Fluke 410B - Overcurrent Protection; Main Output Voltage; Feedback Amplifier; Auxiliary Supplies

410B
closes
if S2
is
set.
to the
STANDBY-RESET
position.
This
energizes K302,
which
opens
K302A,
^moving
K301
from
the
circuit,
and
closes
K302B
and R30
2.
When
switch S2
is
set
to
the
HIGH
VOLTAGE
ON
position
after
K302C
is
closed,
the
primary
of
transformer
it
is
energized
and
transformer
voltage
is
applied
to
GR4U
and
CR402.
3-8.
OVERCURRENT
PROTECTION
3-9.
Relay
K303
provides
overcurrent
protection.
If
the
output
current
exceeds
approximately
12
amperes,
K303
is
actuated,
which
closes
K303A.
This
de-energizes
K302,
which
closes
K302A
.
and
opens
K302B
and
K302C,
extinguishing
the
HIGH
VOLTAGE
Uin
lamp.
The
primary
circuit
of
T2
is
thus
opened
by
K302C
and
high
voltage
is
removed
from
the
output
terminals.
Before
high
voltage
can be
reapplied
to
the
output
terminals,
switch S2
must
be
set to
the
STANDBY
-
RESET
position.
After
the
time-delay
cycle is
complete,
voltage
will
be
available
at
the
output
connector
when
the
HIGH
VOLTAGE
switch,
S2,
is
set
to ON.
3-10.
MAIN
OUTPUT
VOLTAGE
3-11.
The
main
output
voltage
is
developed
by
CR401,
CR402,
C3,
C4,
and R402
through
R411.
This
is
a
full-wave,
voltage
-doubler
circuit,
which
applies
recti-
fied,
filtered,
dc
voltage
to the
plate
of V202.
The
taps
on
the
primary
of T2
provide
approximate
control
of
the
unregulated
dc
voltage,
which
minimizes
power
dissi-
pation
in V202
at
low
output
voltage.
needed
to the
junction
of
two
half-wave
rectifiers
and
fillers.
The
positive
auxiliary
voltage
is
obtained
from
CR201,
CR202,
and
C202.
Auxiliary
voltages
of
+95
volts,
+20 volts,
and
+10 volts
are
obtained
from CR205,
CR206,
and
CR207,
respectively,
which are
connected
across
the
positive
auxiliary
voltage.
The
negative
auxiliary
voltage is
obtained
from
CR203,
CR204,
and
C203.
An
auxiliary
voltage
of
-110 volts Is
obtained
from
CR208.
This
arrangement
of the
+95V,
+20V,
+
10V,
and
-110V
auxiliary
supplies
provides
shunt re-
gulation of
these
supplies.
3-16. REFERENCE
VOLTAGE
3-17. Reference
voltage
for the
instrument Is
obtained
from V201,
a
gas-filled
reference
tube.
Resistor R219
has
a
temperature
coefficient
of
+0.
45%/°
C,
which
provides
temperature
compensation
for
V2Q1.
Tran-
sistors
Q205
and
Q206
are
used as
a
differential
ampli-
fier.
Transistor
Q203 is
operated as
an
emitter-follower
voltage
regulator,
which
provides
100
volts
for the
collector
of
Q205
and the
emitter
of Q204.
The
base of
Q205
samples
the
output
voltage
of the
+125 volt
auxiliary
supply,
through
the
voltage
divider R213
and R214.
The
output
from
Q206
represents
the
amplified
difference
between
the
reference
voltage
and
the
voltage
at the
base
of
Q205.
Transistor Q204
provides
further
amplification
of
this
voltage
difference
for
application to
series
regu-
lators
Q202
and
Q201.
Neon
lamps DS
201
and DS202
are
safety
devices
to
prevent
possible
burn-out of
Q201
and
Q202
if
the
output
of
the
instrument
becomes
short-
circuited.
3-12.
FEEDBACK
AMPLIFIER
3-13.
The
feedback
amplifier
consists of
essentially
five
stages
composed
of
seven
silicon
transistors.
Tran-
sistors
Q213
and
Q214
are
connected
as a
differential
amplifier.
The
output
is
taken
from
the
collector
load
of Q214
and is
applied
to
the
common-emitter
amplifier
Q212.
Output
from the
collector
of
Q212
is ac
coupled
back
to the
base
of
Q213
as
negative
feedback
for
loop
stabilization.
The
output
of
Q212 is
applied
to the
input
of the
compound-connected,
emitter
-follower
pair
of
Q210 and
Q211.
This
stage
provides
current
gain
and
impedance
matching
between
the
high
collector
imped-
ance
of
Q212
and the
low
emitter
impedance
of
Q209.
Transistors
Q209
and Q208
are
common
base
stages
which
provide the
voltage
gain
and
large
voltage
swing
required by
the
grid
of V202.
C215
and R230
in
the
emitter
of Q209
form
a
lead
network
for loop
compensa-
tion.
3-14. AUXILIARY
SUPPLIES
3-15.
AC
voltage
for
both
the
positive
and
negative
auxiliary
supplies
is
obtained
from
one
transformer
winding.
One
terminal of
this
winding is
connected
to
the
positive
output
of the
410B,
which is
the zero
volt
reference.
The
other
terminal
of
the
winding
is
con-
3-18, ACCU
RACY
3-19. The
main
sampling
string
resistors in
the
410B
are
accurate
to
within
±0.
1%.
However,
the
accuracy
of
the 410B
is
specified
as
±0.
25%,
because the
cali-
bration
accuracy
also
depends
upon the
stability
of the
reference
voltage.
The
voltage
of
reference
tube V201
(83A
1)
changes
slightly
due to
aging.
The
accuracy
of
the
supply
will
remain
within
±0.
25%
for
at
least 30
days.
The
calibration
accuracy
may
be
maintained
at
better
than
0. 25%
if the
instrument
is
recalibrated
more
often
than
the
usual
calibration
period of 30
days.
3-20. All
calibrated
power
supplies
have an
accuracy
limit
(floor) as
the
output
voltage
approaches
zero.
This
floor is
caused
by
zero
shift
in the
error
amplifier,
contact
resistance
in the
sampling
string
circuit,
and
the
accuracy
of
the
sampling
string
resistors
used
for
the
least
significant
digits.
The
accuracy
floor may
be
reduced
by
using
more
expensive
components
and ad-
ditional
circuitry.
However,
this
results
in
greater
initial cost
and
longer
calibration
time.
Thus,
very
low
accuracy
floors
are
usually
found
only
in
precision
cali-
brators.
The
410B
has
an
accuracy
of
±0. 25%
or
250
millivolts,
whichever
is
greater,
when
the
first
four
voltage
controls
are
used.
Thus,
the ±0.
25%
accuracy
applies
from
10,
000
volts
to
100 volts.
3-2