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1953-54 Theatre Catalog, 11th Edition, Page 100 (66)

1953-54 Theatre Catalog, 11th Edition
1953-54 Theatre Catalog
1953-54 Theatre Catalog, 11th Edition, Page 100
Page 100

1953-54 Theatre Catalog, 11th Edition, Page 100

Fig. 2. Open circuit voltage applied to the lamp

tor the same point as Fig. l. The high peak

voltage is a result of the steep rise on the voltage

at Fig. I. applied to the L-C combination in the

ballast. This peak insures starting and rte-ignition

oi each cycle. This oscillogram illustrates this voltage condition.

trolled by the grid voltage. A sine wave, variable in phase relationship to the line voltage is applied to the grid of the tube and the tube will fire or conduct when its plate is positive with respect to the cathode and its grid voltage is'essentially zero. Referring to the circuit diagram of Figure 3, each grid derives its voltage from two coils in series, one on T., which is constant in voltage and phase, and one on T: which is variable in voltage and phase. The sum of these two voltages result in a sine voltage of constant

Fig. 4. Vector diagram-grid circuit voltages.

amplitude but variable in phase relationship. In this way the point in the cycle at which the grid is zero volts with respect to the cathode can be controlled. Figure 4 represents a vector diagram of the grid circuit. Vector AC represent the phase of the applied line voltage. V1, is the voltage of one of the secondaries of T: and Vm the secondary voltage on T. in series with V].. Va is the proportional drop across the total resistance in series with the primary of T2. The voltage 0 B is applied to the grids of the tubes properly phased for control. Point B will follow the semi-circular locus as R is varied resulting in complete 1800 control of the conduction angle. The control elements of the circuit are resistors R... and R. one of which can be adjusted so the lamps will not be taken below the minimum brightness and the

-...."..........r............;.....p........,..... ) g '

Fig. 3. Circuit diagram ot the dimming control. Each grid deriVes its voltage from two coils in series.

other selects the brightness at any point in the range. Rotating the control, changes the resistance in series with the primary of T2. Since both tubes are controlled by the secondaries of T2 this single control regulates both tubes.

A small capacitor (C. and C2) is connected between grid and cathode of each thyratron tube to suppress line surges. Without suppression these surges can cause the tubes to fire prematurely in the cycle giving a iiash of light. The resistors R. and R2 are used to limit the grid current to a small value while the grids are positive. R3 tends to stabilize the lamp operation at very low levels by draining off any residual charge from the yellow line after each pulse of current.

The angle at which the tubes start conduction is a function of the ratio of

Fig. 5. Current waveshape corresponding to 20%. 40%, 60%, 80%, and 100% brightness.

the sum of R., R5, R5 and the primary resistance of T2 divided by the inductive impedance of the primary of T:.

Figure 5 shows the entire cycle of current waveshape for five ratios of R/XL (.850, 1.08, 1.32, 1.65 and 1.93). These points correspond to the 20 percent, 40 percent, 60 percent, 80 percent and 100

Fig. 6. Light output waveshapes corresponding to points 01 Fig. 5. Bottom line is zero light reference.

percent light output points. Figure 6 shows the light output wave-shapes for the corresponding ratios of Figure 5. The bottom line of this figure is the zero light reference level. Figures 7, 8, and 9 show the relation of 1!. M. S. lamp current, average light output and conduction angle versus this ratio of Rg'XL.

Lamp flicker at full light output corresponds to single lamp operation with

1953-54 Theatre Catalog, 11th Edition, Page 100