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Lesson 1: Diodes and Applications

electronics fundamentalscircuits, devices, and applications

THOMAS L. FLOYD

DAVID M. BUCHLA

Clamper Circuits (Diode Clampers)

A clamper adds a dc level to an ac voltage.Clampers are sometimes known as dc restorers.Figure 2-45 shows a diode clamper that inserts a positive dc level in the output waveform. The operation of the circuit can be seen by considering the first negative half-cycle of the input voltage.When the input voltage initially goes negative , the diode is forward-biased, allowing the capacitor to charge near the peak of the input (Vp(in)) – 0.7V), as shown in Figure 2-45(a) Figure 2-45 Positive clamper operation.


Just after the negative peak, the diode is reverse-biased.The net effect of the clamping action is that the capacitor retains a charge approximately equal to the peak value of the input less the diode drop.The capacitor voltage acts essentially as a battery in series to the input voltage.The dc voltage of the capacitor adds to the input voltage to produce the output voltage as shown in Figure 2-46(b)

Figure 2-45 Positive clamper operation.


If the diode is turned around, a negative dc voltage is added to the input voltage to produce the output voltage as shown in Figure 2-46.

Figure 2-45 Positive clamper operation.

Special-purpose diodes

Special purpose diodes include

Zener diodes – used for establishing a reference voltage

Varactor diodes – used as variable capacitors

Light-emitting diodes – used in displays

Photodiodes – used as light sensors


Zener Diodes

The zener diode is a silicon pn junction device that differs from the rectifier diode in that it is designed for operation in the reverse breakdown region.

FIGURE 16–48 Diode V-I characteristic illustrating the operation of a zener diode compared to a rectified diode. The slope of the reverse-breakdown region is exaggerated to show detail.


Zener Diodes

FIGURE 16–50 Zener equivalent circuits. The slope is exaggerated to clarify the definition of ZZ.


Varactor Diodes

Varactor diodes are also known as variable-capacitance diodes because the junction capacitance varies with the amount of reverse bias-voltage.

A varactor is basically a reverse-biased pn junction diode that utilizes the inherent capacitance of the depletion region.

FIGURE 16–52 The reverse-biased varactor diode acts as a variable capacitor.


Varactor Diodes

FIGURE 16–53 Varactor diode capacitance varies with reverse voltage.

Applications A major application of varactors is as a part of a tuned circuit in communications device. Example is the FM modulator.


Light-Emitting Diodes

FIGURE 16–55 Light energy Electroluminescence in a forward-biased LED.

The basic operation of an LED (light-emitting diode) is as follows:

When the device is forward biased, electrons cross the pn junction from the n-type material and recombine with holes in the p-type material.

When recombination takes place, the recombining electrons release energy in the form of heat and light.


Light-Emitting Diodes

FIGURE 16–57 Light-emitting diodes (LEDs). FIGURE 16–56 Symbol for an LED.


Photo Diodes

FIGURE 16–58 Photodiode.

The photo diode is a pn junction device that operates in reverse bias.The photo diode has a small transparent window that allows light to strike the pn junction.


Photo Diodes

FIGURE 16–60 A photodiode circuit used in a system that counts objects as they pass on a conveyor belt.

Troubleshooting power supplies

Begin troubleshooting by analyzing the symptoms and how it failed. Try to focus on the most likely causes of failure.

7805

FD1

D2C1

D3

D4C2

1000 mF 1 mF

IC regulator

A power supply has no output, but was working until a newly manufactured PC board was connected to it. (a) Analyze possible failures. (b) Form a plan for troubleshooting.


7805

FD1

D2C1

D3

D4C2

1000 mF 1 mF

IC regulator

The supply had been working, so the problem is not likely to be an incorrect part or wiring problem. The failure was linked to the fact that a new PC board was connected to it, which points to a possible overloading problem. If the load was too much for the supply, it is likely a fuse would have blown, or a part would likely have overheated, accounting for the lack of output.


