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1-8 | Model 120HD Hydrogen Analyzer
Electrical Circuit
The Model 120HD analyzer's electronic circuits include five power supplies, a
linearization circuit, a digital display, and the hydrogen sensor.
Power Supplies
The circuit requires five power supplies: +5V, -5V, +1.25 and -1.25 V, and a
120 mA constant current source.
Hydrogen Sensor
The hydrogen sensor is a thermal conductivity-type sensor. It has a zero (Screw
#4) and a gain (CURRENT ADJUST) adjustment. The output of the hydrogen
sensor is non-linear.
Hydrogen Sensor Circuit
The hydrogen sensor requires a 120 mA constant current source. CUR-
RENT ADJUST is used to adjust the value of the current source.
A two-degree polynomial . . .
is used to linearize the hydrogen sensor output.
x =
Amplified output
x
=
Multiplier output
2
a =
Fixed reference voltage
The summing amplifier adds the fixed reference (a), the amplified sensor
output (bx), and the squared sensor output (cx
linearized output signal to the digital display.
Components on the PCB interconnect board used in linearization are
R21 whose adjustment cancels offset voltages in the multiplier circuit,
and R24 which is used to cancel offset voltages in the summing ampli-
fier. These components should not be adjusted by the user.
Digital Display
The digital display is a 3-½ digit backlit LCD display. The backlighting of the
display is selectable by using the second TEST position with the "light bulb"
icon. All three decimal points light up to indicate "low battery" voltage.
Display Input = (a + bx + cx
)
2
). This output provides the
2
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