Fluke 8021B - Page 45

8021
B
AC/
DC
junction
switches are
both pressed in.
When a measurement
is
made,
continuity
is
indicated by
ar, audible
tone. No tone
indicates an open
circuit or
a
circuit resistance above
the threshold of
the range
selected.
3-29. The
continuity circuit
consists of a
comparator, a one-shot, and a
tone
generator.
See Figure
3-3
D.
During a
measurement,
R pull-up and
V
source
develop
a
voltage across
the
measured
resistance. The
comparator compares this
voltage against an
internal 100
mV threshold
reference. If the
input voltage is
greater than the 100
mV threshold
reference, the
lone generator
is not enabled, a
no-continuity indication.
Conversely,
an
indication
of less
than 100
mV
causes
the comparator
to enable the tone
generator which
emits an
audible continuity
indication.
3-30.
Since the
values of
V source and R pull-up
vary with the
selected range,
it is
convenient to think
of the 100
mV threshold as a
resistance threshold.
The
resistance
threshold and the
V source/ R pull-up
values for each
continuity range
are
given
in the
following
list:
RANGE
R pull-up
V source
(VOLTS)
THRESHOLD
RESISTANCE
200 ft
4 kilohms 1 .
2 .
volts
360
ohms
2 kil 3
kilohms
2.8
volts
1 10
ohms
2 mS 3 kilohms
2.8
volts
1 10
ohms
20
kil 100 kilohms
1.2
volts
9
kilohms
200 kil
100 kilohms
1.2
volts
9 kilohms
2000
kft 10
kilohms
1.2
volts 900
kilohms
20
MO 10
kilohms
1 .2
volts 900
kilohms
200
nS 10 kilohms
1.2
volts 900 kilohms
3-31. Extremely
short
changes in a
continuity condition
(intermittent open or
short
circuits) are
detected by
the
one-shot and the
appropriate indication
is maintained
for
approximately
200 ms.
This
pulse stretching effect
ensures that a
reliable audio tone
is
generated for
continuity
changes as
short as
50
gs
3-7