DC VOLTAGE MEASUREMENTS

1-3 Operation and use of common test equipment was covered in NEETS Module 16, Introduction to Test Equipment, NAVEDTRA B72-16-00-95. It is recommended that you review this module before continuing. VOLTAGE MEASUREMENTS Most Navy technical manuals provide voltage charts that list correct voltages at all primary test points in a piece of equipment. Voltage measurements, when compared with these charts, provide a valuable aid in locating troubles quickly and easily. However, if the sensitivity of the test equipment differs from that of the test equipment used in preparing the chart, the voltage measurements may not reflect true circuit conditions. You must keep in mind that a voltmeter with low sensitivity used on a low range may disturb circuits under test or provide a false indication. Most technical manuals will tell you what type and model of test equipment was used to prepare the voltage charts. As a rule of thumb, the input impedance of the voltmeter should exceed the impedance of the circuit by a ratio of at least 10 to 1. Technicians have spent uncounted hours of wasted time because they have selected improper test equipment. Q-2. The input impedance of your test equipment should exceed the impedance of the circuit under test by what ratio? DC VOLTAGE MEASUREMENTS Direct current voltage may be steady, pulsating, or have ac superimposed on it. The average value of a dc waveform depends on the symmetry of the wave and other aspects of the wave shape. It can vary from 63.6% of peak value for a rectified full sine wave to 50% of peak value for a triangular wave. For a superimposed sine wave, the average value can be zero. Regardless of whether the dc is steady, pulsating, or the ac is superimposed on the dc, a rectifier form of measuring device will indicate its average value. Voltages are usually measured by placing the measuring device in parallel with the component or circuit (load) to be measured. The measuring device should have an infinite internal resistance (input impedance) so that it will absorb no energy from the circuit under test and, therefore, measure the true voltage. The accuracy of the voltage measurement depends on the total resistance of the measuring device compared to the load being measured. When the input impedance of the measuring device is 10 times greater than the load being measured, the error usually can be tolerated. If this error cannot be tolerated, a high input impedance measuring device, such as a vacuum tube voltmeter (vtvm), should be used. Alternatively, using two voltmeters in series increases the voltage range and, because of the increase in total voltmeter resistance, provides a more accurate measurement of voltage across the load. If the voltage to be measured is sufficiently high, more than two similar voltmeters can be connected in series across the load to provide greater accuracy; the total voltage measurement is the sum of the individual meter indications. Q-3. What are the advantages of using two voltmeters in series? Multimeter Method A common piece of test equipment used in the Navy is the Simpson 260 analog multimeter, as shown in figure 1-2. It is capable of measuring both ac and dc voltages of up to 5,000 volts.