Radio age (Jan-Dec 1926)

RADIO AGE for January, 1926 direction and then in the other without intermission. In Fig. 1 is a graphical representation of an alternating current showing the violent periodic changes that take place. All of the loops (P) above the reference line (X-X) are considered "positive" in direction, while the loops (N) below the line are "negative" and flow oppositely to the loops (P). The height (VP) above the reference line (X-X) indicates the maximum voltage or "amplitude" of the positive waves and (VN) is the voltage of the negative system, both sets of waves being equal, but occurring at different times. As the positive and negative waves are equal and opposite it is evident that their sum is zero. In "single wave" rectification all waves of a given polarity or direction are entirely suppressed leaving only half the waves effective. Thus, in Fig. 2 the negative waves (N) are stopped with only the positive waves (P) remaining. These are spaced widely apart by the full width (n) of the missing negative waves so that the impulses are intermittent and jerky. The average voltage is now only equal to the height (n) which is far below the value of the A.C. current, with the line (a-a) indicating the average D.C. output. The shaded portions between the waves (P) indicate the energy that must be supplied by the electrical inertia of the filter to maintain the voltage over the spaces (n) at which the supply voltage is zero. If it were not for the flywheel effect of the filter, the line (a-a) would also drop to zero at every point (n) and then cause a periodic humming in the receiver. Fullwave or "Two wave" rectification is shown in Fig. 3. In this case the positive waves (P) are allowed to pass freely through the rectifier as before, but instead of suppresing the negative waves they are turned up by an independent rectifier and made positive waves. The converted negative waves (n-n-n) are then inserted between the positive waves so that we now have twice the energy that we had in the half-wave system, and further, the application of the energy is more uniform with correspondingly less demand on the filter system. With an equal amount of filter inertia, indicated by the shaded areas, the mean or average D.C. voltage is increased to (m2) which means a greater effect than with the half-wave system, and the rectified voltage (a2-a2) is more nearly equal to the input voltage. For maximum results, full-wave rectification must be employed. Classes of Rectifiers Probably the simplest form of rectifier is the electrolytic cell rectifier shown in Fig. 4. This consists of a lead rod or negative electrode (Pb) and a positive electrode (Al) consisting of an aluminum rod. Both rods are immersed in a solution of borax or similar solution which will maintain a film of oxide on the surface of the aluminum rod. When the aluminum plate is formed, current will pass througn the cell in only one direction as indicated. The single cell performs halfway rectification but when two cells, or cells in multiples of two are connected up, full wave rectification can be had. This is identical with the rectifier used in charging storage "B" batteries. An electron tube, a common example of which is the Tungar bulb, is very commonly used as a rectifier. In this device a heated filament throws off electrons within an exhausted vessel and current is conveyed only in one direction in the electron stream. In Fig. 5 is a two element rectifying tube similar to a Tungar bulb. The filament (F) is heated to the point where electrons are freely emitted and the electrons form a path from the filament to the plate (P). A transformer (T) connected to the 110 volt A.C. lighting circuit supplies current for heating the filament and the two connections (A) and (B) are the terminals from which the half-wave rectified current is drawn. An ordinary three element radio receiving tube, such as a 201A, can also be used as a rectifier tube by connecting it according to the diagram in Fig. 6. The grid and plate are connected together to form a single electrode while the remaining connection is brought out from the filament transformer as before. Such an arrangement also functions as a half-wave rectifier, but its output is limited to a few milliamperes and hence is not desirable The Magazine of the Hour 39 for use as a rectifying medium in a "B" eliminator. For proper operation, a special rectifying tube should be used from which 50 to 60 milliamperes can be drawn without over-taxing the tube. Such tubes are now on the market but they have a filament which is subject to deterioration. Further, the tube types shown are of the half-wave type and for full rectification two tubes must be connected together. By the use of two independent filaments in one bulb full rectification can be obtained but there is only one tube of this type now on the market and that tube is difficult to obtain. The Raytheon tube, a full-wave rectifier, is one of the most interesting and efficient rectifying devices yet devised, and while it has the external appearance of an ordinary tube, yet it has no filament and takes no heating current in the ordinary sense of the word. Owing to the absence of a filament, the life of the Raytheon is indefinite, thousands of hours of operation being recorded during which there was no appreciable drop in the output. The action is rather peculiar and rather difficult to describe within this limited space so that our readers must be content for the time being with a description of its practical workings and application to the "B" eliminator. A general view of the Raytheon assembly is shown by Fig. 7. The elements are enclosed in the usual type of pear shaped glass tube which contains a highly exhausted atmosphere of helium gas, and at the bottom is the usual four prong socket which fits into a standard tube socket. On closer examination we see the elements are very different from those in a filament heated tube. There are two very small anode wires (el) and (e2) which are imbedded in the lava block (L) so only the ends are exposed, and above the anodes is the cap shaped cathode (C) which forms the abutting element. We therefore have the three elements necessary for full-wave rectification, the two anodes and the cathode, and these are connected to the prongs in the base of the tube. The discharge takes place between the anodes and the cathodes by a process of ionization of the residual helium gas, and (Turn to Page 42) Figure 1. The completed power supply device, looking from the rear.