Gibilisco S. Teach Yourself Electricity and Electronics

Подождите немного. Документ загружается.

Gunn-diode oscillators are often tuned using varactor diodes. A Gunn-diode oscillator, con-

nected directly to a microwave horn antenna, is known as a Gunnplexer. These devices are popular

with amateur-radio experimenters at frequencies of 10 GHz and above.

IMPATT Diodes

The acronym IMPATT comes from the words impact avalanche transit time. This, like negative re-

sistance, is a rather esoteric phenomenon. An IMPATT diode is a microwave oscillating device like

a Gunn diode, except that it uses silicon rather than gallium arsenide.

An IMPATT diode can be used as an amplifier for a microwave transmitter that employs a

Gunn-diode oscillator. As an oscillator, an IMPATT diode produces about the same amount of out-

put power, at comparable frequencies, as a Gunn diode.

Tunnel Diodes

Another type of diode that will oscillate at microwave frequencies is the tunnel diode, also known as

the Esaki diode. It produces enough power so it can be used as a local oscillator in a microwave radio

receiver, but not much more.

Tunnel diodes work well as amplifiers in microwave receivers, because they generate very little

unwanted noise. This is especially true of gallium arsenide devices.

Energy Emission

Some semiconductor diodes emit radiant energy when a current passes through the P-N junction in

a forward direction. This phenomenon occurs as electrons fall from higher to lower energy states

within atoms.

LEDs and IREDs

Depending on the exact mixture of semiconductors used in manufacture, visible light of almost any

color can be produced by diodes when bias is applied to them in the forward direction. Infrared-

emitting devices also exist. The most common color for a light-emitting diode (LED) is bright red.

An infrared-emitting diode (IRED) produces energy at wavelengths slightly longer than those of vis-

ible red light.

The intensity of the radiant energy from an LED or IRED depends to some extent on the for-

ward current. As the current rises, the brightness increases, but only up to a certain point. If the cur-

rent continues to rise, no further increase in brilliance takes place. The LED or IRED is then said

to be in a state of saturation.

Digital Displays

Because LEDs can be made in various different shapes and sizes, they are ideal for use in digital dis-

plays. You’ve seen digital clock radios that use them. They are common in car radios. They make

good indicators for “on/off,” “a.m./p.m.,” “battery low,” and other conditions.

In recent years, LED displays have been largely replaced by liquid crystal displays (LCDs). The

LCD technology has advantages over LED technology, including lower power consumption and

better visibility in direct sunlight. However, LCDs require backlighting when the ambient illumina-

tion is low.

Energy Emission 331

Communications

Both LEDs and IREDs are useful in communications because their intensity can be modulated to

carry information. When the current through the device is sufficient to produce output, but not

enough to cause saturation, the LED or IRED output follows along with rapid current changes.

Analog and digital signals can be conveyed over light beams in this way. Some modern telephone

systems make use of modulated light, transmitted through clear fibers. This is known as fiber-optic

technology.

Special LEDs and IREDs produce coherent radiation. These are called laser diodes. The rays from

these diodes aren’t the intense, parallel beams that most people imagine when they think about

lasers. A laser LED or IRED generates a cone-shaped beam of low intensity. But it can be focused

into a parallel beam, and the resulting rays have some of the same advantages found in larger lasers,

including the ability to travel long distances with little decrease in their intensity.

Photosensitive Diodes

Virtually all P-N junctions exhibit conductivity that varies with exposure to radiant electromagnetic

energy such as IR, visible light, and UV. The reason that conventional diodes are not affected by

these rays is that they are enclosed in opaque packages. Some photosensitive diodes have variable dc

resistance that depends on the intensity of the electromagnetic rays. Other types of diodes produce

their own dc in the presence of radiant energy.

Silicon Photodiodes

A silicon diode, housed in a transparent case and constructed in such a way that visible light can

strike the barrier between the P-type and N-type materials, forms a silicon photodiode. A reverse-bias

voltage is applied to the device. When radiant energy strikes the junction, current flows. The cur-

rent is proportional to the intensity of the radiant energy, within certain limits.

Silicon photodiodes are more sensitive at some wavelengths than at others. The greatest sensi-

tivity is in the near infrared part of the spectrum, at wavelengths just a little bit longer than the wave-

length of visible red light.

When radiant energy of variable intensity strikes the P-N junction of a reverse-biased silicon

photodiode, the output current follows the light-intensity variations. This makes silicon photodi-

odes useful for receiving modulated-light signals of the kind used in fiber-optic communications

systems.

