Scion Instruments 436-GC Service Manual page 94

436-GC/456-GC
Bridge voltage regulator and monitor
The TCD bridge excitation voltage is set by the bridge voltage control DAC, U3. AR1 converts the current
output from the DAC to a voltage, which varies from 0 to -10V at the amplifier output. C16 reduces digital
noise interference. The DAC output is shifted and attenuated by R18 (pins 3, 4, 13, and 14) to create a
control voltage that remains within the common mode range of buffer AR2, which is powered from the
floating power supply. The high impedance input of AR2 virtually eliminates any current from the ground-
referenced DAC circuitry from flowing into the floating circuitry, which would cause a signal offset as the
current flowed back to ground through the bridge. Two more sections of R18 divide the 10-volt reference by
two, and the other section of AR2 buffers the connection to the floating circuit. A differential control voltage,
which varies from 0 to -5V between AR2-7 and AR2-1, is thus available to set the bridge excitation voltage.
The remaining sections of R18, combined with R19, R20, and AR3, form a differential amplifier with a gain
of 5.62, referenced to the return side of the floating supply. The feedback is taken from the supply side of
the TCD bridge, so the amplifier output tries to force the bridge voltage to be 5.62 times the differential
control voltage between R18-5 and R18-6. The feedback path is completed from AR3-1 through CR11,
VR1, and R6, to the supply side of the bridge. Regulator VR1 has internal circuitry which holds its output
(pin 2) 1.25 volts above its adjust pin (pin1). It also supplies current limiting and thermal overload
protection, in the event of excessive load. C6 provides a low impedance at the input of VR1 at high
frequencies, to maintain stability of the regulator. C12 stabilizes the overall loop response.
Excitation voltages, bridge current, and signal voltage are all available for measurement at the card edge
connector. Analog switch U5 connects one of these four voltages at a time to P1-3, depending upon which
output of decoder U8 is low. The selection is made by latching the appropriate value into U6, pins 12 and
15. The bridge supply and return voltages are divided by 50.9 through resistors R33, R34, R35 and R36.
The bridge signal voltage is divided by 11 through R38 and R37, in order to minimize the disturbance to the
signal amplifier when a direct measurement is made through switch U5. Bridge current is sensed by
measuring the voltage drop across R6 with differential amplifier AR4 (pins 57). The amplifier gain is 1, so
the scale factor of the voltage at P1-3 is 200mA/V.
Temperature limit circuit
In normal operation, the filaments of the thermal conductivity detector bridge rise and fall in temperature as
the surrounding gas composition changes. Since the filaments have a positive temperature coefficient of
resistance, their peak temperature can be limited by reducing the bridge excitation voltage whenever a
filament reaches a preset maximum resistance. Resistors R9, R10, and R6 form a bridge circuit with the
bridge element connected between J1-5 and J1-3. With K1 closed, the inputs of amplifier AR1 (pins 2 and
3) are connected across this bridge. When the filament resistance reaches about 82Ohm, the temperature-
sensing bridge is balanced, and the output of AR1 (pin 1) begins to drop. This pulls down the adjust pin of
VR1 through CR6 and U2, decreasing the detector excitation, and holding the filament resistance at the
preset limit.
The other half of AR1 amplifies the output of a similar bridge, which senses the resistance of the filament
connected between J I -5 and J 1-2, using resistors R6, R13, and R14. The outputs of both halves of AR1
are combined with the output of the main control amplifier, AR3, through diodes CR11, CR6, and CR7.
Whichever amplifier calls for the lowest bridge excitation controls the loop, with the other amplifier outputs
both at the positive supply rail. C2 stabilizes the loop when one of the temperature-limit amplifiers is in
control. Opening K1 effectively raises the point on R6 where the inverting inputs of AR1 are connected,
thereby lowering the filament resistance which is required to cause limiting.
As long as AR3 is in control of the loop, the 1.25V differential between pins 1 and 2 of VR1 appears across
the base-emitter junction of Q2 and R7. Q2 sources about 6mA into optocoupler U2, turning it on, and
bringing its output (pin 7) low. When temperature limiting occurs, sending the output of AR3 to the positive
rail, CR10 becomes reverse biased. This turns off Q2 and U2, and indicates the limit condition with a high
level on the output of U2.
To maintain normal bridge voltage control with no detector plugged in, the temperature-limit circuit must be
defeated. U2 disconnects the outputs of the limit amplifiers from VR1 when it is turned off, by setting a logic
high at U6 pin 2.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 93