TCD electronics
The TCD is a heated resistance measuring bridge.
The sample gas changes two of the bridge resist-
ances if components dissipate heat better or worse
than the carrier gas. The bridge offset is amplified
and is used as the input signal.
Monitoring of bridge current
The bridge current is held constant by a controller. It
can be adjusted from 0 to 350 mA by the microcontrol-
ler. The voltage on the bridge is monitored by a com-
parator. The microcontroller switches the bridge current
off if the resistance of the TCD bridge becomes too
large. The resistances increases e.g. if the carrier gas
fails.
The bridge current is also switched off if the carrier gas
monitoring function or the Ex overtemperature switch
is triggered.
Range
Each range has a dynamic response of 10
6
. Five ranges
can be selected for the TCD. The ranges result in an
attenuation from 2
0
to 2
4
. The table lists the maximum
detector signal in each range.
Range Detector signal => Full-scale
at recorder range
0
0
1
2
3
4
0 ... 30,5
µ
V
0 ... 31,25 mV
0 ... 62,5 mV
0 ... 125 mV
0 ... 250 mV
0 ... 500 mV
0
10
11
12
13
14
X8
X201
X202
Detector A
X 202
X 201
Detector B
X 202
X 201
X8
RUN
1
Bridge current ON/OFF
Adjust bridge current
Bridge temperature
monitoring
max.
+ 27 V
TCD
30
A
D
A
D
A
D
Input signal
Compensation
and autozero
Bridge current
supply
7,5 V
+ 24 V
=
Voltage
limiting
GND
GND
GND
GNDGND
Mm
20 bits
GND
Mm
GND
Mm
U
U
U
U
Mm
+ 15 V
- 15 V
+ 5 V
Switch-off
Detector electronics
6
9DETEKT.CHP, Stand Dezember 22, 1999
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