Campbell CR23X, Operator'S Manual

Page: 236 / 374

SECTION 14. INSTALLATION AND MAINTENANCE
14-10
supply terminal, and 1 continuous 5 Volt (5V)
supply terminal. Voltage on the 12V and SW12
terminals will change with the CR23X supply
voltage. The 5V terminal is regulated and will
always remain near 5 Volts (
±
4%)so long as the
CR23X supply voltage remains above 11 Volts.
The 5V terminal is not suitable for resistive
bridge sensor excitation, however. Table 14.8-1
shows current sourcing limitations of the 12 Volt
and 5 Volt ports. Table 14.8-2 shows current
requirements for several CSI peripheral. Other
devices normally have current requirements
listed in their specifications. Current drain of all
peripherals and sensors combined should not
exceed current sourcing limits of the CR23X.
Table 14.8-1 Current Sourcing Limits
Terminals Current Source Limit
SW12 < 900 mA @ 20°C
< 729 mA @ 40°C
< 600 mA @ 50°C
< 567 mA @ 60°C
< 400 mA @ 80°C
12V + SW12 < 1.85 A @ 20°C
< 1.50 A @ 40°C
< 1.30 A @ 50°C
< 1.17 A @ 60°C
< 0.85 A @ 80°C
5V + CSI/O < 200 mA
Make certain that the primary source of power
for the CR23X can sustain the current drain for
the period of time required. Contact a CSI
applications engineer for help in determining a
power budget for applications that approach the
limits of a given power supply’s capabilities. Be
particularly cautious about any application using
alkaline batteries and some form of telemetry,
applications using solar panels and cellular
telephone or radio, applications requiring long
periods of time between site visits, or
applications at extreme temperatures.
14.9 CONTROLLING POWER TO
SENSORS AND PERIPHERALS
Controlling power to an external device is a
common function of the CR23X. Whether
control is required as part of the function of a
system function or to conserve power, the
principles of power control are the same.
Many devices can conveniently be controlled
with the SW12 (Switched 12 Volt) port on the
CR23X. Table 14.8-1 shows the current
sourcing limits of the SW12 port. Applications
requiring more control channels or greater
power sourcing capacity can usually be satisfied
with the use of Campbell Scientific’s A21REL-
12 Four Channel Relay Driver, A6REL-12 Six
Channel Relay Driver, SDM-CD16AC 16
Channel AC/DC Relay Module, or by using the
control (C1-C8) ports as described in Section
14.9.1
14.9.1 USE OF DIGITAL I/O PORTS FOR
SWITCHING RELAYS
Each of the eight digital I/O ports can be
configured as an output port and set low or high
(0 V low, 5 V high) using I/O Instruction 20, Port
Set, or commands 41 - 68 associated with
Program Control Instructions 83 through 93. A
digital output port is normally used to operate an
external relay driver circuit because the port itself
has a limited drive capability (2.0 mA minimum
at 3.5 V).
Figure 14.9-1 shows a typical relay driver circuit
in conjunction with a coil driven relay which may
be used to switch external power to some
device. In this example, when the control port is
set high, 12 V from the datalogger passes
through the relay coil, closing the relay which
completes the power circuit to a fan, turning the
fan on.
In other applications it may be desirable to
simply switch power to a device without going
through a relay. Figure 14.9-2 illustrates a circuit
for switching external power to a device without
going through a relay. If the peripheral to be
powered draws in excess of 75 mA at room
temperature (limit of the 2N2907A medium
power transistor), the use of a relay (Figure 14.9-
1) would be required.
Other control port activated circuits are possible
for applications with greater current/voltage
demands than shown in Figures 14.9-1 and 2.
For more information contact a Campbell
Scientific applications engineer.