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Multimeter Method As previously stated, an analog multimeter's usefulness is limited by its low input impedance and poor accuracy (typically 2%). However, rugged construction and ease of operation make analog multimeters extremely useful whenever poor accuracy and low input impedance can be tolerated. When performing ac voltage measurements with a multimeter, be certain that the frequency of the signal being measured falls within the upper and lower frequency limitations of the meter.Oscilloscope Method A major advantage of using an oscilloscope for ac voltage measurements is that the waveform can be observed; consequently, errors in measuring complex peak voltages are minimized. An oscilloscope may be used as a high-impedance ac voltmeter. In standard oscilloscopes, the vertical amplifier input impedance is generally greater than 1 megohm, making it possible to measure voltages in high-impedance circuits. If the signal is applied directly to the plates, rather than at the vertical amplifier input, the input impedance is increased considerably. Voltage measurements are most easily made when the deflection of the trace extends across the major portion of the oscilloscope screen; whenever possible, the trace should cover at least 60% of the vertical viewing area of the screen. If the amplitude of the measured voltage is very low, the trace dimensions may be small. If a voltage to be measured is large and cannot be attenuated to a usable value by attenuation circuits within the oscilloscope, an external resistive or capacitive voltage divider can be used. Such voltage dividers are often furnished with oscilloscope test sets and are called HIGH VOLTAGE PROBES. When the voltage of pulses or other complete waveforms is being measured, the high voltage probe selected must be so designed as not to distort the measured signal. Most probes have adjustable (compensating) capacitors that are used to adjust the symmetry of the displayed waveform. You adjust the probe by monitoring either the calibrator output of the oscilloscope or a known good signal and adjusting the probe for a symmetrical display. Oscilloscopes are calibrated to display peak-to-peak values. To determine the rms voltage of a sinusoidal signal, divide the number of graticule units from the positive to the negative peaks by two and multiply this value by 0.707. When using the oscilloscope for ac voltage measurements, ensure the upper frequency range of the oscilloscope is not exceeded; otherwise, inaccurate values will be displayed. Most commonly used oscilloscopes have a Frequency response from dc up to 100 megahertz. Q.7 Ideally, an oscilloscope presentation should cover what vertical portion of the
screen? Digital Multimeter Method As previously mentioned, digital multimeters present a high input impedance to the circuit under test and are fairly accurate. Many earlier models had very limited frequency responses. Even today the upper frequency limitations of digital multimeters vary from 20 kilohertz to over 300 kilohertz, depending on the model. Their upper frequency limitations can, however, be significantly extended by using optional rf probes. When you perform ac voltage measurements with a digital multimeter, remember that they are true rms indicating devices. Differential Voltmeter Method Most differential voltmeters can be used to measure both ac and dc voltages. The differential voltmeter method of measuring ac voltage is the most accurate of the common measurement techniques. Typical accuracies are 0.05% when operated in the ac mode.
CURRENT MEASUREMENTS Unless an ammeter is already an integral part of the circuit under test, current measurements are rarely taken. In the case of a high-resistance circuit, it will contain such a small amount of current that it cannot be measured accurately with ordinary field test equipment. In lower resistance circuits, current measurements can be taken only if the ammeter is placed in series with the circuit under test. These measurements require that a circuit connection be unsoldered or otherwise opened to insert the meter in series with the circuit. An easier method you may use to obtain a current measurement is to take a voltage measurement across a known resistance and calculate the current with Ohm's law. The accuracy of current measurements depends on the internal resistance of the meter as compared with the resistance of the external circuit. If the total circuit current is decreased by increasing the load, then the percentage of error will decrease. Therefore, greater accuracy is obtained if the meter resistance is considerably less than the load resistance. A method of obtaining greater accuracy of current measurement is to decrease the total internal meter resistance with respect to load resistance. This is accomplished by connecting two ammeters in parallel with each other and in series with the circuit in which the current is being measured. Additional ammeters may be connected in parallel in the same manner for increased accuracy. This method also increases the range of measurements that can be taken. The arithmetical sum of the indications of all the parallel meters represents the total current flow in the circuit. You should note that this is not a common test method and that your test equipment may be damaged if connected incorrectly. MULTIMETER METHOD As previously mentioned, current measurements are usually taken by breaking the current path of the circuit under test and electrically inserting a meter in series. This is normally accomplished by disconnecting a wire from a terminal or unsoldering one end of a component and electrically inserting the meter in series using the meter leads. This method is both time consuming and usually requires the use of a soldering iron, which can damage components. Most analog multimeters cannot be used for measuring ac current and are only accurate to within 2% on dc ranges. Q.8 What are the advantages of connecting ammeters in parallel when performing current
measurements? DIGITAL MULTIMETER METHOD Unlike the analog multimeter, the digital multimeter will measure ac current as well as dc current. Again, current measurements are taken by breaking the current path and inserting the meter in series. Regardless of whether you're using an analog multimeter or digital multimeter, this procedure for measuring current is time consuming. However, there is a major advantage to be gained by using the digital multimeter - its high degree of accuracy. The Fluke 8000A digital multimeter, for example, is accurate to within 0.3% when measuring dc current and 1% when measuring ac current. These accuracies are representative of most medium-priced digital multimeters. CURRENT TRACERS For the purpose of discussion, we have selected the Hewlett-Packard 547A, shown in figure 1-7, as a representative current tracer. A current tracer will not actually measure current; it is designed to indicate the presence of current and the relative magnitude of one source of current as compared to another. The Hewlett-Packard 547A is a hand-held probe that enables you to precisely localize low-impedance faults in a circuit. The probe senses the magnetic field generated by a pulsing current and lights an indicator lamp near the current tracer tip. The brightness of the indicator lamp is proportional to the magnitude of the current. The sensitivity of the indicator lamp can be adjusted with a thumb-wheel potentiometer located on the probe. Figure 1-8 depicts a typical logic circuit application for a current tracer. Current tracers are ideally suited for locating shorted or opened printed-circuit-board runs, wires, or components. In the absence of a suitable pulsing current to drive the current tracer, a logic pulser or pulse generator may be used as a signal source. The inherent disadvantage of a current tracer is that it requires an external power supply. They can, however, be connected to the power supply of the equipment under test if the voltage is correct. Figure 1-7. - Current tracer.
Figure 1-8. - Current tracer application.
CURRENT PROBES Current probes, as shown in figure 1-9, are primarily designed to be used with an oscilloscope or milliammeter for measuring current. Although not used very often by Navy technicians, current probes are available. The primary advantage in using a current probe is that it does not need to be in series with the current being measured. Unsoldering wires or connections to terminals is not necessary; current probes are designed to be clamped onto insulated conductors. They are able to sense, through inductive action, the magnitude of the current flowing in the conductor. Current probes are designed for performing small ac current measurements. Also, when you use them in conjunction with current probe amplifiers, the capabilities of the current probe are extended to measurement of both ac and dc currents with large magnitudes. Current probes are extremely useful when you measure the current drain on a power supply, start-up current of a motor, or current flow in relays. These probes can be divided into three basic types: passive, active, and Hall effect. Each type has advantages and disadvantages peculiar to its method of operation. Prior to using a current probe, you should thoroughly understand its instructions. Figure 1-9. - Current probe used with electronic ammeter.
Q.9 What is the primary advantage of using a current probe? |
 
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