Resistance; Continuity - Fluke 8021B Instruction Manual

8021
2-39
Full-scale
burden
voltage (voltage
drop
across the fuse
and
current shunt)
for
ail
ranges except
2000
mA
is
less
than 300
mV. The
2000
mA
range has a
full-scale
burden
voltage
of
less
than
900
mV. These
voltage
drops can affect
the
accuracy
oi
a current
measurement,
if
the current source
is
unregulated and the
shunt
plus fuse resistance
represents
a significant
portion (1/1000
or
more) of the source
resistance.
If
burden
voltage
does present
a problem, the
percentage of error
can be calculated
using the
formula
in
Figure
2-12.
This
error
can be
minimized by
using the highest current
range
that
gives the
necessary
resolution.
For
example,
if20
mA
is
measured on the
2000
mA
range
the
burden
voltage
is
approximately
5
mV
2-40.
Resistance
2-41.
Six direct
reading resistance scales are
provided on
the
802!
B:
20
Mfl, 2000
kft,
200
kfl,
20
kfl,
2 kfl
and 200ft. All scales
employ
a
two-wire
measurement technique.
As
a
result, test
lead resistance
may
influence
measurement accuracy
on
the
200fl
range.
To
determine
the error,
short the
test
leads together
and read the lead resistance.
Correct
the
measurement by
subtracting the lead resistance
from
the
measurement
reading.
The
error
is
generally
on the
order of 0.2 to 0.3
ohms
for
a standard pair
of
test
leads.
2-42.
Three resistance
ranges
(2
kft,
200
kft,
and 20
Mfl) have
a high
enough open
circuit
voltage to
turn on
a silicon junction.
These ranges can
be used
to
check
silicon
diodes and
transistors
The
2 kfl
range
is
preferred for this
function
and
is
marked with a diode
symbol.
On the three alternate ranges (200ft,
20
kft,
and 2000
kft),
the
full
scale
voltage
is
not
high
enough
to
turn a silicon
junction on.
Use
these
ranges to
make
in circuit
resistance
measurements.
Maximum
full
scale voltage
and
short circuit
current for
each
resistance
range
is
given
in
Table 2-3.
All values
shown
are
referenced to the
COMMON
input
terminal;
i.e.,
the
V/fl terminal
is
positive.
MOTE
Any
change
(greater than
one
or
two
digits
)
in
apparent resistance
when
test
leads are
reversed
may
indicate either the
presence of a diode junction or
a
voltage in
the circuit.
CAUTION
Turn
test
circuit
power
otf
and
discharge
all
capacitors before
attempting
in-
circuit
resistance
measurements.
2-43.
Continuity
2-44.
Audibly confirmed continuity
measurements
may
be
made
using
any of the
resistance
or
conductance ranges.
The
2
kft
range
is
recommended
for this
mode
since
it
provides the lowest
resistance threshold,
approximately
110
ohms.
On
this
range
a
measured
resistance
lower
than
110
ohms
initiates
the
audible tone.
To
determine
the
exact threshold,
use a variable
resistor
and note
the display
reading
at
which
the tone
switches
on and/or
off.
2-19