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1. The Need for Rectification
Most power sources supply alternating current (ac) because of the numerous advantages connected with its generation and transmission. Practically all electron tube circuits and many industrial processes, however, require DC supply voltages and currents, and thus the need arises to convert alternating current to direct current.
A rectifier is capable of changing alternating current into pulsating direct current; to obtain smooth DC power a filter system must be added. A complete rectifier system usually also contains a voltage divider for providing direct currents at various desired potentials, and a voltage regulator to minimize the effect of load and supply voltage variations on the DC output voltages.
2. Basic Principle of Rectification
Rectifiers change AC into pulsating DC by eliminating the negative half-cycles or alternating the AC voltage. Only a series of sinewave pulsations of positive polarity remain. An ideal rectifier may be thought of as a switch that closes a load circuit whenever the polarity of the alternating voltage is positive, and opens the circuit whenever the alternating voltage is of negative polarity.
A switch operating with such synchronism would have effectively zero resistance for that half of the time when the circuit is closed during positive half-cycles and infinite resistance for the other half of the time when the circuit is open during negative half cycles. Practical rectifiers do not attain this goal, but come close to it. Some vacuum tube rectifiers may have almost infinite resistance during the nonconducting interval (back resistance), but the resistance during the conducting interval (forward resistance) is never zero or even constant. Regardless of their approach to the ideal, all rectifiers must provide a one-way path for electric current; that is, conduction must take place primarily in one direction only. This is called unilateral conduction, or a unidirectional characteristic.
3. The Diode as Rectifier
A diode is a device consisting of two elements or electrodes: one is an electron emitter (cathode), the other an electron collector (anode or plate). Since electrons in a diode can flow only from emitter to collector, the diode provides unilateral conduction and hence, rectification. The voltage-current characteristics of an ideal diode and of three practical diodes are illustrated in Figs. 1 through 4.
Ideal Diode. For comparison, Fig. 1 illustrates the characteristic of an ideal diode. It acts like the synchronous switch discussed above. When the voltage (Eb) at the electron collector is negative, the back resistance (Rb) of the device is infinite and no current (I_b) can flow. When Eb is positive, however, the device conducts, the forward resistance (Rr) is zero and the current (h) is effectively infinite. In practice, the current magnitude is limited, of course, by the internal resistance of the power source and the resistance of the load circuit.
Thermionic Vacuum Diode. The I_b-E_b characteristic of a thermionic vacuum diode 1s shown in Fig. 2. Such a diode consists ...
... of a directly heated cathode (emitter) and a plate or anode (collector), both enclosed in a highly evacuated glass or metal envelope. When the cathode is heated to its proper operating temperature it emits from its surface a copious supply of electrons which form a negative space charge between cathode and plate.
When a potential (E_b) is placed on the plate that is positive with respect to the cathode, electrons are attracted away from the space charge and travel toward the plate as a plate current (I_b)· The higher the value of the positive plate voltage, the greater is the flow of plate current through the tube. At each plate voltage value, the total plate current that flows is limited by the amount of the negative space charge present. When the positive plate potential is made very high, however, a point is reached where the entire space charge is attracted toward the plate and electrons reach the plate in the same proportion as when they were emitted. The plate current then levels off and emission saturation takes place. Further increases in the positive plate voltage cannot cause an increase in the plate current since the entire supply emitted by the cathode is already being drawn to the plate. In practice, thermionic diodes are always operated in the space charge - limiting region to avoid damage to the cathode and to keep tube voltage drop (E_b) at a reasonable value.
When the plate voltage of a diode is made negative with respect to the cathode, the electrons in the space charge are strongly repelled from the negatively charged plate and some are actually driven back into the cathode. Since no electrons reach the plate, no plate current flows, and the tube acts essentially as an open circuit. From the above it may be concluded that electron flow within a diode is always from cathode to plate, provided the plate is made positive with respect to the cathode.
Since the plate current is limited when the diode conducts, the tube exhibits a certain forward resistance (Rr) defined as E_b/I_b for E_b>0. This forward resistance varies from about 100 to 1000 ohms for practical vacuum diodes. The back resistance (Rb) DC fined as Eb/I_b for Eb<0, has a range from about 100 to 1000 megohms, or about a million times as high as the forward resistance. Vacuum diodes have practically perfect unilateral conduction and effectively present an open circuit to negative plate voltages Thermionic Gas Diode. The construction of a thermionic gas diode is similar to that of vacuum diodes, except that a certain amount of gas is purposely introduced into the tube envelope.
Ordinarily mercury vapor in equilibrium with liquid mercury is used, but occasionally an inert gas such as helium, neon, or argon may be employed. The presence of a gas radically affects the behavior of the tube.
The I_b-E_b characteristic of a typical gas diode is illustrated in Fig. 3. Note that plate current (I_b) increases gradually at first with increasing (positive) plate voltage, as in a vacuum diode.
