Campbell Scientific CR6 CR6 Measurement and Control System - Page 110
Grounding, ESD Protection
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Section 7. Installation SW12 terminal will change with CR6 supply voltage. Two CRBasic instructions, SW12() and PortSet(), control the SW12 terminals. Each instruction is handled differently by the CR6. SW12() is a processing task. Use it when controlling power to SDI-12 and serial sensors that use SDI12Recorder() or SerialIn() instructions respectively. CRBasic programming using IF THEN constructs to control SW12, such as when used for cell phone control, should also use the SW12() instruction. PortSet() is a measurement task instruction. Use it when powering analog input sensors that need to be powered just prior to measurement. A 12 Vdc switching circuit designed to be driven by a U or C terminal is available from Campbell Scientific and is listed in the appendix Relay Drivers (p. 599). Note SW12 terminal power is unregulated and can supply up to 1100 mA at 20 °C. See table Current Source and Sink Limits (p. 108). A resettable polymeric fuse protects against over-current. Reset is accomplished by removing the load or turning off the SW12 terminal for several seconds. The SW12 terminals may behave differently under pipeline (p. 153) and sequential (p. 154) modes. See CRBasic Editor Help for more information. 7.6 Grounding Grounding the CR6 with its peripheral devices and sensors is critical in all applications. Proper grounding will ensure maximum ESD (electrostatic discharge) protection and measurement accuracy. 7.6.1 ESD Protection ESD (electrostatic discharge) can originate from several sources, the most common, and most destructive, being primary and secondary lightning strikes. Primary lightning strikes hit the datalogger or sensors directly. Secondary strikes induce a voltage in power lines or sensor wires. The primary devices for protection against ESD are gas-discharge tubes (GDT). All critical inputs and outputs on the CR6 are protected with GDTs or transient voltage suppression diodes. GDTs fire at 150 V to allow current to be diverted to the earth ground lug. To be effective, the earth ground lug must be properly connected to earth (chassis) ground. As shown in figure Schematic of Grounds (p. 111), signal grounds and power grounds have independent paths to the ground lug. Nine-pin serial ports are another path for transients. Provide communication paths, such as telephone or short-haul modem lines, with spark-gap protection. Spark-gap protection is usually an option with these products, so request it when ordering. Spark gaps must be connected to either the earth ground lug, the enclosure ground, or to the earth (chassis) ground. A good earth (chassis) ground will minimize damage to the datalogger and sensors by providing a low-resistance path around the system to a point of low potential. Campbell Scientific recommends that all dataloggers be earth (chassis) grounded. All components of the system (dataloggers, sensors, external power supplies, mounts, housings, etc.) should be referenced to one common earth (chassis) ground. 110