31
3. Service
data
3.1. CIRCUIT
DESCRIPTION (see fig.
25)
The
measuring bridge PM 6302
is
made of:
—
a switchable bridge arrangement
—
a
switchable
zero indicator
—
a supply voltage
generator, 1 kHz/100 Hz and 1.5 V d.c.
—
a conversion facility for searching and measuring
—
a
power
supply.
3.1.1. Bridge
arrangements
There
are three selectable measuring ranges, viz:
R measurements, C measurements and L measurements.
The basic set-up of the arrangements for
C and L measurement (for reactances) is illustrated in figure 7.
In this
case, the four bridge parts, the supply source and the indicator are illustrated for C measurement as far
as the potential is concerned.
The same arrangement
applies for L measurement, except for the fact that C515 and R713 are interchanged.
This illustration can be used combined with fig. 8 for R measurement; assume C515 and R713 interchanged,
fixed resistor R634 instead of
R713 and bridge supplied
from
a
floating
d.c.
voltage
source.
3.
1. 1. 1.
R
measurement
The Wheatstone bridge (Fig.
8)
comprises resistors
701-71
1 for measuring ranges in decade steps. They
determine the coarse measuring range and form together
with
the sample (actual
resistance)
one
arm of the
bridge.
The other
one
is
formed
by
components 634
and
71
3
+ 636.
Fixed resistor 634 is adjusted with 635 and
658,
while the initial resistance of potentiometer 713 is adjusted
with resistors 637 and 659 and 636. Both adjustments determine measuring points 1
and
10 of the linear
scale.
The bridge for R measurement is supplied with an internal d.c. voltage.
The diagonal voltage passes through
a dc-ac converter before entering the input of the
indicator amplifier.
Arrangement in searching mode for R measurement, see
3.1.4.
3. 1.
1.2.
C measurement
The dc Sauty-bridge (Fig.
9)
comprises the same measuring range resistors
701-711 and mostly complex sample
C
x
in the (left) arm of the bridge. The
other
arm
comprises components 713 + (636//637//659) and 515 in
series with 712/A or 712/B.
As a
result
of
this both bridge arms are made
of RC components which divide
the supply voltage equally and in the same phase, provided the
time constants are equal.
The diagonal voltage between P and
earth is then
0
V.
The results in adjusted and not adjusted state are vectorially shown in figure 10b and 10c. These figures apply
for loss-free samples.
For lossy capacitances a phase adjustment by means
of
tandem
potentiometer
712/..
is required.
This
adjustment simulates an attenuation of adjusting capacitor 515. For correctly adjusted phase, the
simulated attenuation equals that of the sample
(Fig.
1
1-b) and
its angle loss and tangent values
are
equal;
potentials
0
(i) and P compensate each other.
Tandem potentiometer 712/..
is
marked
in
tan5
=D
values; The attenuation range is switchable with push-
button D= 801/F.
The end
values of the ranges
are adjusted with
resistors
633//712/A
and 632//712/B. The frequency
of supply
generator
312-314
is converted from about 1 kHz
to about
100 Hz by depressing button D (see
also 3.1.3.}.
The
calibration of 712/.. in
D
values
depends on the frequency
of the supply voltage
to the
bridge; therefore
pay attention to this in case
of checking or
external
bridge supply. The calibration
of
adjusting potentiometer
71
3
is
almost independent of
the
frequency of the
supply
voltage.
Two examples of a not-adjusted de Sauty-bridge are shown in Fig.
11-c by dotted
and
chain-dotted vectors.
The other vectors represent the
voltage division of
the bridge arm, in which the
sample
is present.
In the
first case, the simulated losses are greater,
while in the second
case they are
smaller than
the losses (attenuation)
of
the
sample.
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