is shown, whilst you
can also see just how thin the screen is. Compare
this to the size of a normal television.
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