Campbell 21X Instrukcja Operatora Pobierz pdf (Strona 160)

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

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