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In the field, an earth ground may be created
through a grounding rod. A 12 AWG or larger
wire should be run between a Terminal Strip
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 CR510 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 CR510 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 CR510 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 CR510 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 CR510
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 CR510
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 TERMINAL STRIP
The Terminal Strip provides transient
protection, improves excitation voltage
accuracy, and makes convenient, positive
connections to power, sensors, and peripherals
(refer to Figure 14.7-1). Terminal Strip transient
protection is discussed in Section 14.7.
SECTION 14. INSTALLATION AND MAINTENANCE
The Terminal Strip carries two lines between
the CR510 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 CR510 and the excitation port on the
Terminal Strip (see Figure 14.7-1).
A 5 V output terminal is available on the
Terminal Strip 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 CR510 is provided in Appendix D.
14.9 USE OF DIGITAL I/O PORT (C1)
FOR SWITCHING RELAYS
The I/O port (C1) 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 associated with 41, 51, 61
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.
In other applications it may be desirable to
simply switch power to a device without going
through a relay. Figure 14.10-2 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 14.10-1) would be required.
Other control port activated circuits are possible
for applications with greater current/voltage
demands than shown in Figures 14.10-1 and 2.
For more information contact Campbell
Scientific's Marketing Department.
14-7
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