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INTERNATIONAL PROJECTIONIST
VOLUME VIII
NUMBER 5
MAY 1935
STEP-BY-STEP ANALYSIS OF SOUND REPRODUCING EQUIPMENT
Aaron Nadell
X. Control Circuit Analysis
FIGURE 1 is the schematic drawing of a voltage control cabinet designed to correct variations in commercial a. c. supply to the projection room. It looks simple but is really a bit tricky, and is ideally suited for those projectionists who are interested in analyzing circuits.
The two tricky items in Figure 1 are the transformer and the lower left-hand switch. The transformer layout is unusual in that all its windings seem to be connected to the source of power and consequently there may be some difficulty in determining just which winding is the primary. The lower left-hand switch, KS-6203, is extraordinary in that it is wired precisely like a polarity-reversing switch in a d. c. circuit, when it obviously operates only on a. c.
The key to understanding this drawing lies in studying Coil 5-6 of the trans
former. This coil is connected in series with both the load and the line. Obviously, current flowing from the line to the load must pass through this coil, and it is equally plain that any SeCOnd
LINE 'IIOV AC SUPPLY
LOAD 9 9
1*16625 SWITCH
[, HART MTG CO
I 20 AMP
rusts
0-»30 ./AC WESTON MOCE-. *42« [
vOLTMCTEB
KS-6203 SWITCH
k.S-6204 SwiTCl
Figure 1 [7]
ary voltage generated in that coil must either aid or oppose the line voltage.
Coil 5-6, then, is the secondary of the transformer, the purpose of the polarity-reversing switch in an a. c. circuit being revealed at once. By reversing the polarity to the transformer primary, the voltage generated in Coil 5-6 will either increase the line voltage (because it is both in series and in phase with the line), or it will reduce the line voltage because it is in series but 180 degrees out of phase, and therefore "bucking" the line.
Function of Switch KS-6204
One small puzzle remains to be solved, and that is the function of the right-hand switch, KS-6204. The fact that the primary of this transformer is wound as two separate coils, 1-2 and 3-4, suggests the answer. This must be a step-down transformer, since 110 volts flows in the primary and only a small bucking or reinforcing voltage is desired in Coil 5-6.
Now, the degree of step-down must