Micros Systems Workstation 5, Руководство по обслуживанию

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Workstation 5 Field Service Guide 2-39
Workstation 5 System Board Technical Descriptions
System Board Power Distribution
Brightness Control
The controller uses a ‘burst’ dimming technique to control lamp brightness.
An analog voltage applied to the
BRIGHT
input determines the duty cycle
of a digital pulse width modulated (DPWM) in the range between 180Hz
and 440Hz.
When the DPWM signal cycles high, the lamp is driven at the selected
lamp frequency. This is also called the ‘burst’ period because of the lamp
frequency burst that occurs during this time.
During the low DPWM cycle, the controller disables the MOSFET gate
drivers, causing current to stop flowing in the lamp, but not long enough to
de-energize the lamps. Brightness is increased/decreased by modulating
the burst period duty cycle.
PWMBL from the system board supplies an analog voltage to the BRIGHT
input of the controller. The SVM (System Voltage Monitor) input to the
controller is connected across resistor network R18 and R19 and clamping
diode D2 to fully de-energize the lamps and reset the controller when the
backlights are off.