Fluke 415B - Page 33

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