section
111
Resistance Range
.
c. Deenergize circuit
to
be
measured.
OpenCircuit Volts
(at
1
on top scale)
d. Connect
412A
into circuit,
using
MA/OHMS
and
COM leads.
e.
Energize circuit,
set
POLARITY
switch
for
up-scale indication, and read current.
NOTE
When making current measurements,
make
sure
there
is
no connection between
the circuit common
6)
and power line
ground (&)terminals on rear panel.
3-10.
RESISTANCE
MEASUREMENT.
a.
Set
FUNCTION switch
to
OHMS.
b.
If resistance
to
be
measured
is
part of a circuit,
make sure
power
is
turned
off.
c.
Connect
412A
across circuit
or
component
to
be
measured, using
MA/OHMS
and COM leads.
XI
XI0
XI00
X1
K
XlOK
XlOOK
XIM
XlOM
XlOOM
Model
41
2A
d.
Set
RANGE switch
so
that meter indication is as
near center scale as possible, and read resistance on
OHMS
scale.
3-1
1.
When measuring the resistance
of
non-linear devices
such as transistors and diodes, it is often desirable
to
know
what voltage and current were present in obtaining the
resistance measurement. Table
3-1
lists the open-circuit
voltage and shortcircuit current available at
the
412A
leads
during resistance measurements. Voltage and current are
read on the top scale
of
the meter
for
all
resistance ranges.
The meter reading is directly proportional
to
the voltage
across the device being measured, and inversely
proportional
to
the current.
As
an example, assume that
a
device being measured on the
XI0
range gives a reading
of
.25
on
the resistance scale. The meter
also
indicates
.2
on
the
top
scale. Since full-scale voltage for the
XI0
range is
100
mV (Table
34),
this indicates a voltage
of
20
mV
across the device. To determine the current, subtract the
meter reading
of
.2
from the full scale reading
of
1,
and
multiply by the shortcircuit current listed in Table
3-1
.
Since the current shown
for
the
XI0
range
is
10
mA,
this
indicates
a
current
of
8
mA through the device being
measured. The resistance measurement, then, indicates an
equivalent resistance
of
2.5
ohms, with an applied voltage
of
20
mV, and a current
of
8
mA.
Table
3-1.
Voltage and Current in Resistance Measurements
1
10
mV
100
mV
IV
1v
1v
IV
IV
1v
1v
ShortCircuit Current
(at
0
on tot, scale)
10
mA
10
mA
10
mA
1
mA
l00pA
IO
pA
lPA
0.1
pA
0.01
pA
3-2
00
1
29-5
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