Introduction; Block Diagram Analysis; Circuit Descriptions - Fluke 415B Instruction Manual

415B
Section
3
heory
of
Operation
3-1.
INTRODUCTION
3-2. This section describes the theory
of
operation
of
the
Model
41 5B.
Refer
to
the block
diagram, Figure
3-1,
and
to
the functional
schematic diagram
in
conjunc-
tion
with
this text.
The schematic
is
located
in the
rear
of this
manual, following Section
V.
3-3.
BLOCK
DIAGRAM ANALYSIS
3-4.
The Model
41
5B
high voltage
power
supply
con-
sists
basically
of
a
power
transformer,
high-voltage
rectifier,
series pass
regulator,
error
amplifier,
reference
divider
and
voltage control resistor string.
It
also includes
a
time delay
circuit, auxiliary
power
supply, +125
volt
regulator,
temperature-compensated
reference
voltage,
overcurrent
protection circuit
and
a voltmeter.
3-5.
The
input line voltage
passes through
the
power
transformer and
is
converted
to
output dc
voltage
and
auxiliary dc support
voltages.
A
30-second delay
is
imposed
on application of
ac
to
the high voltage
rectifier.
3-6.
The
high voltage
dc appears
across the series
pass tubes
and
the
output
terminals.
The
magnitude of
output voltage
is
set
by varying
the resistance
of
the
voltage control string,
and
is
maintained
at
this level
by varying
the
conduction
of
the
series pass
tubes in
response
to
control signals
from
the
error
amplifier.
3-7. The voltage control circuits include the reference
tube, reference network, voltage control resistor string
and
the
error
amplifier.
The
reference
tube
provides a
constant current
to
the
reference network and
voltage
control resistors
which are connected
in
series
to the
common
output buss.
One
input
to
the
error amplifier
is
the
summation
junction
of the
reference network and
the voltage control string.
The
other input is
connected
to the positive output buss.
Any
voltage difference be-
tween
the two inputs is
amplified,
and used
to
drive the
series pass
tubes
which maintain the output voltage con-
stant.
The
front
panel
meter
provides an
indication
of
output
level.
3-8.
CIRCUIT DESCRIPTIONS
3-9.
Time
Delay
3-10.
When
the
POWER
switch
is
set
to
ON,
ac
is
applied to the
series pass tube filaments and
to the
auxiliary supply.
There
is a 30
second delay before
high voltage
can be
supplied.
This permits
the auxil-
iary voltages to
reach operating
level
and insures
that
voltage control circuits
have
control
when
high voltage
is
applied.
This
delay
is
provided, by
K1 on
the
High
Voltage
Rectifier
Assembly; K1
operates only
if
the
HIGH
VOLTAGE
switch
S2
is in
STDBY/RESET
position.
The
dc output
common
buss
is
also open circuited
when
power
is first
turned
on.
To
close
the
output buss, the
time delay
must be completed and
relay
K4 operated by
the
HIGH
VOLTAGE
switch.
3-11. In referring to the functional schematic,
it
should
be noted
there are two ground
return
symbols
in the
primary
circuit of the
power
transformer. The
ac re-
turn
(
^
ac)
connects
the
low
side
of
the
primary
to
the
low
side of
the
power
line
through
the
POWER
switch
SI.
The
dc return
(
^
R)
completes
the return path
of
the
half-wave
rectifier
consisting
of
CR1,
Rl,
R3 and
Cl. This rectifier
supplies dc
to
operate relays K2,
K3
and K4 on
the
High Voltage
Rectifier
P/C
Assembly.
When
the
time delay cycle
is
completed, dc
is
supplied
to K2.
Contact
K2B
opens and removes
the
time
delay
relay
from
the circuit.
Contacts
K2A
and
K2C
close and
K2A
illuminates the
STDBY/RESET
lamp.
When
the
HIGH
VOLTAGE
switch
is
set to
ON
position, the
HIGH
VOLTAGE
lamp
illuminates
and
relays
K3
and
K4
are
operated
from contact K2C.
Contacts
K3A
and
K3B
complete
the
ac circuit
of
the high voltage
rectifier.
Contacts
K4A
and
K4B
complete
the
dc output
circuit.
3-12.
High Voltage
Rectifier
and
Series Pass
3-13.
The multitap high voltage winding
of
the
power
transformer provides
voltage
to the high voltage
recti-
fier
according
to
the position
of the
first
OUTPUT
VOLTAGE
switch. The
multitap feature
provides ap-
proximate control
of the
unregulated dc output of
the
Rev.
1
3-1