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' iEORY
OF OPERATION
INSTRUCTION
MANUAL
Digilal
Multimeter
Models 172, 173
5-5.
CURRENT BOARD.
Schematic
27478D
contains
all
of
the
current
circuitry
which
is
( Nntained
in
the
Model
173 and also
in
the
172.
The
current
circuitry
for
the
Model
172,
I mown at
the
bottom
of
the
schematic
consists
of
a 2 ampere
fuse
and
a one ohm resistor.
It
is
switched
in
front
of
the
ac voltmeter
or
dc voltmeter
for
ac or
dc amps.
it
allows
200 milliamperes
on the
300 millivolt
range,
and up to
2 amperes
on
the
3 volt
range.
/
limit
of
2 amperes
limits
the
power
in
the
I ohm resistor.
The
input
impedance
is
approximately
I.3
ohms on both
the
300 milliampere
and
the
2 ampere
range.
Above
I
ampere
the
self-heating
effect
and TC of
the
resistor
causes
the
reading
error
to
ICreaSE!.
rur
Model
173 the
voltmeter
is
held
on
the
300 millivolt
range
and
resistors
are
switched
by means
of
relays
K501,
502,
503,
504,
or
505.
KS01 connects
the
input
on the
300
I
croampere
range,
K502 on the
3 milliamp
range,
K503 on 30 milliamps,
K504
on 300 milli-
,ps,
and
K505 on the
3 ampere
ranqe.
The
current
resistors
are
connected
in
series.
ix502
is
connected
to
the
high
of
the
voltmeter
(high
sense).
R510 goes
to
signal
low (low
me).
The measurement
is
thus
done
four
terminal.
The
3 ampere
range
is
adjusted
with
>tentiometer
R512,
the
300 milliampere
range
by R509,
and
the
30 milliampere
range
with
R506.
Input
protection
is
provided
by diode
bridge
CR501,
which
limits
the
voltage
to
7 diode
drops
in
either
positive
or
negative
polarity.
Unity
gain
amplifier
US01 guards
'me center
connection
of
these
diodes.
This
assures
good
performance
on
the
300 micro-
ampere
range
which
nas
IO nanoampere
sensitivity.
A 3 amp medium
acting
fuse
limits
the
maximum
current.
The
fuse
is
rear
panel
accessible.
The
relay
drive
tran-
istors
are
driven
by U502.
Spike
suppression
diodes
across
the
relay
coils
prevent
Jmage to
the
transistor
drivers.
Range
lines
from
the
LSI
chip,
lines
A,
B, and
C
autorange
the
ohms function,
and
the
Model
173 current
function.
They
are
decoded
in
-:O
to decimal
decoder
U502.
See Table
5-7.
There
is
a possibility
of
a small
period
of
;me during
ranging
where
all
relays
may be open.
This
will
not
present
an open
circuit
to
the
source
because
the
diode
bridge
will
conduct.
DiodesCR507,
508,
509 block
the
"gnal
lines,
A,
B, and
C from
reverse
biasing
the
inputs
of
U502 when
at
-12V.
Resistors
;l5,
516, 517, 513G, 513H
and
513F
are
input
dividers
to drop
+8 volts
to
5 volts
to
protect
U502.
Resistors
R513D,C,
A,
B and
E limit
base
current
to
the
transistor
drivers.
Resistors
for
the
top
three
current
ranges
are
adjusted
in shunt
rather
than
in
series.
lthough
adjustment
ranges
are
equal
plus
and minus
from
the
nominal
value,
the
adjust-
dnts
are
non1 inear.
Resistors
R5l 1, R508,
and R505
limit
the
range
of
the
adjustment
in one direction.
The
pots
plus
these
resistors
limit
the
adjustment
range
in
the
2posite
direction.
U5Ol
hasamaximum
input
current
of
7 nanoamperes
(less
than
1 digit
3 the
300uA
current
range).
TABLE 5-7.
173 Current
Rangina
I
RANGE
RANGE
ACA t DCA
RESISTANCE
300~A
IKR
I
3mA
1 OOR
3A
IOR
I
300mA
IO
3A
0.1n
RELAY
OPERATED
KS01
KS02
KS03
KS04
A
+8v
+8v
-12v
-12v
-12v
UGE LINE
.EVELS
B
-12v
-12v
+8v
+8v
-12v
+8v
-12v
+8v
- 12v
+8v
5-12
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