True-Rms Measurements; Waveform Comparison; Transistor Leakage Test; Transistor Beta Test - Fluke 8010A Instruction Manual

OPERATION
APPLICATIONS
2-61
.
TRANSISTOR LEAKAGE TEST
2-62,
Use
the following procedure
to
test transistors for
leakage
(Ices):
1.
Install
the transistor,
and
connect
the
test
fixture to the
Multimeter
(see
preceding
paragraphs).
2.
Set the switch
on
the
test fixture to
ICES.
3.
Select
the conductance
function, 2
mS
range
on
the Multimeter.
4.
A
reading of
more
than 0.0020
(6
juA) indicates
a faulty transistor
(silicon).
2-63.
TRANSISTOR BETA TEST
2-64.
Use
the following
procedure
to
test
the beta of a
transistor:
1.
Install
the transistor
and
connect the
test
fixture to
the
Multimeter
(see
preceding
paragraphs).
2.
Set the switch
in
the
test
fixture to
BETA.
3.
Select the
conductance
function, 2
mS
range
on
the
Multimeter.
4.
Note
the display
reading
on
the Multimeter,
then shift
the decimal point three places to the
right.
This
will
be the
beta of
the transistor.
NOTE
Beta
is
a temperature-sensitive
measurement.
Allow
sufficient
time
for each
tested transistor
to
stabilize.
Avoid
touching the transistor case
with
your
fingers while
making
beta
measurements.
2-65.
True-RMS Measurements
2-66.
One
of the
most
useful features
of
the
Multimeters
is
the direct
measurement
of true-rms ac voltages
and
ac
current.
Mathematically,
rms
is
defined
as the
square root
of the
mean
of
the
squares of the instantaneous
voltages.
In
physical terms,
rms
is
equivalent to the
dc value
that
dissipates the
same amount
of heat
in
a
resistor as the
original
waveform. True-rms
is
the
effective
value
of
any
waveform and
represents the
energy
level
of the
signal.
It
is
used directly in the relationships
of
Ohm's
Law
and
provides
a reliable basis for
comparisons
of dissimilar
waveforms.
2-67.
Most
multimeters
in
use
today have
average-
responding ac converters rather
than true-rms converters
like
the
8010A and
80 12
A. Usually the gain
in
average-
responding meters
is
adjusted so that
the
reading
gives the
rms
value,
provided the input signal
is
a harmonic-free
sinusoid.
However,
if
the signal
is
not
sinusoidal, the
average-responding meter does not
give
a
correct
rms
reading.
2-68.
Y
our Multimeter's
ac converter calculates the
rms
value through analog computation. This
results in
accurate
rms
values for
mixed
frequencies,
modulated
signals,
square waves, sawtooths, 10%-duty-cycle
pulses,
etc,
when
these signals are
measured with your
Multimeter.
2-69.
Waveform Comparison
(RMS
vs
Averaging
Meters)
2-70.
Figure 2-14
shows
the relationship between
common
waveforms and
their
displayed
value, as they
appear on
the
80
10
A
or
8012A, compared
to average-
responding meters. Figure 2-14
also
illustrates
the
relationship
between
ac
and dc measurements
for ac-
coupled meters.
For
example, the
first
waveform
(in
Figure
2-14)
is
a
sine
wave
with a peak
voltage of 1.414V.
Both Fluke Multimeters
(801
OA
and 8012A) and
the
average responding meters display the correct rms
reading of
l.OOOV
(the
dc component
equals
0).
The
1.414V
(peak)
rectified
square
wave
also
produces
a
correct
dc reading (0.707V)
on
all
the multimeters,
but
only the
Fluke Multimeters
correctly
measure the ac
component
(0.707V).
The
average responding meter
measures
the ac
component
of
the
rectified
square
wave
as
0.785V,
which
is
an
error of 5.6%.
2-71.
Waveform
Crest Factors
2-72.
The
crest factor of
a waveform
is
the ratio of
the
peak
to
rms
voltage. In
waveforms where
the positive and
negative half-cycles
have
different
peak
voltages, the
higher voltage
is
used
in
computing
the
crest factor.
Crest
factors start at 1.0
for a square
wave
(peak voltage equals
rms
voltage).
2-73.
Y
our Multimeter can measure
signals
with a
crest
factor
of
3.0
or
less,
at
full scale.
Figure 2-15
illustrates
some
typical signals
and
their crest factors.
The
waveforms
in
Figure 2-15
show
that a signal with a
crest
factor of greater
than
3.0
is
not
common.
2-74.
To
ensure that a
signal
measured
with your
Multimeter has a
crest factor
below
3.0,
measure
the
peak
value with
an
ac coupled
oscilloscope.
If
the
peak
value
is
not
more
than three times the true-rms reading of your
Multimeter, then
the crest factor of the signal
is
3.0
or
less.
Another
method
of verifying the error caused by
the crest
factor of a signal
is
to
compare
the reading of your
Multimeter with a reading
on
the next higher range of
your Multimeter.
The
crest
factor capability of your
Multimeter
increases
(from
3.0)
for
readings
less
than
full-scale.
The
crest factor capability of
your Multimeter
is
shown
by the following equation:
Crest
Factor
Capability
—
3
The
error
caused by exceeding the
crest
factor of
3 .0
at
full
scale, will
be reduced
significantly
on
the next higher
measurement
range of
your
Multimeter.
The
crest factor
capability at 1/10 scale
approaches
10.
2-13