Operator`s manual

SECTION 14. INSTALLATION AND MAINTENANCE
14-5
FIGURE 14.6-1. Connecting Vehicle Power
Supply
14.7 USE OF DIGITAL CONTROL
PORTS FOR SWITCHING RELAYS
Each of the six digital control ports can be set
low or high (0V low, 5V high) using I/O
Instruction 20, Port Set, or commands 41 - 78
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 (1.5mA at 3.5V). Figure 14.7-1 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, 12V
from the datalogger passes through the relay
coil, closing the relay which completes the
power circuit to a fan, turning the fan on.
Campbell Scientific offers the A21REL-12 Four
Channel Relay Driver (12V coil) and the A6REL-
12 Six Channel Relay Driver with manual
override (12V coil) for use with the 21X.
FIGURE 14.7-1. Relay Driver Circuit with
Relay
In other applications it may be desirable to
simply switch power to a device without going
through a relay. Figure 14.7-2 illustrates a
circuit for switching external power to a device
without going through a relay. If the peripheral
to be powered draws in excess of 75 mA at
room temperature (limit of the 2N2907A
medium power transistor), the use of a relay
(Figure 14.7-1) would be required.
FIGURE 14.7-2. Power Switching without
Relay
Other control port activated circuits are possible
for applications with greater current/voltage
demands than shown in Figures 14.7-1 and
14.7-2. For more information contact Campbell
Scientific's Marketing Department.
14.8 GROUNDING
14.8.1 PROTECTION FROM LIGHTNING
Primary lightning strikes are those where
lightning hits the datalogger or sensors directly.
Secondary strikes occur when the lightning
strikes somewhere near the system and
induces a voltage in the wires. The purpose of
an earth ground is to minimize damage to the
system by providing a low resistance path
around the system to a point of low potential.
ANY DATALOGGER IN A FIELD
INSTALLATION MUST BE EARTH
GROUNDED. All components of the system
(datalogger, sensors, external power supplies,
mounts, housings, etc.) should be referenced to
one common earth ground.
All input and output terminals on the 21X are
transient protected with spark gaps. The 21X
must have a good earth ground for the transient
protection to work.
The spark gaps consist of a thin air gap
between the conductor and a heavy copper bar
tied to ground. Depending on the conductivity
of the air, current will be diverted to ground at
roughly 400 to 600V.
A modem/phone line connected to the 21X
provides another path for transients to enter and