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32
Points
O
and
O'
do
not
coincide with point
P,
the diagonal
voltages
Uo
and Uo'
are
finite.
By
alternate
use of
71
2
and
713, point
O
can be
made
to coincide
with point
P.
The
r.m.s.
supply
voltage
is
reduced
from 700
mV
to
70
mV
by means
of
the measuring-range
selector.
Input
DGBI AS
can be
used to
superimpose
the supply voltage with an
external
d.c.
voltage.
The
d.c.
voltage
is
supplied to the
sample
via
the
output winding
of 751
and
the measuring-range
resistor
in
operation.
Connection
conditions:
—
maximum
dc.
voltage
60
V, included
the eventual
common
mode
voltage.
Pay
attention to
the
maximum
test
voltage
and
the
polarity
of
the sample.
—
specific d.c. internal resistance
of source
>
50
fi/1
V
d.c.;
smallest value
30
for
60
V.
—
maximum
a.c.
internal resistance
(output impedance) of the
d.c.
voltage
source
<
10
SI.
—
if
possible,
supply the
(battery)
d.c.
voltage
symmetrically
and
floating.
Set
up
the
symmetry
parts
(e.g.
18
kfl
resistors
conform
to
Fig.
9)
and
the by-pass capacitor very
close to
sockets
DC-BIAS
(Fig. 7).
Electronically
stabilized
and
other mains
parts
can not be
used.
Arrangement
in
searching
mode
forC
measurement,
see 3.1.4.
3.
1.
1.3.
L measurement
The
Maxwell-Wien
bridge
(Fig.
12)
comprise
the
same
measuring-range
resistors
701-71
1
as
for
C
measurement.
Reference
reactance 515,
with
coarse
and
fine
attenuation
adjustment by
means
of
712/A
and
—
/B,
forms
In series
with
adjusting part
713 + 636
the
second
arm.
However,
compared
to the
C
measurement,
the
parts
are interchanged.
As
a
result
of
this,
the voltage
divider
has the
same
frequency
ratio as
that
of
the sample.
After
adjustment
of the
bridge,
the
time constants
L
x
/Rm
and (R71 3
+ R636)
C515
are identical;
the
diagonal
voltage
amounts
to
0
V, provided the phase
has
been
adjusted.
The
phase adjustment can be performed
by means
of adjustable
attenuation
of reference
reactance 515.
Two
ranges
are selectable
by
means
of
push-button
Q=
801/E.
Q
=
1/D
=
1/tan 5
can be
compensated from
<
1
to 200. Values
1-14
can be
read
directly
from
the
scale
of
712/..;
the value indicated
when
push-button
Q
is
not
depressed
must
be
inverted.
Fig.
13.
illustrates
an
adjusted
Maxwell-Wien
bridge.
The
losses
of sample
L
x
are
supposed
attenuation
resistor
R
x
in
series
with the
ideal
inductance
(fig.
13a).
The
reference reactance
515
can be attenuated
in
a
variable
way
by means
of
resistor
712/B,
which
is
cohnected
in parallel
with
515
for
depressed push-button Q. This
applies for values
from
1
to
14
(fig.
13c).
In
the other
case, resistor
712/A
is
in
series
with
the reference reactance 515.
The
quality range
Q
covers 7
to
>
100.
In
this case,
the
result
depends on
the frequency
of the
supply voltage
(see
also
figs.
13b
and
d).
For the
three lower ranges the
supply
voltage
is
reduced
from
700
mV
to
70
mV
by
means
of range switch
822/..
but
the
frequency remains
the
same.
Arrangement
in
searching
mode
for
L
measurement,
see 3.1.4.
3.1.2.
Zero
indicator
(fig.
25)
The
zero
indicator
comprises
a d.c.-a.c.
converter with
stages
301 and 302, impedance
converter
303
and
304,
searching
stage
305
as
well as
a
control
part Furthermore,
the zero indicator includes
a
voltage-feedback
operational amplifier
306
with bridge
rectifier
401404
and measuring instrument
821.
3.
1.2.
1.
D.c.-a.c.
converter
As
the diagonal of
all
bridge
arrangements
is
earthed
at
one
side,
the input
is
asymmetrical.
For
R
measurement
transistors
301 and
302
are
opened and
blocked
by means
of
overloaded
amplifier
307/308
in
feedback
loop.
When
301
is
blocked
for instance,
conductive
transistor
302, the operating point
of
which
is
related to
transistor
303,
takes over the
function of
transistor
301 and forms
a
conductive connection
to earth
via
100
kfi.
The
diagonal
voltage of the not-earthed bridge
arm, point
P,
reaches
impedance
converter
303
during
its
"on"
phase, also
via
100
kS2
and
301.
Voltage feedback
of
impedance
converter
303/304
is
performed by
resistors
603
and
604
via
304
resulting
in
an
amplification of
2.
For
the three
first
measuring
ranges,
which
are
operated with
a
10
times
reduced
supply
voltage, the amplification of the
impedance
converter
is
magnified 10 times
as a result
of
603
by-passing 505.
This
procedure does not
apply
for the
R-searching
mode.
Searching
stage
305
between
the
output
of the
impedance
converter
and
the input of the operational
amplifier suppresses shortly
the
signal
lor approximately
1
0
%
of the
period
time
at
every changeover.
When
using an
a.c,
voltage supply,
305
is
open
(Fig. 14).
The
control part
delivers
two
antivalent search
signals
with
a
period
time
of
20
ms
and
a
pulse
signal
with
a
frequency of
1
00
Hz.
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