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Model 200E Instruction Manual Theory of Operation
The core part of the M200E PMT cooling system is a thermoelectric cooler (TEC), which
keeps the PMT temperature at a constant, low level (7
± 2 ° C) for optimum performance.
Thermoelectric coolers are solid state (semi-conductor) heat pumps, which transfer heat
from one side to the other when a DC current is passed through its circuits. Heat absorbed
at the cold junction (the PMT cold-block) is pumped to the hot junction (the cooling fins) at
a rate proportional to the current passing through the TEC circuit and the number of
junctions. The TEC is just one of many components in the M200E’s PMT cooling system as
shown in Figure 11-7.
10.3.2.4. TEC Control Board
The TEC control printed circuit assembly is located 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, it sets the drive for the thermoelectric cooler. The warmer
the PMT gets, the more current is passed through the TEC.
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.
10.3.2.5. PMT Preamplifier
The PMT preamplifier board (Figure 10-14 and Figure 11-3) converts and amplifies the PMT
current signal into a useable analog voltage (PMTDET) that can be used by the CPU to
calculate the NO concentration of the gas in the reaction cell.
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
that cannot be rectified with the software calibration. See Section 11.6.5 for this hardware
calibration.
The PMT temperature control loop maintains the PMT temperature around 7° C and can be
viewed as test function
PMT TEMP on the front panel.
The electrical test (ETEST) circuit generates a constant, electronic signal that simulates the
output of the PMT. By bypassing the detector’s actual signal, it is possible to test most of
the signal handling and conditioning circuitry on the PMT preamplifier board. See Section
6.7.6 for instructions on performing this test.
The optical test (OTEST) consists of an LED inside the PMT cold block that creates a light
signal, which can be measured with the PMT. If zero air is supplied to the analyzer, the
entire measurement capability of the sensor module can be tested including the PMT and
the current to voltage conversion circuit on the PMT preamplifier board.
044100102 Rev A 175