Accuracy - Fluke 415B Instruction Manual

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