Fluke 825A - Power Supply

3-4. 500 VDC REFERENCE POWER SUPPLY
a.
When
the
825A
is
used
for
differential
voltage
mea-
surements,
an
internal
DC
voltage
is
nulled
or
matched
against
the
unknown
voltage.
Ап
extremely
accurate
ге-
ference
voltage
is
therefore
required.
This
is
obtained
from
the
500
volt
DC
reference
supply
b.
A
half
wave
rectifier
system
consisting
of
three
diodes
(CR101,
CR102,
and
CR103)
and
a
filter
network
(R105,
R106,
C101,
and
C102)
supply
unregulated
DC
voltage
to
series
passing
tube
V102.
The
voltage
is
re-
gulated
by
comparing
the
voltage
from
reference
tube
V104
with
a
voltage
tapped
off
a
divider
string
(R119,
R120,
R121,
and
R2)
іп
а
differeniial
amplifier
V105.
The
output
of
differential
amplifier
V105
is
fed
to
dif-
ferential
amplifier
V106
which
drives
the
grid
of
series
passing
tube
V102.
In
this
way
the
voltage
drop
across
the
passing
tube
is
controlled
such
that
the
reference
supply
is
maintained
at
500
(+0.
005)
volts.
c.
With
RANGE
switch
set
to
500,
sections
S2B
and
S2F
(just
S2F
before
serial
number
185
on
825A
and
serial
number
144
оп
825AR)
connect
500
volts
directly
to
the
Kelvin-Varley
divider.
Іп
the
50,
5,
and
0.
5
volt
posi-
tions,
range
resistors
controlled
by
S2E
divide
the
ref-
erence
voltage
to
50,
5,
and
0.5
volts.
These
voltages
are
switched
to
the
Kelvin-Varley
divider
by
S2F.
The
50,
5,
and
0.
5
range
resistors
networks
are
set
accur-
ately
during
calibration
by
means
of
R323,
R326,
and
R329.
d.
Varıable
resistor
R121
ıs
used
during
calibration
to
set
the
reference
supply
to
500
volts
with
CALIBRATE
control
(R2)
se’
at
its
center
of
rotation.
This
allows
the
reference
supply
to
be
adjusted
to
500
volts
by
means
of
the
CALIBRATE
control
at
any
time
deemed
necess-
ary.
With
the
OPERATE-CALIBRATE
switch
held
at
CALIBRATE,
a
fixed
percentage
of
the
reference
supply
is
compared
to
the
precise
potential
of
an
internal
standard
cell
or
Zener
diode
(in
the/AG
models).
Any
difference
in
potential
is
fed
to
the
null
detector
so
that
the
reference
supply
can
be
set
with
the
CALIBRATE
control.
The
fixed
percentage
of
the
reference
supply
is
set
accurately
during
calibration
by
means
of
R318.
e.
For
proper
operation
a
highly
stable
and
accurate
balance
of
amplification
must
be
maintained
between
the
Input
Attenuator
Null
Detector
825A
two
halves
of
differeniial
amplifier
V105.
The
filament
supply
for
V105
must
be
regulated
to
maintain
this
ра1-
ance.
In
earlier
models,
regulation
was
provided
ру
ballast
tube
V101
which
-upplied
a
constant
current
to
the
filament
of
V105.
Thus,
the
ballast
tube
absorbed
any
secondary
winding
voltage
changes
caused
by
line
fluctuations.
In
later
models,
a
transistor
regulator
is
used
to
provide
even
better
‘egulation
than
could
be
obtained
by
using
a
ballast
tube.
A
filtered
half-wave
rectifier
(CR104
and
C106)
drives
a Darlington
сопгес-
tion
of
Q1
and
Q2.
A divider
string,
R101
and
R102,
uses
the
plate
of
V104
to
maintain
approximately
6.3
volts
on
pin-9
of
V105
while
pin-4
and
pin-5
are
main-
tained
at
approximately
0 volts
through
the
emitter-
base
junctions
of
Q101
and
Q102.
Thus
any
change
in
the
half-wave
rectifier
voltage
causes
a corresponding
change
in the
voltage
across
the
emitter
collector
junc-
tion
of
Q101
so
that
the
filament
voltage
of
V105
remains
stable.
3.5. KELVIN-VARLEY DIVIDER
a.
Inorder
to
null
unknown
voltages,
each
one
of
the
four
precision
voltages
available
from
the
reference
sup-
ply
must
be
made
ad
ustable.
The
five
Kelvin-Varley
decade
resistor
strings,
R401
through
R449
accomplish
this
function.
b.
Note
that
each
str
ng,
with
the
exception
of
the
first,
parallels
two
resistors
of
the
string
that
preceeds
it.
Between
the
two
wipe:
s
of
S6
(voltage
dial
A)
then,
there
is
a
total
resistance
of
40K
(80K
paralleled
by
80K).
With
the
RANGE
switch
in
the
500
volt
position,
a
total
vol-
tage
of
100
volts
DC
will
appear
across
these
two
wipers.
Also,
there
will
be
10,
1,
and
0.
1
volts
DC
across
the
wipers
of
S7,
S8,
and
S9
respectively.
Voltage
dial
E
(S10)
picks
іпсг
те
ts
of
0.
01
volts
DC
from
the
last
decade.
These
voltages
are
reduced
by
a
factor
of
10
for
each
lower
voltage
range.
c.
All
the
resistors
of
each
decade
are
matched
and
all
decades
are
riatched
for
each
instrument.
providing
an
overall
divider
accuracy
of
0.
005%
absolute.
d.
With
the
NULL
switch
in
any
null
range,
the
out-
put
of
the
Kelvir-Varley
divider
is
connected
in
series
with
the
input
а
tenuator
as
seen
from
the
input
terminals.
Polarity
1
І
І
1
'
+
я
/
€
1
Internal
Reference
Voltage
і
3
І
|
І
І
І
І
I
І
І
1
)
І
N
І
|
1
і
1
Ја
Figure 3-2. FUNCTION OF POLARITY SWITCH
3-3