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INSTRUCTION MANUAL
Digital Multimeter
Models 172A. 173A
THEORY OF OPERATION
e.
Current-to-Voltaqe
(I/V)
Converter.
I)
Model
173A I/V
Converter.
(Figure
5-5).
The converter
is
shown
on the
simplified
schematic
An input
current
is
converted
to a voltage
directly
proportional
to
the
input
current
by measuring
the
voltage
drop
across
a resistance
placed
in series
with
the
current.
This
is
accomplished
by closing
the
appropriate
range
relay
(K501
through
K505)
to give
an output
voltage
between
0 and 300 millivolts.
For example:
Consider
that
K503 is
closed.
The current
will
now flow
through
9E + 0.9r:
+ O.lc
= IOQ, and
30mA = 300mV.
Since
a dc current
is
converted
to a dc voltage
and an ac current
is
converted
to an ac voltage,
the
output
of
the
I/V
converter
is
routed
through
either
the
DC attenuator
circuit
or
the
AC/DC converter
before
being
applied
to
the
A/D
converter.
Signal
routing
is
controlled
by the DC/AC pushbutton
and the
gains
employed
are
the
same as those
given
in Table
5-2
for
the
300mV range.
input
protection
for
the
I/V
converter
consists
of a 3 ampere
medium
acting
fuse
(F5Ol)
and a diode
bridge
(CR50
The fuse
is
rear
panel
accessible.
CR501 limits
the
voltage
to 2 diode:drops
in either
positive
or negative
polarity.
Unity
gain
buffer
amplifier
U5Ol guards
the
center
connections
of
the
diodes
so that
less
than
one digit
error
is
introduced
on the
300uA
range,
which
has
IO nanoampere
sensitivity.
Refer
to Schematic
27478D
for
the
remainde
of
this
discussion.
Range control
lines
(A,
B and C from
LSI
chip
UP'JI)
autoranqe
the
Model
l73A
current
function.
The
lines
are
decoded
by the
BCD to decimal
decoder
U502
(See Table
5-3).
The output
from
U502 turns
on the
applicable
relay
driver
transistor
to energize
its
range
relay.
Diodes
are
used
to block
the
range
lines
when -12V
is
applied
and
input
dividers
are
used
to drop
+8V to +5 volts
for
application
to U502.
Spike
suppression
diodes
are
used
across
the
relay
coils
to prevent
damage
to
the
relay
drivers.
Potentiometers
~506,
R509 and R512 provide
adjustments
for
calibrating
the
I/V
converter.
With
the
exception
of
the
fuse,
all
of
the
I/V
converter
circuitry
is
physically
located
on current
~~-406
and shown
on Component
Layout
diagram
27884C
in
Section
7.
TABLE 5-3.
173A Current
Ranging
RANGE
RESISTANCE
IK
I OOR
IOR
IR
O.lQ
RANGE LINE
RELAY
LEVELS
OPERATED
A
B
c
I
2)
Model
l72A
I/V
Converter.
This
current
converter
(shown
in
Figure
5-6)
consists
of
a I ohm resistor
(Rl54)
and a 2A orotection
fuse
(F501).
The
input
current
passes
through
the
resistor
developing
an output
voltage
from
0 to 2 volts.
For example:
a
475mA input
will
produce
an output
voltage
of
475mV.
Since
an ac current
input
will
produce
an ac voltage
and a dc input
will
produce
a dc voltage,
the
output
of
the
I/V
converter
is
routed
through
either
the
DC attenuator
or
the
AC/DC converter
before
beir
applted
to the
A/D converter.
Routing
is
controlled
by the
position
of
the
DC/AC push-
button,
and the
gains
used
are
the
same as those
given
in Table
S-2 for
the
300mV (3000
5-i
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