Advertisement
iEORY
OF OPERATION
INSTRUCTION
MANUAL
Digital Multimeter
Models 172.173
5.
The ohms switching
FETs
(Q703,
Q702)
are
driven
by transistor
Q701 and
the
C-line,
Q710
is
driven
from
the
A-line
and Q707
is
driven
from
the
B-line.
These
lines
are
also
used
and decodes
to operate
the
Model
173 autoranging
current.
Amplifier
U702
is
a
FET input
amplifier.
This
amplifier
is overload
protected
by Q709 base
emitter
junction
connected
to
the
signal
ground,
clamp
transistor.
and
Q708 base
emitter
junction
connected
through
Q706
Thus
the
input
swing
is
clamped.
the
output
side
of
U702 are
allowed.
Very
large
voltage
excursions
on
This
is
done
by Q704,
Q705,
and
CR703.
Under
normal
operating
conditions
Q704
is operated
in
the
saturated
mode,
However,
under
overload
conditions,
Q704 becomes
a current
source
with
its
collector
to
emitter
break-
down
handling
large
voltage
excursions
when
signals
are
positive
with
respect
to
the
ohms source
high.
However,
if
signals
are
negative
with
respect
to
the
ohms source
high,
CR703 reverse
biases,
cutting
off
that
portion
of
the
circuit.
All
of
these
high
voltages
will
cause
current
to
flow
through
R703
(IZOKR)
which
is
connected
to
the
+I5
volt
supply.
The maximum
limitation
of
250 volts
rms
is
a function
of
the
power
rating
of
R703
(a half
watt
resistor),
and
Q704's
400 volt
breakdown.
TABLE 5-3.
OHMS RANGES
I
I
I
RANGE
RANGE
REF
HIR
LOR
R
"I
3;:
300::
3.3444K
3.3444"
3.3444K
-33444"
300K
3::
334.44K
3.3444"
3M
300K
334.44K
.33444"
3OM
33.444M
3.3444"
300M
3::
33.444M
.33444"
/
OHMS
CURRENT
I mA
IOOuA
IOuA
IvA
100 nA
IO nA
9.
Range
Selection.
RANGE LINES
Ranging
is
accomplished
by RSA t RSB lines
going
to
LSI
circuit
UlO3.
by the
front
panel
switches
AUTO/MAN,
DN & UP, according
to Table
5-4.
They
are
controlled
RSA & RSB are
edge
sensitive,
causing
range
changes
when
their
levels
are
changed.
This
causes
one
uprange
or
one downrange
per
button
push.
TABLE 5-4.
Range
Selection
Range
Switches
Range
Lines
AUTO/MAN
DN
UP
RSA
RSB
Action
OUT
X
X
0"
+8"
AUTORANGE
IN
OUT
OUT
0"
0"
HOLDRANGE
IN
IN
OUT
+8v
0"
DOWNRANGE
IN
OUT
IN
+8"
+8"
UPRANGE
X E DON'T
CARE
5-8
AA
Advertisement
