Lesson 10:

Power Supplies

Objectives:

By the end of Lesson 10 the student should have a good understanding of the design and operation of power supplies as they pertain to Amateur Radio, including:

Apparatus:

  1. The demonstration circuit of fig 9.1

  2. Some capacitors of values commonly found in small power supplies and appropriate voltage
    (10µF, 100µF, 1000µF at 25 volts working)

  3. A range of resistors from 10 ohms to 1000 ohms with sufficient power ratings to serve as a load on the power supply

  4. Clip leads

  5. Voltmeter

  6. Oscilloscope

  7. If available the following would be useful:

Handout:

Power supply block diagram. Printable copy. Note that this block diagram has the blocks filled in with the various ways they may be implemented. Some, such as the switched power supply are not on the IC Basic exam, although examples abound in the real world.

Lesson Plan:

Fig 10.1; demo circuit previously used in lesson 9
showing connections to test equipment

  1. Set up the circuit of fig 9.1 that you used last week. Connect the oscilloscope and display the half-wave rectified waveform. Fig 10.1 shows the equipment connections

  2. If the audio amplifier is available, connect its input through a capacitor of 0.5 µF or so to the rectified output of the demo circuit, set the volume relatively low and let the students hear the sound of raw, unfiltered DC

  3. Measure the unfiltered output voltage and record it

       For 4 through 6, refer to fig 10.2

  4. Connect one of the capacitors across points A and B using clip leads. The students should observe that:


    Discuss the reasons why. (hint - start with energy storage in the capacitor)

  5. Using the resistors, connect one at a time across points A and B to vary the load on the circuit, illustrating that:


    Explain why

  6. Increase the size of the capacitor and observe that the voltage is more constant and that the ripple decreases at high loads

  7. Remove the filter capacitor and replace the single diode with a full-wave or bridge rectifier. Explain how to connect it in the circuit properly, using fig 10.3 as a reference. The students should observe that:


  8. This is a good point to introduce the concept of regulation

  9. Place a 5 V monolithic regulator in series with the circuit. Measure the voltage under light and heavy loads. The students should note that:

Fig 10.2 Connection points for resistance loading
and capacitance filtering demonstration

Fig 10.3 Same circuit as fig 10.1 but with bridge rectifier

Homework:

  1. Read Chapter 10

  2. Do the Chapter 10 Quiz

Chapter 10 Quiz:

Printable copy of the Review Quiz

Answers to Chapter 10 Quiz
1 B 5 C 9 D 13 A
2 C 6 D 10 A 14 A
3 A 7 A 11 B 15 A
4 B 8 A 12 A 16 A

Overhead Slide Versions of the Diagrams

  1. Slide 1
    10.2 Power Supply, block diagram

  2. Slide 2
    10.3 Half Wave Rectifier, schematic
    10.4 Half and Full Wave Rectifier, Waveforms
    10.5 Full Wave Rectifier schematic
    10.6 Bridge Rectifier, schematic

  3. Slide 3
    10.7 Simple Power Supply Filter
    10.8 Output Waveform for Simple Filter

  4. Slide 4
    10.9 Voltmeter Placement