B&K 281 - Theory of Operation; Notes on 10 Ohm Range

When
using
the
10
ohm
range,
the
third
digit
of
the
281
may
indicate
"
SPARE
opting
FUSE
some
small
resistance
when
the
test”
M
RE
n.
leads
are
shorted
together.
This
is
the
actual
resistance
of
the
test
probe
assembly.
If
this
indicated
value
is
considered
significant,
it
should
be
subtracted
from
the
re-
sistance
being
measured.
DO
МОЯ
=
ATTEMPT
TO
CORRECT
THIS
RESIDUAL
READING.BY
AD:
JUSTING
THE
INTERNAL
ZERO
ADJUST
CONTROL
OF
THIS
INSTRUMENT.
.
OHMS FUSE F101
= POWER USE
F102
Figure 2.
E.
MODEL
281
DIGITAL
MULTIMETER
THEORY
OF
OPERATION
(Refer
to
Figure
3.
Block
Diagram)
The
Model
281
digital
multimeter
operates
on
largely
the
same
principles
as
the
common
FET
VOM,
differing
basically
in
the
readout
mode.
The
281
incorporates
a
direct
reading
digital
display.
The
basic
unit
consists
of
a
dc
voltmeter
with
100
mV
capacity
and
100%
overrange
capability.
The
higher
voltage
ranges
are
attained
by
the
use
of
a
precision
switchable
attenuator.
The
ohms,
ac
volts,
and
current
functions
are
developed
by
circuits
which
convert
the
unknown
to
be
measured
to
a
corresponding
dc
voltage.
The
unknown
can
then
be
handled
as
a
dc
voltage
by
the
basic
voltmeter.
The
digital
dc
voltmeter
consists
of
an
impedance
converter,
an
analog
to
digital
converter,
a
digital
counter,
and
readout
indicators.
Wrong
polarity
and
overrange
sensors
are
included
to
aid
in
correct
use
and
interpretation
of
the
readout.
Circuit
designations
for
major
devices
of
each
section
are
on
the
block
diagram.
The
impedance
converter
is
a
voltage
follower
that
lowers
the
impedance
of
the
input
voltage
without
changing
its
magnitude,
polarity
or
zero
reference.
The
analog
to
digital
converter
changes
the
output
of
the
impedance
con-
verter
to
a
series
of
pulses
the
number
of
which
is
proportional
to
the
input
12