(source: Electronics World, Aug. 1964)
By ROBERT JONES
CONVERSION of meter readings for different waveforms aids in checking
equipment against a diagram's reference voltages when limited test equipment
is available. There are many meter types ranging from the hotwire ammeter
to the vacuumtube voltmeter. Each of these instruments may have a slightly
different reading when used on waveforms which are not pure sine waves.
The problem becomes more bewildering when some meters respond to one
value of a waveform (for example, the average) and the scale supplied
with the instrument reads another value ( for example, the r.m.s.) .
In this case the calibration is always performed on a sine wave, but
the scale reading will be correct only for this particular waveform.
The picture becomes more complicated when a square wave is applied to
this same instrument, as it will now be found to be reading high by 1.15
times the true r.m.s. value of the wave. To further complicate matters,
the reading error changes with each change of waveform from sine waves
to isoceles waves, sawtooth waves, square waves, and pulses.
AverageResponding Meters
Meters responding to the true average value of a waveform are quite
common. A typical example is the pocket type of meter or the a.c. reading
v.o.m.'s. In fact, any meter using copper oxide, magnesium copper sulphide,
selenium, silicon, or a crystal diode as a rectifier, either as a halfwave
or fullwave rectifier and without a capacitor across the output of the
rectifier, responds to true average values for any waveform. The scale
provided with common commercial instruments reads in r.m.s. values. This
means the true average value can be found by multiplying the meter scale
by 0.903 in order to obtain the correct average value for any other waveform
that we might be interested in.
R.M.S.Responding Meters
These meters include the moving ironvane meters, thermocouple meters,
hotwire meters, electrodynamometer meters, and electrostatic meters.
For this group of meters the meter scale, if r.m.s., matches the meter
movement and the instrument will read true r.m.s. values for any waveform.
PeakResponding Meters
Peakresponding instruments include the slideback vacuumtube voltmeter,
cathoderay oscilloscopes, rectifiertype instruments with a capacitor
across the output of the rectifier, and some types of v.t.v.m.'s. The
response of these meters is true peak for any waveform and most will
have scales calibrated in peak values. These meters give correct readings
for any waveform except for pulses of short duration compared to the
repetition frequency.
If the meter scale is calibrated to read r.m.s., then the true value
can be found by multiplying the scale value by 1.414. If, however, the
scale is calibrated to read average values, then the scale reading must
be multiplied by 1.571 to give true peak value of the waveform being
measured.
VacuumTube Voltmeters
The determining factor in this type of meter is the bias supplied to
the tube or transistor which does the detecting or rectifying. The bias
sets the operating point and by this means the conventional v.t.v.m.
becomes peak, average, or r.m.s. responding. Hence, the common v.t.v.m.
without a special input circuit cannot usually be relied upon to read
true values when anything but a true sine wave is applied.
Consider the waveform as applied to the meter from the detector. The
amplitude is dependent on the amplification provided by the tube and
on the bias supplied to the control grid, while the waveshape or distortion
at the cutoff end of the tube characteristic curve is dependent only
on the bias setting. Since tubes of the same type vary from tube to tube
and with usage, any given bias setting cannot be relied upon to make
the conventional meter read the true value of peak, average, or r.m.s.
This does not reduce the usefulness of the v.t.v.m.being calibrated
on sine waves and reading sinewave valuesbut it does make it difficult
to correct for other waveforms that we must measure.
True Values & Corrections
Once the true values of a voltage or current are known, be it average,
rootmeansquare, or peak, the remaining two values can be computed provided
the waveshape is known. For example, if we have a sine wave and know
the r.m.s. value, then we can find both the average and peak values by
simply applying appropriate correction factors.
This same approach can be applied to many waveforms, but of course the
correction factor changes with each different waveshape.
The ratio r.m.s. to average values is generally called the form factor
of a wave. This ratio is different for each waveshape, being 1.11 for
sine waves and 1.00 for square waves.
Another ratio, peak to average, is known as field form factor and is
1.57 for a sine wave and 1.00 for a square wave.
The peak to r.m.s. ratio, known as the peak or crest factor, is 1.414
for a sine wave and 1.00 for a square wave.
Fig. 1. Relation between average and r.m.s. voltage values.
Fig. 1 shows the relationships between average and r.m.s. values for
various waveshapes. Fig. 2 relates r.m.s. and peak values for various
waveforms. Fig. 3 relates average and peak values. In all cases the peak,
r.m.s., and average values are measured true values.
These charts are used by placing a ruler or straightedge from the known
value in one column through the known waveform to the required answer
in the second column. In using the charts, be sure that you read values
on the same sides of the various voltage scales. Also, all scales can
be extended by dividing or multiplying by 10, 100, 1000, etc.
The technician without a particular type of test instrument called for
in a service diagram, can forecast the value that would be measured by
the proper meter. This, of course, requires that the diagram show the
waveform or that the technician be able to forecast the probable waveshape
at the point of measurement. If a scope is the measuring instrument,
then both waveform and true peak values are available and the charts
can provide conversion.
Charts permit the conversion of peak r.m.s., and average meter readings
so that existing meters can be used to check a variety of applied waveforms.
Fig. 2. R.m.s. and peak voltage values for various waveforms.
Fig. 3. The relation between average and peak voltage values.
