John Fluke 3330B Manual page 25

3330B
Figure 3-2.
BASIC INSTRUMENT
Cl
RCUITRY
{DRAWN WITHOUT THE PREREGULATION
LOOP)
nished
by
the reference supply
and
ladder
network
pro-
vides
a
current through the range
resistor
equal
to the
dialed
output.
The
voltage
developed
across the range
resistor
furnishes
a signal
to the control amplifier that
determines the
rate
of conduction of
the
series
pass
element.
The conduction
level
of the
series
pass
element
then
establishes
the appropriate
output
voltage or current.
3-16.
Two
major loops actually
form
the
instrument
circuitry.
One
loop
consists
of an ac pre-regulator
and
the
other
is
the
main
dc
regulator.
Each
of
these
loops
is
de-
scribed separately
in
the following paragraphs.
3-17. Voltage
Mode
3-18.
PRE-REGULATOR.
This loop
consists of the
Pre-regulator
and the Voltage Controlled
Oscillator
(VCO)
shown
in
Figure 3-3.
The
VCO
samples the voltage present
across the
series
pass element
and
supplies
pulsed timing
information
related to
the load
power
requirement.
These
pulses are
synchronized
to
the
start
of the
line
frequency
cycle
and
are applied to the pre-regulator.
The
pre-regu-
lator then,
initiates line
current
flow to the high voltage
supply
in only the
amount
necessary to
meet
the actual
load requirement, thereby
improving
the efficiency
of
the
instrument.
3-19.
MAIN
REGULATOR
LOOP.
The main
regulator
loop
consists
of
a
Reference
Supply and Ladder Network,
a
Control Amplifier, the
Series
Pass
Element, and
a
High
Voltage
Power
Supply.
The
configuration
is
shown
in
Figure 3-3 and
is
redrawn
in
bridge
form
for
explanation
in
the
same
illustration.
3-20.
The
reference supply /ladder
network and
RSOURCE form one
side
of the
bridge,
while the
series
pass
element/high
voltage
supply and
RraNGE
forni
the
other
side.
What
the operator
is
doing
when
he
dials
an
output
voltage
is
to vary the
Ladder
Network which
effec-
tively produces a variable EreFERENCE.
The
current
from
EreferenCE
is
determined by
the
Ladder and
the
effective voltage
of
Eref.
This current passes through
RraNGE
developing
a voltage across
it
which
is
equal to
the
dialed output voltage.
When
the voltage across
RsOURCE
equals
Eref, and
the voltage across
EranGE
equals EoUT, a stale
of balance
exists across the bridge,
and
there
is
zero
potential across the inputs to
the
amplifier.
When
a
new
output
voltage
is
dialed into the instrument,
a
state
of imbalance
will
momentarily
exist
because EquT
will
no
longer
be
equal to the voltage across RraNGE.
If,
for
example,
the
instrument
is
operating
at
100
volts
and
IS
subsequently
dialed to
500
volts,
a positive error
voltage
will
appear
at
the
summing
junction
and
at
the
input to the
amplifier.
This amplified positive error vol-
tage (the control amplifier
is
non-inverting)
is
applied to
the
series
pass
element
causing
it
to
conduct harder
and
resulting in a
lower
voltage
drop
across
it.
Through
the
action of the
series
pass
and
the pre-regulator
(mentioned
later),
the
output
voltage
will rise until
it
equals the
vol-
tage across RraNGE, or
until
the bridge
is
again balanced.
The
gain of the control amplifier
is
very high
and
provides
the bulk of the regulation.
3-21.
At
the
same
time the control amplifier
is
causing
the
series
pass
element
to increase in
conduction,
or, in
other
words, producing
a
lower
voltage
drop
across the
series pass,
the
VCO
is
sampling
EgERIES PASS
^ud
affecting the
rest
of the pre-regulator loop to increase the
output of the high voltage supply.
*
3-22. Current
Mode
3-23.
PRE-RIiGULATOR LOOP.
Operation of
the pre-
regulator
loop
is
essentially the
same
in current
mode
as
it
is
in
voltage
mode. The
voltage across the
series pass ele-
ment
is
sampled by
the
VCO
which
causes the pre-regulator
to
supply
power
to the high voltage supply
in
the
amount
necessary to
meet
load requirements.
A
simplified block
diagram of
this
mode
is
shown
in
Figure 3-4.
3-24.
MAIN REGULATOR
LOOP.
The main
regulator
loop,
like
the pre-regulator loop, operates
essentially the
same
in
the current
mode
as in the voltage
mode. The
only
difference
is
that the
loop
acts to
maintain
a
constant
vol-
3-3