Table of Contents
Advertisement
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
accomplished 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
additional
protective
features.
It
is characteristic
of
thermocouples to
be
able
to
with-
stand
overloads of rather
large magnitude for
short
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
event
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 the 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 such
a value as to
start
condition.
Once conduction is started, each transistor assists
the
other
into faster saturation
which
lowers
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
calibrate the
operating
point of the Protection
Amplifier. The diode
bridge
composed
of CR310
thru
CR313
is used to protect
the
transistors 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
protection
circuit
to
"speed-up" the
operation of
the Protection Amplifier
when moderate overloads
occur.
CR307
and CR314
present
a
low resistance
path around R311
when
ever
the
voltage drop across
that resistor exceeds the con-
duction
voltage
of
the diodes.
Without
these
diodes,
the
time
constant
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.
Advertisement
Table of Contents
