8/10/2019 Diode Tutorial.doc

1/3

Diode Tutorial

Diodes have the following properties:

They only allow current to flow in one direction

They have a specific voltage drop, which can be taken as a constant for

most circuits

A diode is represented with the following symbol:

The diode above will pass current flowing from left to right, but block current

trying to flow from the right to the left-hand side. The voltage drop over diodeswill vary from diode to diode, the most common ones are:

"Normal" diodes:Have a voltage drop between .!" and .#"Schottky diodes:They have a low voltage drop $about .%" to .&"'Light emitting diodes (LED): These diodes will emit a light when passingcurrent. Their voltage drop will depend on the colour of the ()D, but is around &"to *".Zener diodes:They behave as a normal diode for current flowing from the left toright, but will allow current to flow from the right to left if the voltage is highenough. +ou get a lot of different ener voltage diodes, e.g. .#" and %&".

Using diodes

elow is a simple circuit with a diode. +ou must always place a resistor in serieswith a diode to limit the current flow, if you don/t do that the diode will act as ashort circuit and draw current until the supply or the diode breaks.

The voltage drop over the diode is .!" $this is a normal diode and .!" is closeenough.' The diode and resistor is in series, thus the current through the diode ise0ual to the current through the resistor.

8/10/2019 Diode Tutorial.doc

2/3

12% 3 " 4 2

3 $5" - .#"' 4 #2

3 6.%mA

Thus the current through the diode e0uals 6.%mA

Here is an e7ample with an ()D $you will notice the symbol of an ()D hasarrows pointing away from it, this is to show that it emits light'

(et/s say the ()D is a red ()D with a voltage drop of &", thus:

12% 3 " 4 2

3 $5" - &"' 4 &&2

3 %*.!mA

The more current an ()D/s conducts, the brighter it will be. To increase thecurrent you can either increase the voltage or decrease the value of the resistor.8ust remember that there is a limit to the amount of current a diode can conduct

before it breaks and for ()D/s this is rather low $about &mA typically'

ener diodes are mostly used to create a constant voltage. 9ay we want an ()Dto be the same brightness, no matter what our input voltage is $the input voltagecould be from a battery that starts out at %" and goes down to 6" before it ischarged again'The circuit below is one way of doing it:

8/10/2019 Diode Tutorial.doc

3/3

2% will determine the amount of current flowing through % $for ener diodes towork correctly, you need a minimum current flowing through them, about %mA isa good value'1f the input voltage is 6":

1d% 3 " 4 23 $6" - .#"' 4 *2

3 %%.*mA

1f the input voltage is %":

1d% 3 " 4 2

3 $%" - .#"' 4 *2

3 &.mA

ecause the ener diode will always have a constant voltage drop over it, the

potential at ";ener will always be .#". The current through D& is $assuming thevoltage drop over D& is &"':

1d& 3 " 4 2

3 $.#" - &"' 4 &&2

3 %&.*mA

And this current will always be the same, whether the input voltage is %", 6" oranything in between.