Session 5

The purpose of this session is to teach you the basics about diodes. We will be investigating the construction and function of a general purpose diodes and a zener diode, using test/measurement equipment and simulation software to measure and compare the forward and reverse characteristics of both.

 

BTEC Outcomes Covered

  • Unit 6 Pass 3

What is a diode?

A diode is an electronic component that behaves in a similar fashion to a one way valve in a water system. That is, it allows current to flow easily in one direction yet blocks current in the other direction.

The following diagram illustrates this:

Forward bias

 Diode Construction (Basic)

Diodes are made out of a semi-conductor material called silicon. To make silicon a more effective semi-conductor the material goes through a process known as doping which is a method of adding impurities to the material. By adding such impurities we can make two different types of semi-conductor material known as n-type and p-type:

  • n-type material has a net negative charge due to an excess of electrons
  • p-type materials has a net positive charge to to a lack of electrons

When we join both types of material together we form something known as a p-n junction. The excess electrons in the n-type material flow across the junction to marry up with the gaps left by the absent electrons in the p-type material. The net result is that there are very few charge carriers present at the junction. The junction forms something known as a depletion layer. The diode will  not conduct until a potential difference of 0.6 volts is applied between the anode and cathode. This potential difference is required to overcome the electrostatic field formed across the depletion layer.

Electrostatic

Getting a diode to conduct by forward biasing

As previously mentioned we can get a diode to conduct by ‘forward biasing’ it. The following schematic shows how this can be done:

Forward Bias Schematic

A diode with also conduct in reverse bias (connected the other way around) for standard diodes the reverse breakdown voltage (the voltage at which a diode begins to conduct) is much higher than the forward bias voltage. For the diode we are using it will be 50 Volts.

This circuit is set up to measure the forward bias characteristic of a diode. We have 5 main elements to the circuit:

  • A dc power supply
  • An ammeter to measure the current flowing through the diode
  • A voltmeter to measure the voltage across the diode
  • The diode itself
  • A resistance box to vary the current flowing through the circuit

A real world circuit would look something like this:

Forward bias real

Assessment Section

Carry out a test to measure the forward and reverse characteristic of a 1N4001 diode.

To carry this out we can use a simulator as shown in the next video, this allows us to quickly take measurements from a virtual component and plot them into a graph which shows the diodes conduction curve. We can also analyze the results of the test as is briefly explained in the video.

 

To do this practically for either the 1N4001 or the Zener diode you can follow the procedure described below for forward bias (you need to test the forward bias of at least one of the diodes practically. Use the following circuit for the test

Forward bias real

  •  Set up the circuit as shown
  • Make sure the resistance box is at its maximum setting with all the dials turned fully clockwise
  • Set the dc power supply to a voltage of 5 volts dc, it may be necessary to adjust this setting
  • Adjust the resistance box dials until the voltage drop across the diode reads 0.5 volts (the video shows how this is done). Record the voltage and current in a table
  • Adjust the resistance box so that the voltage across the diode rises by 0.01 volts. The voltage reading should now be 0.51 volts. Record the voltage and current in a table
  • Continue the process of adjusting the voltage across the diode in steps of 0.01 volts measuring and recording the voltage and current for each step.
  • Stop recording voltage and current if either of the following conditions is met. The voltage across the diode reaches 0.7 volts or the current flowing through the circuit exceed 200 mA
  •  Plot both sets of results (1N4001 & zener) on a graph and label it correctly (This is also shown in the video)

The following video shows how this can be done with a Zener, its exactly the same process with a standard diode. The video also shows how to test the reverse characteristics of a low voltage zener practically. You will not be asked to test the reverse bias of a standard diode practically.

To conclude the session you will need to analyze the graphs produced. Examples of how this can be done are described in the first video above.

Leave a Reply