Campbell Scientific CR10X Measurement and Control System - Manual (Page 224)

SECTION 14. INSTALLATION AND MAINTENANCE
14-8
In the field, an earth ground may be created
through a grounding rod. A 12 AWG or larger
wire should be run between a Wiring Panel
power ground (G) terminal and the earth
ground. Campbell Scientific's CM10 and CM6
Tripods and UT3 Tower come complete with
ground and lightning rods, grounding wires, and
appropriate ground wire clamps.
14.7.2 EFFECT OF GROUNDING ON
MEASUREMENTS: COMMON MODE RANGE
The common mode range is the voltage range,
relative to the CR10X ground, within which both
inputs of a differential measurement must lie in
order for the differential measurement to be
made. Common mode range for the CR10X is
±2.5 V. For example, if the high side of a
differential input is at 2 V and the low side is at
0.5 V relative to CR10X ground, a
measurement made on the ±2.5 V range would
indicate a signal of 1.5 V. However, if the high
input changed to 3 V, the common mode range
is exceeded and the measurement cannot be
made.
Common mode range may be exceeded when
the CR10X is measuring the output from a
sensor which has its own grounded power
supply and the low side of the signal is
referenced to power ground. If the CR10X
ground and the sensor ground are at sufficiently
different potentials, the signal will exceed the
common mode range. To solve this problem,
the sensor power ground and the CR10X
ground should be connected, creating one
ground for the system.
In a laboratory application, where more than
one AC socket may be used to power various
sensors, it is not always safe to assume that the
power grounds are at the same potential. To be
safe, the ground of all the AC sockets in use
should be tied together with a 12 AWG wire.
14.8 WIRING PANEL
The Wiring Panel provides transient protection,
improves excitation voltage accuracy, and
makes convenient, positive connections to
power, sensors, and peripherals (refer to Figure
14.7-1). Wiring Panel transient protection is
discussed in Section 14.7.
The Wiring Panel carries two lines between the
CR10X and each excitation port. One line is for
excitation voltage, the other is for feedback
control of the voltage. The feedback line is
required to compensate for line losses between
the CR10X and the excitation port on the Wiring
Panel (see Figure 14.7-1).
Two 5 V output terminals are available on the
Wiring Panel for powering 5 V peripherals. The
5 V ports can source up to 200 mA. An input
protection transzorb will divert current to ground
at approximately 10 V.
A functional description of the 37 pin connector
located on the CR10X is provided in Appendix D.
14.9 SWITCHED 12 VOLT
A single switched 12 volt output is available for
powering sensors or devices that require an
unregulated 12 volts. The 12 volt output is
limited to 600 mA of current.
A control port is used to operate the switched
12 volt control. Connect a wire from any control
port to the switched 12 volt control (see Figure
OV1.1-2 for location of ports). When the control
port is set high, 12 volts is turned on to the
switched 12 volt port. When the control port is
set low, the switched 12 volts is turned off. See
example 8.12 for more information.
14.10 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 (1.5 mA at
3.5 V).
Figure 14.10-1 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.
Campbell Scientific offers the A21REL-12 Four
Channel Relay Driver and the A6REL-12 Six
Channel Relay Driver with manual override for
use with the CR10X.
In other applications it may be desirable to
simply switch power to a device without going