Campbell Scientific CR3000 Micrologger - Manual (Page 91)

Section 7. Installation
91
See Sensors Requiring Current Excitation
(p. 330)
for precautions when measuring
resistances > 1000 Ω or sensors with leads > 50 feet.
7.4.5.3 Continuous Regulated (5 Volt)
The 5V terminal is regulated and remains near 5 Vdc ( ±4%) so long as the
CR3000 supply voltage remains above 9.6 Vdc. It is intended for power sensors
or devices requiring a 5-Vdc power supply. It is not intended as an excitation
source for bridge measurements. However, measurement of the 5V terminal
output, by means of jumpering to an analog input on the same CR3000), will
facilitate an accurate bridge measurement if 5V must be used.
Note Table Current Source and Sink Limits
(p. 89)
has more information on
excitation load capacity.
7.4.5.4 Continuous Analog Out (0 - 5 Volt)
Continuous analog out (CAO) terminals are regulated and remain near the set
voltage (
±4%) so long as the CR3000 supply voltage remains above 9.6 Vdc.
Measurement of the output from CAO (by means of jumpering to an analog input
on the same CR3000) enables an accurate bridge measurement if a CAO terminal
must be used for excitation.
The CAO sample capacitor is a good example of capacitor dielectric absorption, a
phenomenon wherein a capacitor charge / discharge time is greatly increased
because of a small percentage of dielectric molecules being quite slow to respond.
The CR3000 CAO circuitry uses polypropylene capacitors for the sampling
capacitor, which have the lowest dielectric absorption of commonly available
capacitors, yet the effect is still noticeable for large step changes in the CAO
output voltage. Hence a default 2.6-ms charge time is employed in the CAO
instruction to minimize dielectric absorption errors following CAO voltage step
changes. Using fast update (see
FastUpdate parameter in ExciteCAO()
instruction) following a large step change can result in a fairly large (5% or so)
CAO setting error for the first scan, with the error being reduced upon subsequent
scans after the step change.
7.4.5.5 Continuous Unregulated (Nominal 12 Volt)
Voltage on the 12V terminals will change with CR3000 supply voltage.
7.4.5.6 Switched Unregulated (Nominal 12 Volt)
The SW12V 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. 89).
Voltage on a
SW12V terminal will change with CR3000 supply voltage. Two CRBasic
instructions, SW12() and PortSet(), control a SW12V terminal. Each instruction
is handled differently by the CR3000. 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 SW12V, such as when used for cell phone control, should
also use the SW12() instruction.