At a certain critical plate voltage, known variously as ionization potential, striking potential, or firing point, the plate current suddenly increases to a very large value, almost equal to the total electron emission of the cathode. What happens is essentially this: at a certain value of the plate voltage (striking potential), electrons colliding with gas molecules in their journey to the plate attain enough speed and energy to strip off one or more electrons from the outer shell of the molecules. This leaves the affected molecules with a net positive charge and they are then known as positive ions. The electrons freed from the gas molecules join the original electron stream and liberate still more electrons by colliding with other gas molecules. This process, called ionization, is cumulative and results in the sudden, dramatic increase in the plate current, evident in the I_b-E_b characteristic.
Although full electron emission is reached almost at once with the onset of ionization, the large emission current cannot be utilized in practice because of the presence of the positive gas ions.
These positive ions drift toward the negative cathode, where they combine with electrons to re-form neutral gas molecules. If the voltage across the tube is too high (more than about 22 volts), the heavy, positive gas ions strike the cathode with force which eventually destroys it. For this reason the external circuit of a gas rectifier is always arranged to keep the tube voltage drop at about 15 volts and the plate current safely below the total cathode emission.
Once ionization has started, the action maintains itself at plate to-cathode voltage considerably lower than the ionization potential.
There is a minimum voltage called the de-ionizing or extinction potential, below which ionization cannot be maintained. The gas then de-ionizes and conduction stops. The tube acts as an electronic switch which closes at the striking potential, permitting a large current to flow, and opens at the extinction potential blocking the current flow.
When a negative potential is applied at the plate of a gas diode, ionization cannot take place and the electrons in the space charge are repelled from the plate, just as in a vacuum diode. Then the tube has the unidirectional characteristic required for rectification.
Caution must be exercised when placing gas tubes in AC circuits where the polarity of the plate voltage continually reverses. If the negative (or inverse) plate voltage is too high, or the frequency of the AC supply is too great, the positive gas ions may not have sufficient time to combine with free electrons and stop ionization before reaching the plate. The gas ions will then strike the negative plate, constituting an inverse current flow, or arcback. The tube carries a heavy current on both AC half-cycles, which might destroy it. It is important to know the inverse-voltage rating of the tube at the operating frequency.
Arcback is also affected by the ambient temperature (and corresponding mercury-vapor pressure) of the tube. If the tempera ture (and pressure) is too high, the inverse voltage at which arcback occurs becomes abnormally low. If the temperature (and pressure) is too low, the intensity of ionization is reduced to a point where the space charge is not completely neutralized and the plate current is low. Moreover, the voltage drop across the tube becomes excessive at low temperatures, and cathode disintegration may occur. Normally, the voltage drop across the tube is low (between 12 and 20 volts) and remains constant for large variations of the plate current. This means that most of the available supply voltage will appear as rectified output voltage and little will be wasted across the tube. The low internal tube drop is one of the big advantages of gas diodes.
Among the disadvantages of gas diodes, besides the somewhat erratic behavior and arcback, is the preheating time. The cathodes of mercury-vapor tubes must be brought to normal operating temperatures for one to two minutes before the plate voltage is applied to permit the mercury to be completely vaporized and electron emission to reach its full value. Otherwise, the tube cannot carry its rated plate current and may be damaged.
Because of the relatively heavy plate currents and low voltage drop of gas-filled diodes their forward resistance (Rr) is low, about 1.5 to 100 ohms for representative commercial types. The back resistance (Rb) in the absence of arcback is several hundred megohms. With the exception of the disadvantages stated, there fore, gas-filled diodes make excellent rectifiers.
Crystal Diode. Another important category of diode rectifiers is the crystal or semiconductor diodes. Current conduction is attained in certain semiconductor crystals such as germanium and silicon by a process too complex to be covered here; suffice it to say that definite amounts of impurities (doping) introduced into these crystals will permit an electron current to flow from emitter to collector, but not in the opposite direction. When low positive voltages (forward bias) are applied to the collector, relatively large forward currents flow from emitter to collector, as shown in Fig. 4. The forward resistance of crystal diodes therefore is low, from about I to 500 ohms.
When a voltage of opposite polarity (reverse bias) is applied to the crystal diode, a small reverse current ( I.) flows. This reverse current is merely a few microamperes as long as the reverse bias is not made too high. The corresponding back resistance (Rb) varies from about 100 kilohms to several megohms.
When the reverse voltage is made very high a breakdown occurs and the reverse current suddenly increases to relatively large values.
This is shown in Fig. 4, where for the purposes of clarity the magnitude of the reverse current has been greatly exaggerated.
When the excessive negative voltage is removed, conditions return to normal.
While the circuits of crystal diode rectifiers are similar to those of vacuum diodes, a considerable number of changes must be made in practice to take care of different voltage and current requirements.