The Optoisolator

An LED or IRED and a photodiode can be combined in a single package to get a component

called an optoisolator. This device, the schematic symbol for which is shown in Fig. 20-10, creates a

modulated-light signal and sends it over a small, clear gap to a receptor. An LED or IRED converts

an electrical signal to visible light or IR; a photodiode changes the visible light or infrared back into

an electrical signal.

When a signal is electrically coupled from one circuit to another, the two stages interact. The

input impedance of a given stage, such as an amplifier, can affect the behavior of the circuits that

feed power to it. This can lead to various sorts of trouble. Optoisolators overcome this effect, be-

cause the coupling is not done electrically. If the input impedance of the second circuit changes, the

impedance that the first circuit sees is not affected, because it is simply the impedance of the LED

332 How Diodes Are Used

or IRED. That is where the “isolator” in “optoisolator” comes from. The circuits can be electroni-

cally coupled, and yet at the same time remain electrically isolated.

Photovoltaic Cells

A silicon diode, with no bias voltage applied, can generate dc all by itself if enough electromagnetic

radiation hits its P-N junction. This is known as the photovoltaic effect. It is the principle by which

solar cells work.

Photovoltaic cells are specially manufactured to have the greatest possible P-N junction surface

area. This maximizes the amount of light that strikes the junction. A single silicon photovoltaic cell

can produce about 0.6 V of dc electricity. The amount of current that it can deliver, and thus the

amount of power it can provide, depends on the surface area of the junction.

Photovoltaic cells can be connected in series-parallel combinations to provide power for solid-

state electronic devices such as portable radios. These arrays can also be used to charge batteries, al-

lowing for use of the electronic devices when radiant energy is not available (for example, at night!).

A large assembly of solar cells, connected in series-parallel, is called a solar panel. The power pro-

duced by a solar panel depends on the intensity of the light that strikes it, the sum total of the sur-

face areas of all the cells, and the angle at which the light strikes the cells. Some solar panels can

produce several kilowatts of electrical power in direct sunlight that shines in such a way that the

sun’s rays arrive perpendicular to the surfaces of all the cells.

Quiz

Refer to the text in this chapter if necessary. A good score is at least 18 correct. Answers are in the

back of the book.

1. When a diode is forward-biased, the anode voltage

(a) is negative relative to the cathode voltage.

(b) is positive relative to the cathode voltage.

(d) alternates between positive and negative relative to the cathode voltage.

2. If a diode is connected in series with the secondary winding of an ac transformer, and if the

peak voltage across the diode never exceeds the avalanche voltage, then the output of the complete

transformer-diode circuit is

(a) ac with half the frequency of the input.

(b) ac with the same frequency as the input.

(d) none of the above.

Quiz 333

20-10 An optoisolator has

an LED or IRED

at the input and a

photodiode at the

output.

3. A crystal set

(a) can be used to transmit radio signals.

(b) requires a battery with long life.

(d) is used for rectifying 60-Hz ac.

4. A diode detector

(a) is used in power supplies.

(b) is employed in some radio receivers.

(d) changes dc into ac.

5. If the output wave in a circuit has the same shape as the input wave, then

(a) the circuit is operating in a linear manner.

(b) the circuit is operating as a frequency multiplier.

(d) the circuit is operating as a rectifier.

6. Suppose the two input signal frequencies to a mixer circuit are 3.522 MHz and 3.977 MHz.

At which of the following frequencies can we expect a signal to exist at the output?

(a) 455 kHz

(b) 886 kHz

(d) 1.129 MHz

7. Fill in the blanks to make the following sentence correct: “A spectrum analyzer provides a

display of

as a function of .”

(a) amplitude/time

(b) time/frequency

(d) amplitude/frequency

8. Zener voltage is a specialized manifestation of

(a) forward breakover voltage.

(b) peak forward voltage.

(d) reverse bias.

9. The forward breakover voltage of a silicon diode is

(a) about 0.3 V.

(b) about 0.6 V.

(d) dependent on the avalanche voltage.

334 How Diodes Are Used

10. A diode audio limiter circuit

(a) is useful for voltage regulation.

(b) always uses Zener diodes.

(d) can cause distortion under some conditions.

11. The capacitance of a varactor varies with the

(a) forward voltage.

(b) reverse voltage.

(d) forward breakover voltage.

12. The purpose of the I layer in a PIN diode is to

(a) minimize the junction capacitance.

(b) optimize the avalanche voltage.

(d) increase the current through the diode.

13. Which of these diode types can be used as the key element in the oscillator circuit of a

microwave radio transmitter?

