Voltage Sources; The Display; The Keyboard; Tungsten Lamp Power Supply - Thermo Scientific GENESYS 20 Service Manual

Electrical Circuit Description and Adjustment
op-amp offset voltage. The comparator function is returned to U41 by setting
/ZERO HIGH and the next signal to be measured is selected. The MEASURE
signal is then set HIGH to start the capacitor discharge and enable the output
latch.
The time from setting MEASURE to HIGH to begin the measurement until a
falling edge occurs at the output latch is precisely measured by a microcontroller
timer. Measurements are made directly in terms of absorbance as a result of the
exponential decay. All other conversions to transmittance, concentration, or
voltage are carried out by the microprocessor.

Voltage Sources

U28, U29, and U30 provide +5, +8, and -8 volt references for use by the analog
electronics. Double regulation is used to provide a level of isolation from the
noisy digital electronics.

The Display

The display is a 2 line by 20 character liquid crystal type with a back light. A
contrast potentiometer (R1) is accessible by the customer. U18 latches the data
for the display. All drivers are located on the display itself.

The Keyboard

The ten keyboard keys are arranged in a matrix of rows and columns. A key
press electrically connects the row and column of that key. U20 sends a test
pattern with one LOW bit to the columns and U19 is used to read back data from
the rows. A LOW bit received back indicates a key pressed at the intersection of
the column where the LOW signal was sent and the row where the LOW signal
was read from.

Tungsten Lamp Power Supply

The Tungsten lamp supply is a buck-mode (step-down) switching regulator
controlled by U26 and operating at a frequency of 72 kHz. The lamp is run in
constant-current mode and operates at a voltage near 6.0 volts. A pulse-width
modulated (PWM) signal from the microprocessor is filtered by D3, R9, R10,
and C16 to establish a bias voltage which U27 maintains across the lamp current
sense resistor R12 through feedback with C20 and R14. Increasing the duty
cycle of the PWM signal increases the bias voltage and thus the current through
the lamp. Q1 turns the lamp on and off. Ferrite bead L1 keeps the 72 kHz
frequency and its harmonics from feeding back to other circuits or into the
mains.
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