transformer as the source of alternating voltage. The circuit is shown in figure 1-5. Note that the only real difference in this circuit from the previous ones is the transformer. The transformer secondary is connected in series with the plate circuit - where the plate battery was previously. ">

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  • Diode Operation with an Alternating Voltage on the Plate
  • After experimenting with a positive plate and a negative plate,
  • Fleming replaced the direct voltage of the battery with an alternating voltage.
  • In our explanation, we'll use a transformer as the source of alternating voltage.
  • The circuit is shown in figure 1-5.

Note that the only real difference in this circuit from the previous ones is the transformer. The transformer secondary is connected in series with the plate circuit - where the plate battery was previously.

Remember from your study of transformers that the secondary (output) of a transformer always produces an alternating voltage. The secondary voltage is a sine wave as shown in the figure.

You'll remember that the sine wave is a visual picture, a graph of the change in alternating voltage as it builds from zero to a maximum value (positive) and then drops to zero again as it decreases to its minimum value (negative) in the cycle.

Assume that the polarity across the secondary during the first half-cycle of the input ac voltage is as shown in the figure. During this entire first half-cycle period, the plate's polarity will be POSITIVE. Under this condition, plate current flows, as shown by the ammeter.

The plate current will rise and fall because the voltage on the plate is rising and falling. Remember that current in a given circuit is directly proportional to voltage.

During the second half-cycle period, plate's polarity will be NEGATIVE. Under this condition, for this entire period, the diode will not conduct. If our ammeter could respond rapidly, it would drop to zero. The plate-current waveform (Ip) in figure 1-5 shows zero current during this period.

Here is a summary of effects of applying alternating voltage to the plate of the diode:

  • Diode plate current flows during the positive half-cycle.
  • It changes value as the plate voltage rises and falls.
  • The diode cuts off plate current during the entire period of the negative half-cycle.
  • Diode plate current flows in PULSES
  • because the diode cuts off half the time.
  • Diode plate current can flow in only one direction. It is always a direct current. (In this case PULSATING DC - one that flows in pulses.)
  • In effect, the diode has caused an alternating voltage to produce a direct current.

The ability to obtain direct current from an ac source is very important and one function of a diode that you will see again and again wherever you work in electronics.

The circuits that we have discussed up to this point were chosen to show the general concepts discovered by Edison and Fleming. They are not practical because they do no useful work. For now, only the concepts are important.

Practical circuitry will be presented later in this chapter as you learn specific points about the construction, limitations, and other characteristics of modern diode tubes.

Q.5 An ac voltage is applied across a diode. The tube will conduct when what alternation of ac is applied to the plate? answer.gif (214 bytes)
Q.6 What would be the output of the circuit described in question 5? answer.gif (214 bytes)

DIODE CONSTRUCTION

Diode tubes in present use are descendants of Fleming's valve. There is a family resemblance, but many changes have been made from the original. Diodes are both smaller and larger, less powerful and more powerful, and above all, more efficient and more reliable. The search for greater efficiency and reliability has resulted in many physical changes, a few of which will be covered in the next paragraphs.

Most of what is said here about construction and materials will be true of all electron tubes, not just diodes.

Filaments

Modern filaments in ALL tubes last longer, emit greater amounts of electrons for a given size, and many operate at a lower temperature than in the early days. Most improvements have resulted from the use of new materials and from better quality control during manufacture.

Three materials that are commonly used as filaments are tungsten, thoriated tungsten, and oxide-coated metals.

Tungsten has great durability but requires large amounts of power for efficient thermionic emission. Thoriated-tungsten filaments are made of tungsten with a very thin coat of thorium, which makes a much better emitter of electrons than just tungsten. Oxide-coated filaments are made of metal, such as nickel, coated with a mixture of barium and strontium oxides. The oxide coat, in turn, is coated with a one-molecule-thick layer of metal barium and strontium. Oxide coating produces great emission efficiency and long life at relatively low heat.

A major advance in electronics was the elimination of batteries as power sources for tubes. Except in electronic devices designed to be operated away from the ac power source, alternating current is used to heat filaments.

Voltage may be supplied by a separate filament transformer or it may be taken from a filament winding that is part of a power transformer. The actual voltage may vary from 1 volt up and depends on the design of the tube. Common filament voltages are 5.0, 6.3, and 12.6 volts ac. Filaments may be connected in series with other tube filaments or may be in parallel with each other. This is determined by the equipment designer.

Cathodes

As was mentioned previously, a more formal name for the electron-emitting element in a tube is the CATHODE.

Cathodes in all tubes, not just diodes, are of two general types, either directly heated or indirectly heated. Each has its advantages and disadvantages.

DIRECTLY HEATED. - The filament that has been discussed so far is the directly heated cathode. Directly heated cathodes are fairly efficient and are capable of emitting large amounts of electrons. Figure 1-6 shows this type and its schematic symbol.

Figure 1-6. - Cathode schematic representation.

0008.GIF (11206 bytes)

An added advantage of this type of filament is the rapidity with which it reaches electron-emitting temperature. Because this is almost instantaneous, many pieces of electronic equipment that must be turned on at infrequent intervals and be instantly usable have directly heated cathode tubes.

There are disadvantages. Because of its construction, parts of the filament are closer to the plate than other parts. This results in unequal emission and a loss of efficiency. Another disadvantage occurs when dc is used to heat a filament. The filament represents a resistance. When current flows through this resistance, a voltage drop occurs. The result is that one side of the resistance, or filament, is more negative than the other side. The negative side of the filament will emit more electrons than the positive side; which, again, is less efficient than if the filament has equal emission across its entire surface.

When ac is the source of filament power, it causes a small increase and decrease of temperature as it rises and falls. This causes a small increase and decrease of emitted electrons. This effect is not too important in many diode circuits, but it is undesirable in other tube circuits.

INDIRECTLY HEATED. - Figure 1-7 shows this type of cathode and its schematic symbol. Indirectly heated cathodes are always composed of oxide-coated material. The cathode is a cylinder, a kind of sleeve, that encloses the twisted wire filament. The only function of the filament is to heat the cathode. The filament is often called a heater when used in this manner.

Figure 1-7. - Indirectly heated cathode schematic.

0009.GIF (8976 bytes)

Some schematics do not show heaters and heater connections. Heaters, of course, are still present in the tubes, but their appearance in a schematic adds little to understanding the circuit. The heater is not considered to be an active element. For example, a tube with an indirectly heated cathode and a plate is still called a diode, even though it might seem that there are three elements in the tube.

Because indirectly heated cathodes are relatively large, they take longer to heat to electron-emitting temperature. Once up to temperature, however, they do not respond to the small variations in heater temperature caused by ac fluctuations. Because of the inherent advantages, most tubes in use today have indirectly heated cathodes.

Q.7 Besides tungsten, what other materials are used for cathodes in vacuum tubes? answer.gif (214 bytes)
Q.8 What is the advantage of directly heated cathodes? answer.gif (214 bytes)







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