4. Triode Rectifiers Grid-controlled tubes, or triodes, are sometimes used as rectifiers.
The use of the grid permits regulating the power delivered to the load by controlling the time during which the current flows during each cycle. The tubes used for this purpose are almost exclusively gas-filled types called thyratrons. Since this volume is primarily concerned with vacuum tubes, thyratron rectifiers will not be covered here.
5. Design Ratings of Thermionic Diodes The important design ratings that determine the performance of hot-cathode (thermionic) diode rectifiers and hence influence the selection of tubes for a specific type of service and circuit, are:
(a) The maximum allowable peak plate current (the maximum current which may be allowed to flow at any time) which is determined by the maximum usable cathode emission over the life of the tube. The cathode must at all times maintain a full space charge, hence, the peak plate current is always less than the total emission of the cathode during the useful life of the tube.
(b) The maximum allowable average plate current (the maxi mum value of average plate current which may be carried continuously without overheating), or DC output current. This can never exceed one-half of the peak plate current, since current flows through the rectifier tube only half the time, during positive AC half cycles. The maximum allowable value of the average plate current depends on the permissible plate dissipation (heating of the plate) of the tube. Its value usually turns out to be considerably less than one-half of the peak plate current.
(c) The maximum allowable inverse plate voltage (the largest Voltage which may occur at the negative peak of the cycle without breakdown). It is also the largest negative voltage that may be applied safely to this tube. Its value also determines the maximum positive plate voltage that may be applied to the tube, and thus to the DC output voltage. The exact DC output voltage obtained from the tube depends on the rectifier circuit (as we shall see later), but it is generally less than the allowable inverse plate voltage.
(d) For high-vacuum diodes the voltage drop across the tube (the voltage across the tube during the conduction interval) is determined by the permissible plate current; for mercury-vapor diodes it is more or less constant, as explained before. The product of the tube voltage drop and the allowable plate current is the amount of plate dissipation that must be designed into the tube if it is to handle the rated current.
The tube voltage drop is also an important factor in determining the regulation of the DC output voltage; that is, the value of the DC output voltage for varying load currents. As we have seen, the low, constant tube drop of mercury-vapor tubes provides the advantage of wasting little voltage across the tube, and of keeping the regulation for varying load currents to a minimum change.
Actual design ratings and other data useful for various representative types of high-vacuum and gas-filled diodes are listed in ...
Chap. 5, Table 1. Regulation no-load voltage - full-load voltage
X 100 full-load voltage
• This statement is not strictly correct. The determination of actual plate dissipation in a tube is fairly complicated and depends on the type of rectifier and the type of output filter used. For capacitor input filters, the dissipation per plate is:
WP=K e_d I_b
where WP= watts dissipation per plate
ed = peak voltage drops across the diode with peak current flowing
I_b = DC per plate.
K = a constant having the value 0.84 for vacuum diodes, 0.90 for semiconductor diodes and 1.00 for gas diodes.
For a rectifier with a choke input filter the dissipation is:
K WP=Nlo ed
where N = 2 for single-phase full-wave and 1_phi, bridge circuits
N = 3 for three-phase half-wave and 3_phi, bridge
N = 6 for three-phase full-wave 10 = DC output
ed = diode drop at current I_0
K = a constant whose value depends upon the ratio of peak ripple current to DC. Curves showing the value of this constant are available if desired.
Thus a diode must be designed for the dissipation which would result in the worst case--with a capacitor input filter. For vacuum diodes the formula then would be:
6. Diode Rectifier Circuits
The type of circuit in which diode rectifier tubes are used determines the attainable DC output and voltages, the smoothness of the DC, the amount of filtering required, and other important factors. Rectifier circuits may be classified as either single-phase or polyphase, depending on whether the AC power has one or more phases.
Single-phase circuits are used primarily for the relatively small plate-power requirements of various electronic devices; radio and television sets, public address systems. In contrast, polyphase rectifier circuits are used for high-power requirements demanding economical operation. The capabilities of power transformers and tubes are more fully utilized in polyphase circuits. They have the additional advantage of delivering a DC output that is smoother and requires less filtering than the DC of single-phase circuits.
7. Review Questions
(1) Explain the basic function of a rectifier and the operation of an ideal rectifier.
(2) What is a diode? Distinguish between various types and describe their physical characteristics.
(3) How does the I_lb-E_b characteristic of a gas diode differ from that of a vacuum diode, and why? How do both differ from the characteristic of an ideal diode? What are the advantages of each?
(4) What is meant by the breakdown of a crystal diode and how will the reverse current affect rectifier operation?
(5) Define forward resistance and back resistance and state the approximate range of values for each type of diode. What is their significance for rectifier operation?
(6) Define maximum allowable peak plate current, average plate current, peak inverse voltage and tube voltage drop; explain the significance of these tube design ratings.
(7) Distinguish between single-phase and polyphase rectifiers and describe the major uses and advantages of each.