Fluke 415B - Page 35

415B
high
voltage rectifier
and
minimizes
the
power
dissipa-
tion required of the
series
pass
tubes at
low output
voltage. The high
voltage
rectifier
filter
consists
of
CR2
through
CR23 on the High
Voltage
Rectifier
P./C
Assembly in addition
to
C7, C
8 and RI
through
R6 on
the
Bleeder
P/C Assembly.
Tins
circuit
forms
a volt-
age
doubler
configuration
which
provides
filtered
but
unregulated dc
voltage
to the
series
pass
tubes
VI and
V2.
The parallel-connected
series
pass
tubes
maintain
the
output voltage
at the
value
set on
the
control dials
by
altering tube conduction
in
response
to
control signals
from
the error
amplifier.
3-14.
Error
Amplifier
3-15.
The error
amplifier
consists
of
Q6,
Q7
and
Q8
on the Amplifier
P/C
Assembly and
Ql through
Q4
on
the Series
Pass
P/C Assembly
and
their
associated
circuitry.
Q7
and
Q8
form
a differential
amplifier.
The input
to Q7 is from
the
positive
output
buss;
the
input to
Q8
is from
the
summation
point at
the junction
of the voltage
control
resistors
and
the
reference
net-
work.
Any
voltage
change at
the
output
buss appears
as
a voltage
difference
between
the
bases
of
Q7
and
Q8
and
alters
the collector
current
of
Q
8.
This
change
appea rs
as
error signal
at the
base of the
common
emitter
ampli-
fier Q6. The
output
of
Q6 is
fed
back
to the
base
of
Q7
for loop
stabilization
and
to the
compound-connected
emitter
follower
stage
Q3
and
Q4 located
on the Series
Pass
P/C Assembly
(A
5).
3-16.
Q3
and
Q4
provide
current
drive
and
impedance
match
between.
the
high collector
impedance
of
Q6
and
the low emitter
impedance
of
Q2. The
cascaded
common
base
amplifiers
Ql and
Q2
provide
the
voltage
gain
necessary
to swing
the
grids of the
series
pass
tubes
VI and
V2, through
the required
range.
3-17.
Reference
Tube and
Resistor Network
3-18.
The reference
tube,
VI, is a
specially
selected
and
aged type
83A1.
VI is provided with
a temperature-
compensation
network,
R14 and
R15, so that its
output
is
a
constant
voltage.
Since the
summation
point is always
at
zero
volts,
VI causes
a constant
current to flow
through
the
reference
network.
This same
constant
flows
through
the voltage
control resistors,
and
the
output
voltage
is equal to
the IR drop
across these
re-
sistors.
The output
voltage may thus
be precisely
con-
trolled
by varying
the
resistance of the
voltage
control
string.
3-19.
Auxiliary
Supply
3-20.
AC
voltage
for
both positive and
negative auxil-
iary
dc
supplies
is
provided
by
a
single
transformer
winding.
One
terminal
of this winding
is
connected to
the
positive
output
buss;
the other
terminal
is connected
via
R2
to the
junction
of
CR1 and
CR2.
Diodes
CR1
and
CR2,
on the
Series
Pass P/C
Assembly,
and
Cl and
C2,
on the Amplifier
P/C
Assembly
comprise a voltage
doubler.
The output
of the doubler
is
impressed
across
a
network
consisting
of
R3, R4 and
R5,
and
zener
diodes
CR3
through
CR6. This arrangement
provides
regulated
dc
voltages
of
+95, +20,
+10
and -110
volts.
The raw
dc
+125
volt
output
of
the supply
is
fed
to
the regulator
on
the
Amplifier
P/C Assembly.
3-21.
+125
Volt
Regulator
3-22.
The +12.1
volt
regulator
consists
of
Ql
through
Q5
and
their
associated
circuitry
on the
Amplifier
p/C
Assembly.
CH
and
Q5 comprise
a
differential
amplifier
Q2
is an
emitter
follower
regulator
which
serves
as
a
100 volt
source for the
emitter
of
Q3 and
the
collector
of
Q4.
The bases
of
Q4 and
Q5
are
held
at
a
constant
83
volts
by the
reference
tube
VI. The
base
of
Q4 also
samples
the +125
volts dc
through
the
divider
R8
and
R9. Any
difference
in
voltage
between
the
bases
of
Q4
and
Q5 appears
as error
signal at
the
base
of
Q3.
Q
3
provides
further
gain to
the
signal
which
is
applied
to
the
series
regulator
Ql.
Zener
diode
CR1
provides
regulated
dc
to
Q3
and
bypasses a
part
of the
current
to
decrease
dissipation
of
Ql.
3-23.
Overcurrent
Protection
3-24.
Protection
to the
instrument
from
damage
by
by
excessive
current
is
provided
by relay
K1
located
on the
Bleeder
P/C
Assembly.
Kl
is
in series
with
the
common
output
lead
and
is
normally
adjusted
to
operate
at
32
milliamperes
by
R5
located
on the
High
Voltage
Rectifier
P/C
Assembly.
Adjustment
R5
divides
the
load
current
between
Kl
and the
relay
shunting
re-
sistors
R5 and
R6.
The
contacts
of
Kl are
connected
across
the
coil of
K2 on the
High
Voltage
Rectifier
P/C
Assembly and
de-energize
this
relay.
In
order
for
high
voltage
to
be re-applied
the
overload
must
be
removed
and
the
time
delay
permitted
to
complete
its
cycle.
3-25.
ACCURACY
3-26.
The
main sa.moIin£T strimr rpci in f
415B are
accurate
to within
±0.
1
%. The
accuracy
of the
Model
415B, however,
is
specified
as
±0.25%
because
the
instrument
accuracy
also
depends
upon
the
repeata-
bility
and
stability
of
the
reference
voltage
and
the
length
of
on-time
of
VI.
The
output
voltage
of
VI
changes
slightly
with
time
due
to
aging.
The
accuracy
of
the
instrument
will
remain
within
±0.
25% for
greater
than
30 days.
The
calibration
accuracy
may
be
maintained
at better
than
±0.
25%
if
the
supply
is
recalibrated
more
often
than
the
usual
calibration
period
of
30 days.
3-27.
The
overall
accuracy
of the
meter
in the
Model
415B is approximately
±3% of
meter
input,
including
the
tolerances
of
multiplying
and
shunting
resistors.
How-
ever, the
calibrated
voltage
controls
should
be relied
upon to
indicate
the
value
of the
output
voltage.
For
example,
if
an output
voltage
of
1000
volts
is
selected,
the
meter will
indicate
between
970 volts
and
1030
volts
(±3%
of
1000-30).
However,
the
actual
output
voltage
will
be between
997.
5
volts and
1002.
5
volts
(±0. 25%
of
1000=±2.
5).
3-28.
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, con-
tact resistance
in
the
sampling string
circuit, and the
accuracy of the
voltage
-control
resistors
used
for the
least significait
digits.
The
Model
415B
has an
accuracy
of
±0.
25%
or 100
mv,
whichever
is
greater,
with
the
vernier
at zero.
Thus,
the
±0.
25%
accuracy is valid
down
to
40 volts.
3-2
Rev.
1