Grounding - Campbell 21X Operator's Manual

Vehicle
Vehicle
FIGURE 141.6-1.
Connecting Vehicle Power
I
Supply
+12
+
o_
| Y
l+
/
-=-6wc
/
-=-
EAnERY
{l
t-
|
TilT
14.7
USE OF DIGITAL CONTROL
PORTS FOR SWITCHING RELAYS
Each of the gix
digitalcontrol ports can be set
low or high (gV low, 5V high) using l/O
f
nstruction
2p, Port Set, or commands
41
-78
associated v{ith Program Control Instructions 83
through
93.
lA
digital output port is normally
used
to operpte an external relay driver circuit
because
the port itself has a limited drive
capability (1.SmA at 3.5V). Figure 14.7-'l
shows
a
typical
rela!
driver
circuit
in
conjunction with
a
coildriven
rdlay
which may be used to switch
external power to some device.
In
this
example, when the
controlport
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 Sdientific offers the
A21REL-12 Four
Channel Relpy Driver (12V coil) and the A6REL-
12
Six
Chanfiel Relay Driver with manual
override (12V coil)
for use with the 21X.
SUPPLY MAXMUY
-
40
WC
MlNlilUil
-
REQUIRED
COL \€ITAC€
UAXTUM
CURtrilI IO
COL
lS 75 mA AT
25t,
{
mA
AT
-50'C
FIGURE
Relay
Driver Circuit with
Relay
ln
other it may
be
desirable
to
simply
power to
a
device
without going
through
a
refay. Figure
14.7-2 illustrates a
circuit
for
svrjitching
external power to a device
without going through a
relay.
lf
the peripheral
to be powerpd draws in excess of 75 mA at
room
tempefature (limit of the 2N2907A
SECTION
14.
INSTALLATION AND MAINTENANCE
medium power transistor), the use of a relay
(Figure 14.7-1) would be required.
lOC
MAXIMUM
MAXIMUM CURRENT TO
PERIPHERAL
lS
75
mA
AT
25t,
40 mA
AI -50t
coNlR0t
PORI
FIGURE
14.7-2. Power Switching without
Relay
Other
control port activated circuits are possible
for applications
with greater currenVvoltage
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. Allcomponents
of
the system
(datalogger, sensors, external power supplies,
mounts, housings, etc.)
should be referenced to
one common
eadh ground.
Alf 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
14-5