540B
is
thru
the
MODE
switch
to
the
Search
Amplifier where
the
ac
signal
is
amplified and then rectified and filtered.
R704
is
connected
across
the
input
in
SEARCH
to
pro
-
vide
proper
input impedance. The output of
the
Search
Amplifier
is
then connected
thru
the
MODE
switch
to
the
PERCENT INPUT
meter
where
the
needle deflection
is
calibrated
to
read
in
percentage
of
input
per
range
selected.
3-5.
When
the
MODE
switch
is
in
the
DC
SEARCH
position
the
direct
current
path
is
identical with
the
ac
path
except
that
no
amplification
is
required
and
the
MODE
switch
provides
an
alternate
path
around
the
Search
Amplifier.
Calibration
adjustments
are
pro-
vided
for
the
search
ranges
in
the
Search
Amplifier
and in the alternate de
path
around the
Search
Amplifier.
3-6.
With
the
MODE
switch
in the
AC
or
DC
TRANS-
FER
positions,
the
signal
from
the
input
terminals
passes
thru
the
MODE
switch, then
thru
.
the
first
pro-
tection
relay
(K801),
and
feeds
both
Compensated
At-
tenuators.
The
Search
Compensated Attenuator output
is
connected
to
the
Protection
Amplifier and
the
Search
Amplifier. The
Transfer
Compensated Attenuator feeds
thru
the
MODE
switch
then
thru
the
second
protection
relay
(K701).
From
the
second
protection
relay,
the
signal feeds the Thermocouple. The output of
the
Ther-
mocouple
is
fed
to
the
Galvanometer
in
such
a
manner
that
it
is
opposed
or
balanced
by
the
output
from
the
Reference Supply. The Galvanometer indicates
the
null
or
balance between the two.
3-7
. The
Protection
Amplifier
is
always
connected
to
the
output of
the
Search
Attenuator, and
is
designed
to
function on a
pre-set
level
of
ac
or
de.
When
the
Protection Amplifier
operates
it
causes
relays
K701
and
K801
to
disconnect
the
input
from
the
attenuators
and
open
the
circuit
to
the
thermocouple.
The
Protection
Amplifier
also
causes
relay
K301
to
close
a
circuit
from
the
battery
supply
which
causes
the
PERCENT
INPUT
meter
needle
to
deflect
into
the
OVER LOAD
area
of the
meter
scale.
Once the
Protection
Amplifier
has
operated
it
must
be
reset
manually
by
turning
the
MODE
switch
to
OFF. The
cause
of the overload should
be
located
and
removed
and
the
range
increased
or
other
remedial
action
taken before
returning
the
mode
switch
to
its'
operating
position.
3-8.
When
the
Model 540B
is
operated
in
the
SHUNT
mode, no
protection
is
provided
for
either
the
Model
540B thermocouple
or
for the
current
shunt.
It
is
neces-
sary
to
exercise
extreme
caution when
operating
in
this
mode
to
prevent
damage
to
the
instrument.
3-9.
CIRCUIT DESCRIPTIONS
3-10.
SEARCH CIRCUIT
3-11.
The
search
circuit
is
composed
of
three
basic
elements;
the
Search
Compensated
Attenuator,
the
Search
Amplifier
and
the
Percent
Input
Meter.
The
Search
Compensated
Attenuator
is
composed
of
switch
S501E, R502
thru
R514
and
C501
thru
C510.
This
at-
tenuator
is
compensated
to
equalize
the
output
of
the
Search
Amplifier
in
order
to
give
the
Percent
Input
reading
a
favorable
deflection/frequency
response.
3-2
3-12
.
The
Search
Amplifier
is
a two
stage
transistor
amplifier composed of
Q301
and
Q302
and the
associated
parts.
Low
frequency
adjustment
and
calibration
is
accomplishe
d with R310. High
frequencies
are
cali
-
brated
with C305. CR301 and CR302
rectify
the
ampli-
fied
ac
and
capacitors
C303 and C304
filter
the
resultant
de voltage for application
to
the PERCENT
INPUT
meter
.
The
Search
Amplifier
is
not
used
in
DC
SEARCH and a
path
is
provided by the
MODE
switch
to
apply de
directly
to
the
meter
from
the
attenuator.
3-13
. PROTECTION CIRCUIT
3- 14.
The
Protection
Circuit
is
composed
of an
am-
plifier
and two
relays
which
interrupt
current
flow
to
the
attenuators
and
the
thermocouple,
plus
a
set
of
diodes which
provide
ad
ditional
protective
features.
It
is
characteristic
of
thermocouples
to
be
ab
le
to
wi
th-
stand
overloads
of
rather
large
magnitude
for
shor
t
periods
of
time.
Diodes CR701
thru
CR704
appear
a-
cross
the
input of
the
A54
portion
of
the
540B and
are
biased
in
such
a manner
as
not
to
conduct under
normal
operation.
In
the
even
t of
an
overload,
these
diodes
conduct
and
provide
an
alternate
circuit,
shunting
the
input
to
the
thermocouple until the
protection
amplifier
can
operate
the
protection
relays
.
3-15.
The
protection
amplifier
is
a
single
operation
device designed
to
remain
in a
stable
state
until a
volt-
age
is
applied
to
it
that
is
in
excess
of
the
operating
point. Once overloaded
the
amplifier
operates
the
pro-
tection
relays
and
holds
them
operated
until
power
is
removed from the protection amplifier. The
three
tran-
sistors
in
th
e
Protection
Amplifier
are
Q303, Q304 and
Q305. Under
normal
(non
overloaded
) condition
there
is
sufficient
current
flow
thru
Q305 to hold
relays
K701
and
K801
in
their
operated
positions.
Transistors
Q303
and
Q304
are
cut-off and
remain
so
until
the
voltage
a-
cross
R313
raises
to
suc
h a value
as
to
star
t condition.
Once conduction
is
started,
each
transistor
ass
i
sts
the
other
into
faster
satura
tion which l
owers
the
voltage
at
the
emitter
of
Q303.
This
emitter
is
connected
to
the
base
of
Q305
and
this
voltage being lowered
causes
Q305
to
cut-off
releasing
the
protection
relays
by
stopping
the
current
flow
to
them.
R314
is
used
to
cali
brate
the
operating
point
of
the
Protection
Amplifier. The diode
bridge composed of
CR310
thru
CR313
is
used
to
protect
the
transisto
rs
of
the
Protection
Amplifier
from
over-
loads
and
voltage
surges
which
might
damage
them.
3- 16.
other
features
of
the
protection
circuit
include
the
operation of
K301
when the
circuit
is
tripped.
K301
supplies a fixed voltage
to
the PERCENT INPUT
meter
to
indicate
an
over-load
condition
has
occurred.
Two
diodes have been included
in
the
protec
tion
circuit
to
"
speed-up
"
the
operation
of
the
Protection
Amplifier
when
moderate
overloads
occur.
CR307
and
CR314
present
a low
resistance
path
around
R311 wh
en
ever
the voltage drop
across
that
resistor
exceeds
the
con-
duction voltage
of
the
diodes. Without
th
ese
diodes, the
time
cons
tant
of R311
and
C309
is
sufficiently
long
to
slow
the
operation of
the
Protection Amplifier enough
to
damage
the
thermocouple
or
other
components
under
conditions
of
moderate
overload,
if
the
"speed
-
up"
diodes
were
not
provided.
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