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POWER SUPPLY FOR SOUND SYSTEMS 503
charger circuit of Figure 114. The transformer primary is adjustable between 110 and 125 volts. There are three secondaries, just as before. The rectifier tubes in this particular drawing are marked "V-l" and "V-2." The filaments are shown as inverted "UV rather than as inverted "V"s. The plate secondary of the transformer is tapped at the center, as in the charger. The real difference between Figures 114 and 115 lies in the d. c. circuits, after rectification has taken place; instead of a simple pair of d. c. output terminals, which are to be wired to a storage battery, Figure 115 shows the d. c. line branching into five parallel circuits, four of which are equipped with "brute force" niters.
To study Figure 115, look first (bottom center of drawing) at the terminals marked "24 volts, + and — ." The positive wire runs directly up to the mid-tap of the secondary ; the negative wire runs to the plates of the two tubes. Current, moving from negative to positive, flows from the filament of either tube to the plate of that tube, through the wire marked — , through the external load, back in at the wire marked +, and thence to the mid-point of the secondary. Thus the plates, while always negative with respect to the mid-point of the secondary, are alternately positive and negative with respect to their own filaments, precisely as in the case of the charger.
Branching from these two output wires and running off to the left are two complete filter circuits. The upper one terminates in two binding posts marked "Proj. No. 1, + and — ." This upper circuit filters and supplies d. c. for the exciting lamp and amplifier filaments of that projector. At the right-hand end of the filter is an adjustable rheostat, R-l, which may be set to secure the precise voltage desired. (23) From right to left, follow choke coils L-l, L-2 and L-3, and condensers C-l, C-2 and C-3, which constitute the filter itself. Resistor R-3, of 40 ohms, is a "bleeder" that helps R-l reduce the 24-volt output of the rectifier to the 12 volts required by the projector.
To the left of L-3 is S-l, a protective relay. When