Campbell Scientific AM416 Multiplexer - Manual (Page 11)

AM416 RELAY MULTIPLEXER
5
voltage level must fall below 1.5VDC then
exceed 3.5VDC to clock. Pulse width must be
at least 5 ms. An additional delay is required
before the measurement to ensure adequate
time for the relay to close.
In the 21X and CR7 dataloggers, a switched
excitation is generally used to clock the
multiplexer (Instruction 22 - 5,000 mV
excitation). If no switched excitation channels
are available it is possible to clock using control
ports. See Section 5.1 for additional details.
In the CR10(X) datalogger, a control port is
generally used to clock the multiplexer.
Instruction 86 with the pulse port option
(command code 71 through 78 - generates a
pulse 10 ms in width) may be used to clock the
multiplexer.
4.1.3 GROUND
The multiplexer ground terminal is connected to
datalogger power ground. If a separate power
supply is used, AM416 ground is also
connected to the power supply ground (Figure
3). The datalogger should always be tied to
earth ground by one of the methods described
in the Installation/Maintenance Section of your
datalogger manual.
4.1.4 POWER SUPPLY
The AM416 requires a continuous 9.6 to 16
VDC power supply for operation. The
multiplexer's current drain is less than 100
microamps while quiescent and is typically 17
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 AM416 from the datalogger's battery.
For more power-intensive operations, an
external, rechargeable, 12VDC, 60-AmpHr
source may be advisable. Because of their
ability to be recharged, lead-acid supplies are
recommended where solar- or AC- charging
sources are available. The datalogger alkaline
supply (7.5 AmpHr) can be used to power the
AM416 in applications where the system current
drain is low, or where frequently replacing the
batteries is not a problem. It is advisable to
calculate the total power requirements of the
system and the expected longevity of the power
supply based on the system current drains (e.g.
the datalogger, multiplexer, other peripherals
and sensors) and the expected ambient
temperatures.
The power required to operate an AM416
depends on the percentage of time it is active.
For example, if a CR10(X) makes differential
measurements on 32 thermocouples every
minute, the average current drain due to the
AM416 is about 0.3 mA. Under the same
conditions, a 2 second scan rate increases the
average system current drain to about 8.5 mA.
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
AM416, 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 AM416 (Figure 3).
FIGURE 3. Power and Ground Connections
for External Power Supply.
Low power and high ambient temperatures may
affect the actuation time of the multiplexer
relays (Figure 4). If the relay is not closed when
a measurement is started, the result will be an
inaccurate or overranged value.