ELEC 103 Unit 5 Page 1 PURPOSE To determine whether a circuit is operating properly, or malfunctioning, it is often necessary to observe the electronic signals in the circuit. A VOM or DMM will give the magnitude of an ac current or voltage, but is unable to display the signal or provide information to allow the user to determine if there is distortion on the signal, or if there is an improper phase shift in the circuit. The oscilloscope allows the technician to view these signals and determine the circuit operating characteristics. The purpose of this exercise, is to use the ac meter and the oscilloscope to make voltage and current measurements in an ac circuit. The student will be able to describe the procedure to measure voltage, period, frequency, and phase relationships for ac waveforms with the oscilloscope. EQUIPMENT AND MATERIALS REQUIRED Oscilloscope with 2 probes 2 each DMM’S and VOM’s Powered Protoboard Audio Signal Generator Frequency Counter Resistor, ½W, 5%, 1.5kΩ Capacitor, 35V, 10%, 0.1µF INTRODUCTION There is a wide variety of test equipment available to the technician, which provides information on circuit operation. All test equipment, used to measure voltage in a circuit, must have a high input impedance to prevent the test equipment from loading the circuit under test and thereby provide inaccurate measurements. The VOM usually has a moderately high input impedance while the DMM and oscilloscope have a high input impedance of 1MΩ or higher. This high input impedance assures the measurements will be accurate since loading does not occur. The VOM and DMM are used to measure ac currents and voltages in the same way they were used in the ELEC 111 course experiments. Since the polarity of the waveform is constantly changing, there is no polarity to consider when making ac voltage and current measurements. The oscilloscope provides a visual representation of the voltage at any point in the circuit on a CRT. As a result, the technician may view the display and determine the shape of the waveform, the amplitude of the waveform, the frequency of the waveform, the phase relationships between voltage waveforms at various points in the circuit, or the duration of some event lasting one or more cycles. An ac voltage is one that is continuously changing in magnitude and direction, with respect to time. These ac signals have a frequency of repetition measured in Hertz (cycles per second), which is the number of complete waveforms created in one second. The symbol for frequency is f, and the unit of measurement is the Hertz (Hz). Reference may also be made to the amount of time required for two alternations, which make up one cycle, of the signal, which is called the period of the waveform. The period of the waveform uses the symbol T, and has the second as the unit of measurement. There is an inverse relationship between the frequency and period of the waveform. This may be expressed as: f = 1 T <5 − 1> The analog waveform used to carry information in an ac circuit is the sinewave. These sinusoidal signals may be symmetrical above and below a zero volt reference, or they may be riding on a dc level.ELEC 103 Unit 5 Page 2 The amplitude of a signal is the height of the signal above or below the reference. In most cases, the reference used is zero volts or ground. The polarity of a signal refers to the waveform’s value, with respect to the common level in the system, which is usually ground or zero volts. All parts of the signal that rise above the zero volt level have a positive voltage polarity, and all parts of the signal that fall below the zero volt level have a negative voltage polarity. The sinewave in Figure 5 − 1 has equal excursions above and below the zero volt reference. The maximum positive level from the zero volt reference is called the positive peak voltage, VPP, and the maximum negative level from the zero volt reference is called the negative peak voltage, −VP. Since the variation above and below the reference is the same, the magnitude of VP is equal to −VP. While this is the case for Figure 5 − 1, it is not required. An ac information signal may also ride on a dc level. When the dc level is such that there is no excursion about the zero volt reference, but is always either above or below zero volts, this signal is no longer an ac waveform but a pulsating dc voltage. The amplitude is defined as the voltage level between the positive and negative peaks, and is called the peak−to−peak voltage, or VPP. When the magnitude of VP is equal to the magnitude of −VP, we may determine VPP from the equation: VPP = 2VP The amplitude of the ac signal may be measured and defined in different formats. The instantaneous voltage is the value of the voltage at a particular time. The symbol v is used to denote an instantaneous voltage, and the unit of measurement is the Volt. The time at which we wish to determine the instantaneous voltage may be θ (theta) and is measured in either degrees or radians, or time ( t ) measured in seconds. Both θ and t must be measured with respect to some defined reference of the signal. This is normally when the signal is at zero volts and rising, or zero degrees. When the instantaneous voltage is determined using t, the time must first be multiplied by 2 π f, which is the angular velocity (also known as ω) of the voltage. The units of measurement for ω is rad/s (radians per second). Since the voltage is sinusoidal, the instantaneous voltage may be determined from the equations: v = VP Sin θ v = VP Sin (2 π f t) Another measurement to define an ac voltage or current is the effective or Root−Mean−Square (rms) voltage. This measurement is important, since the rms voltage or current has the same heating value as the corresponding dc voltage or current. The symbol used to denote an rms voltage measurement is Vrms, and is measured in Volts. Current measurements use the symbol Irms, and have the Ampere as the unit of measurement. The VOM and DMM provide rms voltage and current measurements. The rms voltage may be calculated from the peak voltage using the equation: Vrms = V 2 P which equals Vrms = 0.707 Vrms FIGURE 5 − 1ELEC 103 Unit 5 Page 3 The final measurement, used to refer to an ac voltage or current, is the average value. The average value for an ac voltage is the average of all the voltage levels for one−half of a cycle of an ac voltage. This measurement is important