(a) A rectifier diode

(b) A PIN diode

(d) None of the above

14. A Gunnplexer is often used as a

(a) microwave communications device.

(b) low-frequency RF detector.

(d) signal mixer or frequency divider.

15. The most likely place you would find an LED would be in

(a) a rectifier circuit.

(b) a mixer circuit.

(d) an oscillator circuit.

16. Coherent electromagnetic radiation is produced by a

(a) Gunn diode.

(b) varactor diode.

(d) laser diode.

Quiz 335

17. Suppose you want a circuit to operate in a stable manner when the load impedance varies.

You might consider a coupling method that employs

(a) a Gunn diode.

(b) an optoisolator.

(d) a PIN diode.

18. The electrical power that a solar panel can provide in direct sunlight depends on all of the

following factors except

(a) the ac voltage applied to the panel.

(b) the total surface area of all the cells in the panel.

(d) the intensity of the sunlight that strikes the cells.

19. Emission of energy in an IRED is caused by

(a) high-frequency radio waves.

(b) rectification.

(d) none of the above.

20. A photodiode, when not used as a photovoltaic cell, has

(a) reverse bias.

(b) no bias.

(d) negative resistance.

336 How Diodes Are Used

A POWER SUPPLY CONVERTS UTILITY AC TO DC FOR USE WITH CERTAIN ELECTRICAL AND ELECTRONIC

devices. In this chapter, we’ll examine the components of a typical power supply.

Power Transformers

Power transformers can be categorized as step-down or step-up. As you remember, the output, or

secondary, voltage of a step-down unit is lower than the input, or primary, voltage. The reverse is

true for a step-up transformer.

Step-down

Most solid-state electronic devices, such as radios, need only a few volts. The power supplies for such

equipment use step-down power transformers. The physical size of the transformer depends on the

current. Some devices need only a small current and a low voltage. The transformer in a radio re-

ceiver, for example, can be physically small. A ham radio transmitter or hi-fi amplifier needs more

current. This means that the secondary winding of the transformer must consist of heavy-gauge

wire, and the core must be bulky to contain the magnetic flux.

Step-up

Some circuits need high voltage. The cathode-ray tube (CRT) in a conventional home television set

needs several hundred volts. Some ham radio power amplifiers use vacuum tubes working at more

than 1 kV dc. The transformers in these appliances are step-up types. They are moderate to large in

size, because of the number of turns in the secondary, and also because high voltages can spark, or

arc, between wire turns if the windings are too tight. If a step-up transformer needs to supply only

a small amount of current, it need not be big. But for ham radio transmitters and radio or television

broadcast amplifiers, the transformers are large, heavy, and expensive.

Transformer Ratings

Transformers are rated according to output voltage and current. For a given unit, the volt-ampere

(VA) capacity is often specified. This is the product of the voltage and current.

337

CHAPTER

Power Supplies

A transformer with 12-V output, capable of delivering 10 A, has 12 V × 10 A = 120 VA of ca-

pacity. The nature of power-supply filtering, to be discussed later in this chapter, makes it necessary

for the power-transformer VA rating to be greater than the wattage consumed by the load.

A high-quality, rugged power transformer, capable of providing the necessary currents and/or

voltages, is crucial in any power supply. The transformer is usually the most expensive component

to replace.

Rectifier Diodes

Rectifier diodes are available in various sizes, intended for different purposes. Most rectifier diodes are

made of silicon, and are known as silicon rectifiers. Some are fabricated from selenium, and are called

selenium rectifiers. Two important features of a power-supply diode are the average forward current

) rating and the peak inverse voltage (PIV) rating.

Average Forward Current

Electric current produces heat. If the current through a diode is too great, the heat will destroy the

P-N junction. When designing a power supply, it is wise to use diodes with an I

rating of at least

1.5 times the expected average dc forward current. If this current is 4.0 A, for example, the rectifier

diodes should be rated at I

= 6.0 A or more.

Note that I

flows through the diodes. The current drawn by the load is often different from this.

Also, note that I

is an average figure. The instantaneous forward current is another thing, and can

be 15 or 20 times the I

, depending on the nature of the filtering circuit.

Some diodes have heatsinks to help carry heat away from the P-N junction. A selenium diode

can be recognized by the appearance of its heatsink, which looks something like a baseboard radia-

tor built around a steam pipe.

