Campbell Scientific CR6 CR6 Measurement and Control System - Page 109
Switched-Voltage Excitation, Switched Current Excitation, Continuous Unregulated Voltage 12V
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Section 7. Installation 7.5.1 Switched-Voltage Excitation Up to 12 analog-output (excitation) terminals can be configured from U terminals to operate under program control to provide - to mV dc excitation. Check the accuracy specification of terminals configured for exctitation in CR6 Specifications (p. 91) to understand their limitations. Specifications are applicable only for loads not exceeding ±25 mA. Note Table Current Source and Sink Limits (p. 108) has more information on excitation load capacity. CRBasic instructions that control voltage excitation include the following: • BrFull() • BrFull6W() • BrHalf() • BrHalf3W() • BrHalf4W() • ExciteV() Note Square-wave ac excitation for use with polarizing bridge sensors is configured with the RevEx parameter of the bridge instructions. 7.5.2 Switched Current Excitation U terminals can be configured as current-output terminals. These operate under program control to provide -4 mA to 4 mA for making resistance measurements. Check the accuracy specification of these terminals in CR6 Specifications (p. 91) to understand their limitations. CRBasic instructions that control current excitation include the following: • ExciteI() • Resistance() See Sensors Requiring Current Excitation (p. 367) for precautions when measuring resistances > 1000 Ω or sensors with leads > 50 feet. 7.5.3 Continuous Unregulated Voltage (12V Terminal) Use 12V terminals to continuously power devices that require 12 Vdc. Voltage on the 12V terminals will change with CR6 supply voltage. Caution Voltage levels at the 12V and switched SW12 terminals, and pin 8 on the CS I/O port, are tied closely to the voltage levels of the main power supply. For example, if the power received at the -BAT+ terminals is 16 Vdc, the 12V and SW12 terminals, and pin 8 on the CS I/O port, will supply 15.7 Vdc (0.3 V less than main supply) to a connected peripheral. If the connected peripheral or sensor is not designed for that voltage level, it may be damaged. 7.5.4 Switched Unregulated Voltage (SW12 Terminal) The SW12 terminalsare often used to power devices such as sensors that require 12 Vdc during measurement. Current sourcing must be limited to 1100 mA or less at 20°C. See table Current Source and Sink Limits (p. 108). Voltage on a 109