TM 9-4935-282-34
803D
2-11. NOTES ON MEASURING DC VOLTAGES
accuracy of the voltmeter may be reduced. However, with the
polarity switch the upper input post never has to be connected
a. RECOMMENDED NULL RANGES FOR DC
to ground. If the unknown voltage is grounded, always connect
MEASUREMENTS. Certain null ranges are recommended for
the grounded side to the lower input post (middle post) and use
use with each setting of the RANGE switch for the following
the polarity switch to obtain the proper result.
reasons. With the RANGE switch at 500 volts the last voltage
readout dial (E) changes the reference voltage in steps of 0.01
2-12. NOTES ON MEASURING AC VOLTAGES
volt. Therefore, the unknown voltage would have to be an
exact multiple of 0.01 volt if a null is to be obtained on the 0.1
a. ERRORS DUE TO DISTORTION. The AC to DC
or 0.01 volt NULL range. Furthermore, it is unlikely that an
converter in the 803D is an average measuring device
unknown voltage of a few hundred volts will be stable within 10
calibrated in RMS. The converter will put out a DC voltage that
millivolts. Finally, the regulation of the reference supply is
is proportional to 1.11 times the average value of the AC input
approximately +0.0025% (+0.0125 volt) for a 10% change in
voltage. Thus, if the input signal is not a true sinusoid, the
line voltage. Although this is more than adequate when the
803D reading is probably in error because the ratio of RMS to
instrument is used in the recommended way, a badly
average is usually not the same in a complex wave as in a sine
fluctuating line voltage may cause the 803D to meter the
wave. The magnitude of the error is dependent on magnitude
regulation of its own 500 volt reference supply. For example,
of the distortion and on its phase and harmonic relationship
when measuring 500 volts a line change of 10% may cause
with respect to the fundamental. Figure 2-6 indicates how the
the 500 volt reference supply to change as much as 12.5 mv.
accuracy will be affected by various harmonics for different
Although this is small, the 803D will indicate full scale for a
percentages of distortion. If the distortion present in the signal
change as little as 100 mv or 10 mv if attempting to use the 0.1
is composed of even harmonics and is less than 2%, the error
or 0.01 null ranges.
between the 803D reading and true RMS is fairly minor. A
b. EFFECT OF AC COMPONENTS ON DC
larger error can occur if the distortion is composed of odd
MEASUREMENTS. An AC component of several times the
harmonics, especially the third harmonic. Note that for 2% of
unknown DC may be present on the unknown and the 803D
third harmonic distortion the error in the reading could range
will always indicate well within the specifications for
from 0 to 0.687%.
frequencies over a few hundred cycles. An AC component
may have an adverse effect if it is of a low frequency or if it has
%
% Error From True RMS*
a frequency that is a multiple or submultiple of the chopper
Harmonic
Distortion
Maximum
Maximum
frequency. A double section low pass filter (R201, C201,
Positive
Negative
R202, and C202) is used at the input of the null detector to
reduce any AC present on the DC being measured. At lower
Any even
0.1
0.000
0.000
frequencies, this low pass filter is less effective and the
harmonic
magnitude of the AC component is more significant. If this
0.5
0.000
0.0001
frequency is below 100 cycles, the accuracy may no longer be
1.0
0.000
0.005
with specifications. For example, a 60 cycle AC voltage that is
2.0
0.000
0.020
10% of the input voltage will cause an error of approximately
0.01% which is well within the specifications. Also, since the
Third
0.1
0.033
0.033
input attenuation is less for the more sensitive null settings, the
harmonic
accuracy may be affected only on the more sensitive null
0.5
0.167
0.168
settings. When the frequency is very close to a multiple or
1.0
0.328
0.338
submultiple of the chopper frequency (approximately 83
2.0
0.667
0.687
cycles), the meter needle will oscillate at the difference
frequency. If AC components that affect the accuracy are ever
Fifth
0.1
0.020
0.020
encountered, additional filtering will be required. For an AC of
harmonic
a single frequency, a twin-T filter is effective and has the
0.5
0.099
0.101
advantage of low total series resistance. For an AC variable
1.0
0.195
0.205
frequency, an ordinary low pass filter may be used. In either
2.0
0.380
0.420
case, high quality capacitors of high leakage resistance should
*Error depends upon phase relationship between harmonic
be used.
and fundamental, i.e. error can be any value between
c. MEASUREMENT OF NEGATIVE DC VOLTAGES.
maximum positive and maximum negative, including zero.
Because of a polarity switch, voltages which are negative with
respect to ground as well as the more commonly encountered
Figure 2-6. PERCENT ERROR DUE TO HARMONIC
positive voltages may be measured with equal facility. If the
DISTORTION
upper input post is connected to the metal case or line ground,
either at the 803D or at the source under measurement, the
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