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POWER SUPPLY FOR SOUND SYSTEMS 501
is negative with respect to the upper end of the secondary it must of necessity be positive with respect to the lower end of that same winding. Therefore when the automatic vacuum-switch of bulb No. 1 is open that of bulb No. 2 is closed, and vice versa.
When the plate of bulb No. 1 is positive the plate of bulb No. 2 is negative, and vice versa. When current flows across the vacuum of bulb No. 2, none flows across the vacuum of bulb No. 1.
Thus, with both tubes in operation, the d. c. output is nearly continuous because the same instant current stops Mowing across one vacuum it starts to flow across the other.
The complete circuit of Figure 114 is therefore a fullwave rectifier, consisting of two complete half-wave circuits so arranged that each rectifies one-half of the a. c. cycle.
Power Supply Rectifier
If the rectifier-type charger will deliver d. c. to charge a storage battery, which in turn provides d. c. to operate the sound system, .why not use the charger itself to operate the sound system, and thus eliminate the battery?
In the past few years a great many theatre storage batteries have been eliminated by precisely this process.
However, the d. c. furnished by the charger cannot be used for sound purposes without filtering. The ripple in the rectified current would create hum in the sound. The (21) power supply rectifier is simply a full-wave battery charger with a "brute force" filter added.
(22) The schematic circuit of one such device is ^hown in Figure 115. It is an extremely simple circuit that the projectionist can trace without difficulty if he understands and remembers the battery charger circuit just considered, and the explanation of "brute force" filters given on Page 475.
Circuit of a Power Supply Rectifier
The upper portion of Figure 115 is an ordinary fullwave rectifier, identical except in minor details with the