Operational Theory
2000 XTC
membrane. The amount of voltage needed to offset the analog output
(i.e. to null the circuit when the resistance changes due to temperature
effects) is measured and applied. A preprogrammed microcontroller
with EPROM uses the offset voltage from the analog stages to generate
a linearized output voltage signal, which is then converted to a 0-4 mA
current. This signal is fed back to the galvanic isolator for conversion to
a standard 4-20 mA current, to be used as interface with the user’s
readout device or alarm circuit. The linearized output is based on
calibrations performed and adjustments made at the factory for the
specific application at hand.
Certain factors outside of the gas concentrations bear on the
accuracy of the thermal conductivity sensor. The major contributor
involves temperature deviations or transients from a baseline
temperature. For this reason, a pair of controlled heaters is used to fix
the operating temperature of the sample cell. The control circuitry is
capable of maintaining a temperature setting from 18°C to 67°C within
the operating range of the instrument (0 to 50°C). Electronics for the
temperature control are housed in the transmitter module. Power is
supplied to the heaters by pins 2-3 for heater #1 and 1-4 for heater #2 on
the 5-wire interconnection cable.
2.4.2 Galvanic Isolator
The purpose of the safety barrier is to limit power to and from the
transmitter to levels that are incapable of generating enough energy to
cause ignition in a flammable or explosive gas mixture. The galvanic
isolator accepts a 24 volt DC input and provides two separate and
isolated 15 volt DC power outputs, plus receiving a 0-4mA current input
from the transmitter. Both power outputs are mutually isolated and
comply with the CENELEC requirement intrinsically safe to 60V. The
output from the isolator supplies power to the two (2) heaters in the
transmitter. Additionally, it supplies power to the sensor and PC board
for the compensation circuitry within the transmitter.
Each output side of the galvanic isolator is protected with a series
200mA fuse. A pair of zener diodes limits the open circuit voltage to
below 18V and short circuit current to 503mA or less. Power
fluctuations are reduced at the input side of the 24V input transformer
using a crowbar network of diodes.
Teledyne Analytical Instruments
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