TM5-6675-326-14
2-3.2 Detailed Theory of Operation
2-3.2.1 I l l u m i n a t i o n. T w o e n c a p s u l a t e d , c o l d - c a t h o d e , a r g o n m e r c u ry
light grids each provide a maximum of 2500 footlamberts of brightness
through viewing surfaces.
Intensity of light is controlled by a dimming
c i r c u i t .
Light can be reduced to 20 percent of maximum value.
Current (120 v, 50/60 Hz) is passed through the main power switch.
The illumination control panel on the switch operates the fan and light
g r i d s.
The increase potentiometer controls voltage to the dimmer boards.
Alternate current enters the dimmer circuit boards through pin A and
is transferred to pin B through silicon-controlled rectifier (SCR) Q1 or
Q2, which determines the amount of the ac sine wave voltage that is
applied to the external high-voltage transformer to light the grid lamp.
The SCRs prevent all of each ac half-wave from being transferred, but when
t r i g g e r e d,
allow current to flow during the remaining portion of each half
sine wave until the zero crossing point is reached, whereupon the SCR is
turned off and the ac waveform is again blocked.
When the ac voltage enters the control board, it is applied to Cl and
R1 which provide a slight delay in the input voltage, which is applied to
the timing circuit composed of capacitor C2 and resistors R4, R3, R5, and
external potentiometer 1R1.
(Note that resistor R6 is not used in this
application.)
As the delayed ac voltage is applied across capacitor C2, the
capacitor begins to charge at a rate depending
potentiometer 1R1.
The voltage across C2 also
CR1 and trigger diode CR2.
upon the setting of
a p p e a r s a c r o s s r e c t i f i er
2-6
