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:
- voltage conversion
- rectification
- filtering
- the reasons for voltage variations and how to resolve them
Apparatus:
- The demonstration circuit of fig 9.1
- Some capacitors of values commonly found in small power
supplies and appropriate voltage
(10µF, 100µF,
1000µF at 25 volts working)
- A range of resistors from 10 ohms to 1000 ohms with sufficient
power ratings to serve as a load on the power supply
- Clip leads
- Voltmeter
- Oscilloscope
- If available the following would be useful:
- A 0-1 Amp ammeter
- A small audio amplifier and speaker
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
- 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
- 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
- Measure the unfiltered output voltage and record it
For 4 through 6, refer to fig 10.2
- Connect one of the capacitors across points A and B
using clip leads. The students should observe that:
- The waveform is now smoother (more DC-like)
- The volume in the audio amplifier has dropped
- The measured voltage has risen
Discuss the reasons why. (hint - start with energy storage in
the capacitor)
- Using the resistors, connect one at a time
across points A and B to vary the load on the circuit,
illustrating that:
- As the current increases, the voltage drops
- As the current increases the ripple in the output increases
Explain why
- Increase the size of the capacitor and observe that the voltage is
more constant and that the ripple decreases at high loads
- 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:
- The space between cycles of the AC input is now filled
- The output voltage is much higher
- The output voltage tends to be more constant under varying
loads
- The more astute may also note that the ripple frequency has
doubled
- This is a good point to introduce the concept of regulation
- Place a 5 V monolithic regulator in series with the circuit.
Measure the voltage under light and heavy loads. The students should
note that:
- The voltage under load does not change when the load current changes
(resistance of the load changes), up to the point where the regulator
starves for voltage
- If the supply voltage from the output capacitor drops below the
input voltage required by the regulator due to heavy loading, the
regulator will cut off
- If possible, set the load current so that the input voltage to the
regulator drops below the minimum value only during part of the AC
cycle; note the result on the voltmeter and oscilloscope
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:
- Read Chapter 10
- 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
- Slide 1
10.2
Power Supply, block diagram
- 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
- Slide 3
10.7
Simple Power Supply Filter
10.8 Output Waveform for Simple Filter
- Slide 4
10.9
Voltmeter Placement