Fluke 8012A - 3-44. Meter Protection Circuits

APPLICATIONS
Diode Testing
The measurement accuracy
of capacitors less than
5
nF can
be
improved
by
first using
the Relative mode to zero the dis-
play
and
automatically subtract
the residual meter and test
lead capacitance.
Since
the Relative mode also selects
man-
ual
ranging, zero the residual capacitance
only
when
measur-
ing small value capacitors.
Residual
voltage
charges
on the capacitor,
or
capacitors
with
poor
insulation resistance
or poor dielectric absorption may
cause measurement
errors.
To
check
capacitors larger
than
5^,
select fl
with
the rotary
switch (or press
the BLUE
button if you
are in the capacitance
mode). Select an
appropriate range
from
Table
4.
Discharge
the
capacitor, connect
the capacitor to
the meter, and
time the
number
of seconds
it takes
for
the charge
to go
from zero
to
full
scale. At
full
scale,
all
of
the
analog
display segments
are on.
To estimate
the value
of the capacitor,
multiply the
number
of
seconds
times the
charge rate (j/F/sec)
in Table 4.
For exam-
ple,
a
10//F
capacitor takes
about 33 seconds
to
charge in the
4 MQ
range or 3.3
seconds
in the 400 kd range.
To
reconfirm
your estimate,
reverse the test
leads; when
the capacitor
discharges
to zero (the
analog
display
polarity switches
from
-
to
+),
start
timing the recharge
to
full
scale.
22
Diode
Testing
To perform a
diode
or
transistor
junction test
plug
the
test
leads
into
the Vf}-«-
and
COM inputs,
turn
the
rotary
switch
to
-*»-,
and
connect
the test
leads
across the
diode(s).
In diode test,
voltage is
developed
across
the component(s) by
a
test
current
(approximately
1 mA
with the test
leads shorted)
from
the
meter.
Voltage is
read on
a
single
to
+3.000V
range
that can
measure up
to
five
silicon diode
or transistor
junctions
in
series.
For
a
silicon diode,
the
typical
forward
voltage should
be
about
0.6V.
Voltages
greater
than
3.00V
or open
test
leads
produce
an
overload (OL)
reading.
If the digital reading
is
the
same
in
both directions,
the diode junction
is
probably shorted.
If
the display
reads
OL in both
directions,
the diode
junction is
probably open. To
protect
sensitive
devices,
the
open
test
lead
voltage
from the
meter
will
not
exceed
3.9V.
Negative
inputs
(from
an
external
power source,
for
example) are
not
suppressed.
Table 4. Approximate Charge Rate for Capacitors
Range 400Q 4kfl
40 kn
400
kO
4MQ
/uF/sec 3000 300
30
3
0.3