Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 86

Section 7. Installation Note Table Current Source and Sink Limits (p. 84) has more information on excitation load capacity. 7.4.5.3 Continuous Unregulated (Nominal 12 Volt) Voltage on the 12V terminals will change with CR800 supply voltage. 7.4.5.4 Switched Unregulated (Nominal 12 Volt) The SW12 terminal is often used to control low power devices such as sensors that require 12 Vdc during measurement. Current sourcing must be limited to 900 mA or less at 20°C. See table Current Source and Sink Limits (p. 84). Voltage on a SW12 terminal will change with CR800 supply voltage. Two CRBasic instructions, SW12() and PortSet(), control a SW12 terminal. Each instruction is handled differently by the CR800. 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 digital I/O port, is available from Campbell Scientific and is listed in the appendix Relay Drivers (p. 541). Note The SW12 terminal supply is unregulated and can supply up to 900 mA at 20°C. See table Current Source and Sink Limits (p. 84). A resettable polymeric fuse protects against over-current. Reset is accomplished by removing the load or turning off SW12 for several seconds. 7.5 Grounding Grounding the CR800 with its peripheral devices and sensors is critical in all applications. Proper grounding will ensure maximum ESD (electrostatic discharge) protection and measurement accuracy. 7.5.1 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 CR800 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. 88), power ground and signal grounds have independent paths to the ground lug. Nine-pin serial ports are another path for transients. Communications paths, such as telephone or short-haul modem lines, should be provided with spark-gap protection at installation. Spark-gap protection is usually an option with these 86