Diodes can be connected in parallel to increase the current rating over that of an individual

diode. When this is done, small-value resistors should be placed in series with each diode in the set

to equalize the current. Each resistor should have a value such that the voltage drop across it is about

1 V under normal operating conditions.

Peak Inverse Voltage

The PIV rating of a diode is the instantaneous reverse-bias voltage that it can withstand without the

avalanche effect taking place. A good power supply has diodes whose PIV ratings are significantly

greater than the peak ac input voltage. If the PIV rating is not great enough, the diode or diodes in

a supply conduct for part of the reverse cycle. This degrades the efficiency of the supply because the

reverse current bucks the forward current.

Diodes can be connected in series to get a higher PIV capacity than a single diode alone. This

scheme is sometimes seen in high-voltage supplies, such as those needed for tube-type power ampli-

fiers. High-value resistors, of about 500 Ω for each peak-inverse volt, are placed across each diode

in the set to distribute the reverse bias equally among the diodes. In addition, each diode is shunted

by (that is, connected in parallel with) a capacitor of 0.005 µF or 0.1 µF.

Half-Wave Circuit

The simplest rectifier circuit, called the half-wave rectifier (Fig. 21-1A), has a single diode that

chops off half of the ac cycle. The effective (eff) output voltage from a power supply that uses a

338 Power Supplies

half-wave rectifier is much less than the peak transformer output voltage, as shown in Fig. 21-2A.

The peak voltage across the diode in the reverse direction can be as much as 2.8 times the applied

rms ac voltage.

Most engineers like to use diodes whose PIV ratings are at least 1.5 times the maximum ex-

pected peak reverse voltage. Therefore, in a half-wave rectifier circuit, the diodes should be rated for

at least 2.8 × 1.5, or 4.2, times the rms ac voltage that appears across the secondary winding of the

power transformer.

Half-wave rectification has shortcomings. First, the output is difficult to filter. Second, the out-

put voltage can drop considerably when the supply is required to deliver high current. Third, half-

wave rectification puts a strain on the transformer and diodes because it pumps them. The circuit

works the diodes hard during half the ac cycle, and lets them loaf during the other half.

Half-wave rectification is usually adequate for use in a power supply that is not required to de-

liver much current, or when the voltage can vary without affecting the behavior of the equipment

connected to it. The main advantage of a half-wave circuit is that it costs less than more sophisti-

cated circuits.

Half-Wave Circuit 339

21-1 At A, a half-wave

rectifier circuit. At B,

a full-wave center-tap

rectifier circuit. At C,

a full-wave bridge

rectifier circuit.

Full-Wave Center-Tap Circuit

A better scheme for changing ac to dc takes advantage of both halves of the ac cycle. A full-wave

center-tap rectifier has a transformer with a tapped secondary (Fig. 21-1B). The center tap is con-

nected to electrical ground, also called chassis ground. This produces voltages and currents at the ends

of the winding that are in phase opposition with respect to each other. These two ac waves can be

individually half-wave rectified, cutting off one half of the cycle and then the other, over and over.

The effective output voltage from a power supply that uses a full-wave center-tap rectifier is

greater, relative to the peak voltage, than is the case with the half-wave rectifier (Fig. 21-2B). The

PIV across the diodes can, nevertheless, be as much as 2.8 times the applied rms ac voltage. There-

fore, the diodes should have a PIV rating of at least 4.2 times the applied rms ac voltage to ensure

that they won’t break down.

The output of a full-wave center-tap rectifier is easier to filter than that of a half-wave rectifier

because the frequency of the pulsations in the dc (known as the ripple frequency) from a full-wave rec-

tifier is twice the ripple frequency of the pulsating dc from a half-wave rectifier, assuming identical ac

input frequency in either situation. If you compare Fig. 21-2B with Fig. 21-2A, you will see that the

full-wave-rectifier output is closer to pure dc than the half-wave rectifier output. Another advantage

of a full-wave center-tap rectifier is the fact that it’s gentler with the transformer and diodes than a

half-wave rectifier. Yet another asset: When a load is applied to the output of a power supply that uses

a full-wave center-tap rectifier circuit, the voltage drops less than is the case with a half-wave supply.

But because the transformer is more sophisticated, the full-wave center-tap circuit costs more than a

half-wave circuit that delivers the same output voltage at the same rated maximum current.

Full-Wave Bridge Circuit

Another way to get full-wave rectification is the full-wave bridge rectifier, often called simply a

bridge. It is diagrammed in Fig. 21-1C. The output waveform is similar to that of the full-wave cen-

ter-tap circuit (Fig. 21-2B).

340 Power Supplies

21-2 At A, the output of a

half-wave rectifier. At

B, the output of a full-

wave rectifier. Note the

difference in how the

effective (eff) voltages

compare with the peak

voltages.