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Theory of Operation
Spectrophotometer Lamp Supply (SLS) Board
Deuterium Lamp Filament Control
During the ignition cycle two different voltage levels are applied to the
filament of the deuterium lamp by the deuterium lamp filament control
circuit. This control circuit is enabled by the processor on the SPM board.
Deuterium Lamp Current Control
The deuterium lamp current control circuit comprises two parts. One
generates an ignition pulse of 600 V DC for the lamp, resulting in lamp
ignition. After the ignition process this voltage is disabled. The second is a
constant current source of 320 mA at an operating voltage between
50 and 105 V DC for stable operating conditions and light emission of the
deuterium lamp. The deuterium lamp current control circuit is supervised by
the processor on the SPM board.
Igniting the Deuterium Lamp
The deuterium lamp is heated with a voltage of 1.7 V DC for 1 s and
afterwards a voltage 2.5 V DC for 9 s with currents between 2–10 A prior to
ignition. The deuterium lamp current control circuit provides an ignition
pulse to the lamp, resulting in lamp ignition. The filament control circuit
disables the filament voltage if the lamp has been successfully ignited.
If the deuterium lamp has failed to ignite, the whole sequence is repeated. If
after the second attempt the deuterium lamp did not ignite, an error message
occurs.
Tungsten Lamp Voltage Control
Voltage to the tungsten lamp is generated by the tungsten lamp voltage
control circuit which is enabled by the processor, resident on the SPM board.
This circuit provides a constant voltage of 6 V DC with currents between 0.7–
0.9 A to light the tungsten lamp.
Diagnostic A/D Converter
The diagnostic A/D converter senses currents and voltages of the deuterium
and tungsten lamps and converts the analog signals into digital values. The
digital values are transferred via the control bus to the SPM board. When
values are outside of the normal range, an appropriate error message is
generated.
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