@
P"s"ze
SQUARE
WAVE
F igure
6
The AC
meter
scales
are
calibrated to read the
rms
(root'mean-square)
value of
a
pure
sine
wave.
The
rms value
for
a
sine
wave
voltage is
.707
times the
peak
voltage.
For
complexwave-
forms,
such
as
square waves,
sawtooth
waves
or
pulses,
this
ratio
does
not necessarily hold
true.
The
complex
waveform in
Figure
6, for
example, shows
a
spike
that
may be several
times as
large
(peak
value) as the
rms value.
Since the
spike is
of
short duration, the
rms
value of
the
overall
waveform
is
barelyaffected.
On the
other
hand, the
square wave would
pre-
sent
an
rms value equal
to the
peak
value. The
voltmeter
responds
only
to the
positive peaks
of
AC
voltages. Therefore,
when
measuring
non-
symmetrical waveforms,
a
reversal
of
the
test
leads
may
give
a different
reading.
When
connecting
the
Voltmeter to the
circuit
under
test, the
Voltmeter
input resistance
and
in-
put
capacitance
are
effectivelyplacedinparallel
with the
voltage
source.
In
some cases,
this
can
Ioad
the
circuit
and change
the actual voltage to
be
measured.
At
low
frequencies
of 50 or
60
Hz,
the effects
of
capacitance loading
mayusuallybe
disregarded.
Thus, the
load
the Voltmeter
pre-
sents
to the
circuit
being measured
is about
the
same
as
1
megohm resistor.
At
higherfrequen-
cies,
the capacitive reactance
decreases.
At
10
kHz
for
example,
it is
approximately
100
K
ohm,
Such
a
value
may
seriously
affectthevolt-
age at
the
point
of
measurement.
The
amount
of
loading
presented
by the input
capacitance
and resistance
of
the Voltmeter
to
the
circuit
being
tested
is determined
by
the
impedance
of
the
circuit
being tested.Inlowim-
pedance
circuits, such
as
50
to
600 ohm
no
noticeable error
is
introduced in the voltage
reading
through
circuit loading.
These
circuits
will
only begin
to
be loaded at frequencies that
approach the
upper limit
of
the Solid-State VoIt-
meter's
frequency response.
Remember,
as
a
general
rule,
that fr.equency
response and loading may affect
the accuracyof
your
voltage readings.
There will
be
aresistive
loading
of
1 megohm regardless
of
frequeney.
The
amount
of capacitive loading
will depend
on
the
frequency involved.
The
actual cap4citance
of
the
instrument and the
leads
may also aJfect
the
tuning
of low capacitance
resonant circuits.
You
should know
the
values in
the
circuit under
test and the
values of
the input resistance
and
capacitance
of
the
Voltmeter. This
will
permit
you
to
make
valid
readings
over
a wide range
of
impedances within the
full frequency
response
of
the instrument.
The
Heathkit
Solid-State
Voltmeter
is a
very
sensitive electronic
AC
voltmeter and,
since
the
human
body
picks
up
AC
when
near anyAC
wires,
the
meter
will
indicate this
pickup.
Never touch
the
tip
of
the
probe
when
the VOLTS
Range
switch is set to the
lower voltage
ranges.
Zero
should be set with the
probeshortedtothe
com-
mon
clip, using the
DC+
andDC-positionsof the
Function
switch.
ACCURACY
The
accuracy
of
measurements
made with
this
Voltmeter
depends
on
the accuracy
of its cal-
ibration
and the tolerance
of
the
components used
in its
circuits. If the
Voltmeter is
carefully
cal-
ibrated
with
precision
standards,
then the
worst
case
of
error
would
be
the
sum
of component
tolerances.
The
meter
movement
is accurate
to
within
*2
percent
of
full
scale.
On
DC,
the
+1
percent
accuracy
of
the divider
resistors
must
be con-
sidered,
resulting
in an accuracy
within
+3per-
cent
of full
scale. On AC,
the
rectifier
circuit
contributes
variations
which
could result
in a
maximum
error
of +5
percent
of full
sca1e.
The
accuracy
on
the
OHMS
ranges
depends
pri-
marily
on
the multipliers,
which are
LVo,
andthe
meter movement
accuracy
of
ZVo. Becauseof the
nonlinear
OHMS scale,
the
resultingaccuracyis
not
readily expressed
in a
percentage
figure.
NOTE: When
comparing
this
instrument with an-
other
voltmeter,
consider that the error
of
the
other
instrument may
be
in
the
opposite
direc-
tion. Therefore,
when comparing two instru-
ments
of. *570 accuracy, the total difference may
be
*1070.
Critical
comparisons should
only be
made
against
certified laboratory standards.
SI NE
WAVE
COM PLEX
WAVEFORM
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