SECTION 14. INSTALLATION AND MAINTENANCE

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 come complete with ground and lightning rods, grounding wires, and appropriate ground wire clamps.

14.7.2EFFECT OF GROUNDING ON MEASUREMENTS: COMMON MODE RANGE

The common mode range is the voltage range, relative to the CR10 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 CR10 is ±2.5 V. For example, if the high side of a differential input is at 2 V and the low side is at

0.5V relative to CR10 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 CR10 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 CR10 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 CR10 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 purpose of the Wiring Panel is to provide transient protection, improve excitation voltage accuracy, and make convenient, positive connections of power, sensors, and peripherals to the CR10 (refer to Figure 14.7-1). Wiring Panel transient protection is discussed in Section 14.7.

The Wiring Panel carries two lines between the CR10 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 CR10 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 most common use of these terminals is to switch the 5 V to a relay coil through a relay driver circuit which is enabled by one of the eight Digital I/O Ports, C1 through C8 (see Section 14.9 for relay driver circuits). 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 CR10 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.

14.10USE 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

3.5V). 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

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