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INSTRUCTION
MANUAL
Digital
Multimeter
Models
172A.
173A
OPERATING
INSTRUCTIONS
c.
Current
Measurements.
I.
Overloads.
Fuses
are
as follows:
Modell72A:
2A
Model
173A:
3A
2.
Shunt
Resistance.
The Model 173Adevelops
approximately
300 millivolts
across
the
input
terminals
at
full
range.
The172Ahas
1.3
R on all
ranges.
For example,
on the
300 milli-ampere
range
the
shunt
resistor
is
1.3 ohm, which
results
in a voltage
drop
of 0.3
x I.3
= 390 millivolts
at
full
range.
3.
ACCUrXy.
l72A/l73A
accuracy
is
specified
in
terms
of
a % of
reading
and
so many digits.
An additional
reading
error
should
be considered
if
the
source
re-
sistance
is not
greater
than
1000 times
the
shunt
resistor.
The amount
of
error
can
be determined
by the
following
relationship:
where
Rin
= shunt
resistance
of
the
Model
172A/i73A
Rs = source
resistance.
d.
Resistance
Measurements.
I.
Maximum
Allowable
Voltage
Input.
The maximum
input
should
not
exceed
250V ~~15
sine
wave or
?25OV dc.
2.
Polarity
of
Ohms.
The Model
172A/l73A
provides
a positive
voitaqe
at the
PI
terminal.
3.
Maximum
Open-Circuit
Voltage.
When the
HI and LO terminals
are
open
in either
HI or
LO ohms,
the
maximum voltage
developed
between
HI and LO is +5 'dolts.
This
illustration
is
typical
of a 4-terminal
measuring
technique
using
a separate
Voltmeter
and Current
Source.
The Model
1?2A/l73A
combines
these
features.
"HI"
LEADS
;?SOURCE
HIGH
"LOW" LEADS
9 SOURCE LOW
When using
the
4-WIRE method,
connect
one
Pair
Of
'lcurrent"
leads
to R SOURCE input.
connect
a
second
pair
of
"voltage"
leads
to HI and LO.
Then
set
to 4 WIRE.
Care should
be taken
to connect
"current"
leads
and "voltage"
leads
properly.
The LO terminal
and n Source
low must
always
connect
to the
same
side
of
the
unknown
resistance.
FIGURE 3-5.
Typical
4 WIRE Resistance
Measurement.
3-9
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