TM8400B-L-00
1 November 2004
transistor Q101 is turned on, to apply power to and illuminate the front panel "check heater"
l.e.d.
Continuous cooling of the tank is applied through a thermoelectric cooling module. This module
forms part of the temperature control circuit but is powered on permanently to provide cooling at
a constant rate. Power rectifier U109 supplies a rectified nominal 12 V DC to the cooling
module.
Triac DT1 and opto-isolator circuit U110 shown in the schematic of the Power Supply and
Heater Control Card (18962.01.04) act to remotely switch the pump motor power from a low
voltage switch mounted on the front panel.
9.4. CONDUCTIVITY MEASURING CIRCUIT
The following references are to the Chopper and Conductivity circuit board schematic
18961.01.04 and assembly 18961.01.02.
The chopper circuit provides complimentary chopping or reversing signals, and non-overlapping
sampling signals. The timing of these signals is shown in figure 9.1 in the calibration section of
this manual.
A precision voltage reference U201 is provided on the board. The precisely regulated reference
voltage is divided by R248 and R249, and also inverted by U209 providing a negative reference
which is also divided down. The two references are alternatively selected by U211 which is
controlled by the two chopper signals. This reference is buffered and sent to the cell current
comparator circuit, and to the suppression resistor ladder.
The voltage across the conductivity cell potential arms is buffered by U200 and U203. This
potential is compared to the reference signal by U208 which varies the cell current to balance
these potentials. U208B alternatively sources and sinks the cell current from the C1 (current)
arm of the cell, and U208D does the same to the C2 arm. U208D compares the voltage at the P2
arm with signal ground, and drives C2 up or down to hold P2 at ground. This prevents leakage
current from flowing through the heat exchanger which is also at ground potential.
The current from U208B actually flows through some matching resistors before reaching the
current arm. R47 (mounted on the front panel) is the standardize control, R203 is the precisely
matched cell matching resistors, R205 through R209 are the temperature compensating resistors,
which along with R204 and R210 compensate the conductivity signal for the six temperatures in
measurement of the sample. R202 replaces the conductivity cell when the circuit is switched to
standby. The conductivity signal itself is taken across the cell matching resistor (R203), and the
standardizing control (R47).
The conductivity signals are buffered by U205A and U205D which with U205B form a DC and
AC balanced differential amplifier. U205C buffers and sums in the voltage signal from the
suppression chain. These are summed and amplified by the gain stage formed by U207. The
resulting chopped DC conductivity signal is fed to two de-chopping/sampling circuits. U211 and
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