TM 5-6675-316-14
4-3.2 Detailed Theory of Operation.
a.
Illumination.
Two encapsulated, cold-cathode, argon mercury light grids
each provide a maximum of 2500 ft lamberts of brightness through viewing surfaces.
Intensity of light is controlled by a dimming circuit. Light can be reduced to 20-
percent of maximum value.
(1) 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 grids. The
(2) Alternating current voltage 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 ex-
ternal high-voltage transformer to light the grid lamp. The SCR’s prevent all
of each ac half-wave from being transferred, but when triggered 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
4-6
