10
1.4.
DESCRIPTION OF THE BLOCK DIAGRAM (see Fig.
1
)
The bridge circuit comprises
range resistors,
reference
parts
and
a
fine-adjustment potentiometer.
The range resistors in the bridge
circuit are
identical for all measurements
and can be
switched-over
with the
range
selector.
For R-measurement
a resistor is used as a reference, for
C-measurement
a
capacitor with
a series-connected
potentiometer and for L-measurement
a
capacitor
and a
potentiometer; these are series-connected in the
higher Q-range and parallel-connected in the lower Q-range. The fourth
branch of
the
bridge
is the sample
itself.
The bridge circuit is earthed at that end of the measuring diagonal which is connected to the adjusting
potentiometer and the reference parts. In this
wav,
harmful influence of switching and stray capacitances
remains small.
The d.c. voltage supply for the bridge is originated from a separate mains
part,
the input voltage of which is
tapped from
a
extra-screened
secundary winding of the mains
transformer.
The internal resistance of
this
mains
part is
dimensioned
in
such
a
way that low-ohmic measuring objects can
not be overloaded.
The a.c. voltage supply for the bridge is originated from
a
Wien-bridge oscillator, the output of which is coupled
to the bridge circuit via a transducer.
The mater
amplifier
amplifies the voltage across the diagonals of the bridge circuit. For R-measurements a
d.c. voltage appears, which
is
chopped
by an
electronic switch and amplified in an a.c. voltage amplifier
(identical for
all measurements) with high-ohmic input
resistance.
The output voltage of the amplifier is rectified and indicated with a
moving-coil indicator. As
a
result of the
logarithmic characteristic of the amplification, the indication sensitivity is higher
for
lower input voltages, i.e.
near the bridge balance.
Push-button SEARCH
is used to search the correct measuring range. As long as this button is depressed, the
supply voltage of the bridge is connected to
a
series circuit consisting of the sample and the measuring range
resistor. The voltage drop across the sample is indicated. In this mode the meter-amplifier characteristic is
switched-over into
a
linear amplification characteristic.
2.
Directions
for
use
2.1. INSTALLATION
2.1.1. Safety regulations
(see
IEC 348
or
VDE
0411)
Before using the instrument after
stocking and transport, pay attention that the instrument
has
not been
mechanically damaged. If
there is doubt about the safety (of the protection) check this.
If the
protection
is no
longer guaranteed,
disconnect the instrument
from
the mains and do not operate it.
Do not open the
instrument
before
it has
been disconnected from the mains.
Maintenance and service
works, which must be done with the instrument on,
should only
be
performed
by
a
qualified technician.
The mains connector may only be
plugged
in a trains socket with earth contacts, It is not allowed
to
make
this
safety protection inoperative,
e.g.
with an unproper extension cable.
1.2. Controls and sockets
(see
Figs.
2 and
3}
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