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Theory of Operation T400 Ozone Analyzer Operator’s Manual
300
13.3.7.2. O
3
Photometer UV Lamp Power Supply
The photometer’s UV lamp requires a high voltage AC supply voltage to create and
maintain its mercury vapor plasma arc. This AC voltage is produced by a variable
transformer, the primary of which is supplied by the output of a DC regulator (powered
by the instrument’s +15 VDC supply). A circuit made up of a control IC and several
FET’s, turns the transformer on and off converting it into a 30kHz square wave.
The DC regulator is controlled by a drive voltage supplied by an amplifier that adjusts
its output based on the difference between the rectified current output of the lamp and a
constant voltage resulting from a D-to-A converted “set-point” signal sent by the CPU
via the I
2
C bus. If the rectified current output by the lamp is lower than the CPU set
point voltage, the amplifier drives the regulator output voltage higher. If the current
output is higher than the set point voltage, the amplifier decreases the regulator output
voltage.
At start up, when there is no mercury vapor arc and therefore no current being output by
the lamp, the amplifier continues to drive the regulator output (and therefore the
transformer output) higher and higher until the mercury is vaporized and the plasma arc
is created (about 800 VAC). Once the arc is created, current begins to flow and the error
amplifier reduces the regulator/transformer output to a steady 200 VAC.
.
Figure 13-19: O
3
Photometer UV Lamp Power Supply Block Diagram
06870C DCN6332