1. Disconnect power and check the fuse. If it is bad, replace it. Before reapplying power, remove the load, open the power supply case, and look for evidence of overheating (such as discolored parts or boards). If no evidence of overheating proceed.2. Check the new pc board (the load) for a short or overloading of the power supply that would cause the fuse to blow. Look for evidence of overheating.

3. Verify operation of the supply with measurements (see next slide).

Based on the analysis, a sample plan is as follows. (It can be modified as circumstances warrant.)


Reapply power to the supply but with no load. If the output is okay, put a resistive test load on the power supply and measure the output to verify it is operational. If the output is correct, the problem is probably with the new pc board. If not, you will need to further refine the analysis and plan, looking for an internal problem.

The analysis showed that a likely cause of failure was due to an overload. For the measurement step, it may be as simple as replacing the fuse and confirming that the supply works. After replacing the fuse:

Majority carrier

Minority carrier

PN junction

Diode

The most numerous charge carrier in a doped semiconductor material (either free electrons or holes.

Selected Key Terms

The boundary between n-type and p-type semiconductive materials.

An electronic device that permits current in only one direction.

The least numerous charge carrier in a doped semiconductor material (either free electrons or holes.

Barrier potential

Forward bias

Reverse bias

Full-wave rectifier

A circuit that converts an alternating sine-wave into a pulsating dc consisting of both halves of a sine wave for each input cycle.

The condition in which a diode conducts current.

The inherent voltage across the depletion region of a pnjunction diode.

Selected Key Terms

The condition in which a diode prevents current.

Bridge rectifier

Zener diode

Varactor

Photodiode

A diode whose reverse resistance changes with incident light.

A type of diode that operates in reverse breakdown (called zener breakdown) to provide a voltage reference.

A type of full-wave rectifier consisting of diodes arranged in a four corner configuration.

Selected Key Terms

A diode used as a voltage-variable capacitor.

Quiz

1. An energy level in a semiconductor crystal in which electrons are mobile is called the

a. barrier potential.

b. energy band.

c. conduction band.

d. valence band.

Quiz

2. A intrinsic silicon crystal is

a. a poor conductor of electricity.

b. an n-type of material.

c. a p-type of material.

d. an excellent conductor of electricity.

Quiz

3. A small portion of the Periodic Table is shown. The elements highlighted in yellow are

a. majority carriers.

b. minority carriers.

c. trivalent elements.

d. pentavalent elements. Si

B

Al

Ga

P

As

Sb

Ge

C

Sn

N

III IV V

In

Quiz

4. At room temperature, free electrons in a p-material

a. are the majority carrier.

b. are the minority carrier.

c. are in the valence band.

d. do not exist.

Quiz

5. The breakdown voltage for a silicon diode is reached when

a. the forward bias is 0.7 V.

b. the forward current is greater than 1 A.

c. the reverse bias is 0.7 V.

d. none of the above.

Quiz

6. The circuit shown is a

a. half-wave rectifier.

b. full-wave rectifier.

c. bridge rectifier.

d. zener regulator.

Quiz

7. PIV stands for

a. Positive Ion Value.

b. Programmable Input Varactor.

c. Peak Inverse Voltage.

d. Primary Input Voltage.

Quiz

8. A type of diode used a a voltage-variable capacitor is a

a. varactor.

b. zener.

c. rectifier.

d. LED.

Quiz

9. If one of the four diodes in a bridge rectifier is open, the output will

a. be zero.

b. have ½ as many pulses as normal.

c. have ¼ as many pulses as normal.

d. be unaffected.

Quiz

10. When troubleshooting a power supply that has a bridge rectifier, begin by

a. replacing the bridge rectifier.

b. replacing the transformer.

c. making measurements.

d. analyzing the symptoms and how it failed.

Quiz

Answers:

1. c

2. a

3. c

4. b

5. d

6. b

7. c

8. a

9. b

10. d

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