Teledyne 100E - Figure 10-15: PMT Cooling System; PMT Preamplifier; TEC Control Board

Theory Of Operation Model 100E Instruction Manual
182 045150102 Rev XB1
TEC
Control
PCA
PMT Preamp
PCA
Thermistor
outputs temp of
cold block to
preamp PCA
Preamp PCA sends
buffered and
amplified thermistor
signal to TEC PCA
TEC PCA sets
appropriate
drive voltage
for cooler
Heat form PMT is absorbed
by the cold block and
transferred to the heat sink
via the TEC then bled off
into the cool air stream.
PMT
Cold Block
Heat Sink
Cooling Fan
T
hermo
E
lectric
C
ooler
Figure 10-15: PMT Cooling System
10.4.5.2. TEC Control Board
The TEC control printed circuit assembly is located ion the sensor housing assembly,
under the slanted shroud, next to the cooling fins and directly above the cooling fan.
Using the amplified PMT temperature signal from the PMT preamplifier board (Section
10.4.6), it sets the drive voltage for the thermoelectric cooler. The warmer the PMT gets,
the more current is passed through the TEC causing it to pump more heat to the heat
sink.
A red LED located on the top edge of this circuit board indicates that the control circuit is
receiving power. Four test points are also located at the top of this assembly. For the
definitions and acceptable signal levels of these test points see Chapter 11.
10.4.6. PMT Preamplifier
The PMT preamplifier board amplifies the PMT signal into a useable analog voltage (PMT)
that can be processed by the motherboard into a digital signal to be used by the CPU to
calculate the SO
2
concentration of the gas in the sample chamber.
The output signal of the PMT is controlled by two different adjustments. First, the voltage
across the electron multiplier array of the PMT is adjusted with a set of two hexadecimal
switches. Adjusting this voltage directly affects the HVPS voltage and, hence, the signal
from the PMT. Secondly, the gain of the amplified signal can further be adjusted through
a potentiometer. These adjustments should only be performed when encountering
problems with the software calibration that cannot be rectified otherwise. See Section
11.6.3 for this hardware calibration.