Session 15

The purpose of this session is to build upon some of the knowledge gained in session 11 by putting the components to work in a real circuit and using test equipment to measure and analyse their function and performance within those circuits.

BTEC Outcomes Covered

  • Unit 6 Pass 3
  • Unit 35 Pass 1
  • Unit 35 Pass 2

A Typical Application for a Diode

The diode is best known for its ability to rectify an ac supply to dc. Recall ac current flows back and forth between 2 points whereas dc always flows in the same direction. The following video helps to demonstrate the difference.

This ability to change alternating current to direct current is extremely useful. Virtually all electronic circuits require a dc supply to operate. It would be very expensive not to mention completely impractical to try and run computer systems from batteries.

However, electrical power is generated and distributed as alternating current. It enters your house as alternating current. In order to turn your house power supply into some sort of direct current you will required 2 things:

The transformer is used to step down the mains ac voltage to a safe level. Typically this will be 24 volts or lower, for our practical exercise we are going to be using 15 volts ac. This voltage, when connected to a diode circuit produces the following waveform.

Half Wave Rectifier

The negative portion of the sine wave -V is prevented from conducting because the diode is in reverse bias. The positive portion of the sine wave (+V) conducts because the diode is forward biased. If we measure the output of the circuit across R we will note that the amplitude of the waveform is approximately:

Half wave rectifier output = Input – 0.6 volts

That is, we lose 0.6 volts across the diode when it is forward biased just as we saw in session 11. Thus we have created a form of direct current (dc) where the electrons all flow in the same direction albeit in regular pulses.

 This is the first stage in understanding the process of converting ac to dc.

Practical Section 1

Build and test the following circuit in Multisim and practically. For our purposes V1 represents the portable transformers we use in the laboratory for ac power supplies.

Half Wave Multi

  • Measure and record the input voltage
  • Measure and record the output voltage

Full Wave Bridge Rectifier

The half wave rectifier is not the best or most efficient way of using a diode as a rectifier. Another, far more efficient way of carrying out rectification is to use 4 diodes in a network known as a full wave bridge rectifier, shown next.

Full Wave Bridge Rectifier

The diamond shape with 4 diodes is the circuit symbol for a full wave bridge rectifier. The following diagram explains how the circuit works, the diodes are connected in exactly the same way as the diagram above.

Full Wave Bridge Rectifier

To following diagrams explain the operation of a full wave bridge rectifier.

FWB1

FWB2

Now we have a much more efficient rectifier where both the positive and negative components of a sine wave are converted to dc. When measured on an oscilloscope the output waveform should look like this:

FWB Wave

We should note 2 things:

  • The output will equal the input – 1.2 volts
  • The frequency of the output has now doubled to 100 Hz

Practical Section 2

Build and test the following circuit both practically and in Multisim using an oscilloscope to confirm the amplitude and frequency of the output waveform

FWB Circuit

  • Explain the function and operation of the diodes in this circuit

Smoothing Capacitor

To this point we have succeeded in making a circuit that converts all of the sine wave to dc. However the dc output is far too unstable for use with electronic circuits therefore we need to take some more steps. To conclude today’s session we are going to add a smoothing capacitor to our circuit along with a variable resistor.

Practical Section 3

Build and test the following circuit practically and using Multisim

With Smoothing

Tasks

  • Test the circuit with no electrical load i.e. with R1 and R2 disconnected measure and record the output waveform
  • Reconnect R1 and R2 and set R2 to its maximum setting measure and record the output
  • Set R2 to its minimum setting, measure and record the output
  • Explain the function of the smoothing capacitor in this circuit. Why is it more effective at smoothing out the waveform when R2 is at its highest setting?
  • Why is the output waveform virtually flat when no electrical load is connected?
  • Produce a report which includes all the documentary evidence gathered for all 3 practical tasks. Be sure to evaluate the things you have learned by carrying out these tasks.

The following video gives some hints on how to test the circuit in Multisim.

This video shows the process of building the Full Wave Bridge Rectifier on breadboard and will also be useful in the next session.

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