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.
To Next Page
Loading...
