Session 8

The teaching for the next two sessions is primarily aimed at preparing you for D1 for unit 6 but also has a bearing on several outcomes in other units, notably unit 35.

By the end of this session you will have a better understanding of how to:

Analyze the operation and effects of varying component parameters in a power supply that contains a transformer, diodes and a smoothing capacitor.

The circuit we will be working with has no component values!! As you will see in the assignment its your job to select the component values to achieve a given specification. Here is the circuit:

T1

The first thing to note is that this circuit is fed from a transformer which will give a stepped down ac supply. As long as your ac supply is between lets say 10 – 15 volts then the circuit will work out just fine. In terms of the transformer the critical thing is to choose one which gives that output. You would need to state why for example those values would be important, so here’s a couple of hints, firstly think about the reverse breakdown voltage of the diodes, secondly think about the maximum voltage rating of the capacitor.

It then should be obvious why we need to step the voltage down. I have prepared a video on how transformers work which you might find useful. The material in the video will also be useful for P9 which appears in assignment 3.

Next up we have the bridge rectifier:

T2

The bridge rectifier is highlighted by the blue rectangle. Its consists of 4 diodes whose function is to change ac into dc. Diodes 1 and 3 conduct on one half of the ac input and 2 & 4 conduct on the other half. The effect is to cause current to flow in the same direction (dc) in the external circuit. Effectively we have now have an ac input and a dc output.

T3

The picture above illustrates the input voltage and output voltages without the capacitor connected and the places where you would test using an oscilloscope or multimeter.

When you connect a capacitor the output voltage will look something like the next picture but here’s a hint. Its all to do with the value of the capacitor!!

T4

From the diagram you can see that the full wave bridge rectifier charges the capacitor twice for every ac cycle. The capacitor charges instantaneously when the output voltage of the bridge rectifier is rising. When the bridge rectifier output falls the capacitor begins to discharge. However it cannot discharge through the diodes (they are reverse biased) so it must discharge through the other circuit components and thus the rate of discharge will be a function of the time constant formed by the capacitor and the resistance of the external circuit.

That’s plenty of information to get you started. Remember you will need to explain the function and operation of the entire circuit as well as giving reasons for the components you chose.