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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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