ProStar325 Service Manual
74
Publication date 8 May 2003
The Processor PWB assembly contains a number of independent
power supply circuits for regulating the voltage supplies needed
by the instrument. Most of these supplies use linear regulators.
Consequently the Processor PWB only requires low voltage input
supplies for producing all the necessary voltages.
The supplies for the D2 and visible source lamps use switch mode
regulation techniques for supplying their respective lamps.
Linear regulators
U52, U53, U54 and U55 provide the necessary dc voltages to the
rest of the Processor board.
4.2.2
Deuterium (D
2
) lamp power supply
The deuterium lamp supply provides power for the filament and
the arc in the lamp. It allows the flexibility of maintaining some
filament drive when the arc is struck. It consists of a current
limit foldback circuit and a current source. The sequence of
operation to start the lamp is: Assert D2FILON# (this turns on
the filament supply at full voltage and the arc supply with the
current source turned off), Wait 10 seconds for the filament to
warm up, assert D2ON. During the warmup time the arc supply
is charging C226 to approximately 475V to provide the strike
voltage. When D2ON is asserted the current source transistor,
Q8, turns on and tries to sink 300mA. This places 475V across
the lamp via R194 and R241. The lamp starts conducting and is
carried into the negative resistance region by the current from
C226. The voltage on the anode drops below the voltage on D14
anode and the current starts to flow from the run supply. The
current builds up to 300mA and the current source starts to
regulate causing the voltage on Q8 drain to go up and the
controller to reduce the pulse width due to the action of D15.
Filament supply
The filament supply is an isolated flyback converter running at
approximately 220 kHz. Feedback is provided by the network
from D24 to R219 which senses the voltage pulse on the primary
reflected from the secondary. R221 changes the output voltage
for warmup and run modes of operation. D22 turns off the
filament supply when D2FILON is negated. D23 is a snubber to
reduce the spike on the transformer primary due to leakage
inductance of the transformer. U43 senses the average voltage
on R226 and outputs D2FILOK# to the processor board when it is
above a threshold.
Arc supply
The arc supply is a discontinuous current boost converter with an
extra tap on the transformer running at 180kHz. It has 2 modes
of operation. During filament warmup it operates with Q8 off and
generates 475V on D2ANODE and the filament terminals can
float up to a high voltage so the arc is not started. During run
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Summary of Contents for ProStar 325 UV-Vis
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