AM32 MULTIPLEXER
4
switched excitation channels are unavailable, a
control port may be used. However, the pulse
port instruction of the CR7 and 21X has a fixed
pulse time of 100msec. A 100msec delay per
measurement may be unacceptable.
3.1.3 GROUND
If the AM32 is powered by the datalogger
supply, then the multiplexer ground terminal is
connected to datalogger power ground. If a
separate supply is used to power the AM32, the
ground of that supply should be connected to
the datalogger power supply ground or directly
to earth ground (Figure 3-2). The datalogger
should always be earth grounded, and there
should be only one earth ground per system. If
several earth grounds exist, they should be tied
together by a large diameter wire (12 AWG
minimum).
3.1.4 POWER SUPPLY
The AM32 requires a continuous 9.6 to 16 VDC
power supply for operation. The multiplexer's
current drain is less than 15 microamps while
quiescent and is typically 25 milliamps at 12
VDC when active. Power supply connections
are made at the terminals labeled 12V and
GND.
In many applications, it may be convenient to
power the AM32 from the datalogger's battery.
Campbell Scientific offers alkaline and sealed
rechargeable power supplies with the 21X and
CR10(X) dataloggers. The CR7 is available
with a sealed rechargeable supply only. The
datalogger alkaline supply (7.5 AmpHr) can be
used to power the AM32 in applications where
the system current drain is low or where
frequently replacing the batteries is not a
problem. Sealed rechargeable supplies are
recommended where solar- or AC- charging
sources are available. For power-intensive
operations, a higher capacity, external,
rechargeable, 12VDC source should be
considered.
To determine the life of the power supply,
calculate the system current drain and divide it
into the power supply Amp-Hour rating. Table
3-1 provides CR10(X)/AM32 current drains for
given scan rates and measurement integration
times. At a minimum, the power supply must be
able to sustain the system between site visits
over the worst environmental extremes.
If a 21X power supply is used to power the
AM32, all low level analog measurements
(thermocouples, pyranometers, thermo-piles,
etc.) must be made differentially. This
procedure is required because slight ground
potentials are created along the 21X analog
terminal strip when the 12V supply is used to
power peripherals. This limitation reduces the
number of available analog input channels and
may mandate the use of an external supply for
the AM32 (Figure 3-2).
FIGURE 3-2. Power and ground connections
for external power supply.
Low power and high ambient temperatures may
affect the actuation time of the multiplexer
relays (Figure 3-3). A measurement error will
result if the relay is not closed when the
measurement begins. To solve this problem, a
delay can be programmed after the clocking
instruction but, before the measurement
instruction (refer to Section 4).
FIGURE 3-3. Actuation time of relays versus
temperature (
°
C) and battery voltage.
Содержание AM32
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