TOOL BOX ITEMS

Why Alternating Current?

Ac is less costly to generate, is easily transmitted, and can readily be converted to dc. For these reasons, the use of ac upon its implementation changed industry's methods of running machinery. Making use of magnetism principles, the rotation of the generator shaft produces a complete sine wave during each cycle. The electricity from the generator is then increased in voltage value through the use of a transformer. A transformer is a device consisting of two or more coils that are used to couple electric energy from one circuit to another. It maintains isolation and can either increase or decrease voltage values. High-voltage ac is carried by power lines.

Ac transmission uses voltages ranging from 200 to 600 thousand volts. To meet customer demands the power company installs a transformer at different points along the ac power transmission line to lower the output voltage. Various potentials are involved. For a residence, it is lowered to the 220/120 volt level. You will be using these two ac voltage values during your studies.

TV circuits and those of other consumer electronic products use sine wave-like information signals. A public address system uses transducers to convert sound waves to electrical waves and back again. The waveforms have a sinusoidal shape, and the electrical waves travel much faster than sound waves.

Traveling electric waveforms from wire communications are converted by antennas to electromagnetic waves, which, depending upon their frequency, travel great distances. Then they are received and processed to recover the internal information. Sound waves are varied air pressure. Electrical waves are ac voltage and current. Electromagnetic waves are variations of magnetic field strength.

Since waves travel over distance, time is involved. The speed of travel multiplied by the time results in the distance traveled.

AC Wave Shapes

A sine wave is one whose amplitude is the sine of a linear function of time. It is drawn on a graph that plots amplitude against time or radial degrees relative to the angular rotation of the alternator (generator).

Familiarize yourself with the definitions of the following referenced terms.

Amplitude = the magnitude of a wave. Represented by a vector arrow whose length indicates the magnitude and direction.

Peak value = instantaneous maximum value for both the positive and negative alternations. This value may be considered as maximum signal amplitude. Peak value = one half peak-to-peak value.

Peak-to-peak value = the value between positive and negative maximums of either voltage or current. It is twice the peak value of the same waveform. Vpp = peak x 2.

Root mean square (rms) = the effective (dc) value equivalent of ac. Rms = 0.707 × peak value. Variations are peak = rms/0.707, and peak = rms x 1.414. Ac voltages are always given in rms, and from this value peak and peak-to-peak values may be mathematically obtained. This value tells us how well a sine wave will do its job in terms of dc current. Since maximum values are instantaneous, ac voltage or current cannot supply the same power as these values if they were dc. However, 70.7% of the ac amplitude is available for this.

Average value = the vector length of each 1 degree interval of either the positive or negative alternation. This calculates out to be 0.637. The formula for an ac or pulsating dc peak is: Vaverage = 0.637 x peak. The average of a complete cycle is zero, because the positive and negative averages cancel.

Frequency = number of repetitions of a periodic wave in one second. Its symbol is f, and the unit of measure is hertz. Frequency is the reciprocal of time, where f (hertz) = 1/t. When time is known, frequency may be calculated.

Period = the time, t (seconds) = 1/f, taken to complete one full cycle of a repeating waveform. A cycle is the change from zero to the positive peak, to zero, to the negative peak, and then to zero.

Wavelength = physical length of one complete cycle measured in meters. Velocity/frequency determines length (lambda). Because electromagnetic waves travel at the speed of light in air, or at 300,000,000 meters per second, a high frequency means a short wavelength. Lambda = 3 x 10⁸/f(Hz). For sound wave length, Lambda = 344.4 ms/f, since sound is much slower than electromagnetic waves.

Phase relationship = angular relationship between two waves. With an ac circuit, it is normally between voltage and current. A phase shift is a phase angle change. It is between two points. Waveform phase differences are expressed in degrees of lead or lag.

A square wave alternates between two fixed values for an equal amount of time. Each half cycle is equal in time and amplitude but changes in polarity. When they are equal, the duty cycle for each half cycle will be the same. Duty cycle is a percentage term used to describe the amount of ON time against the OFF time. A square wave has a 50% duty cycle. Other nonsymmetrical square waves will have longer or shorter duty cycles. For example, if the positive portion lasted for 50 microseconds and the negative portion for 450 microseconds, the pw/t value would be 11%.

Zero is the average value for a square wave whose positive and negative peaks are the same value. There is, however, an average value for a square wave that does not alternate about the zero reference. A changing rise or fall as the square wave is being generated is not instantaneous. Only 10–90% or 90–10% of the rise/fall is used. This standard way of measurement eliminates distortions made by the circuit-signal generation. To determine the pulse width (pw), the 50% mark is used.

A rectangular or pulse wave has a longer opposite alternation. Pulse generation may be in either the positive or negative direction. The rate at which pulses recur is referred to as the pulse repetition frequency (prf). The reciprocal of this frequency is called the pulse repetition time (prt). It is the time period of one pulse cycle. The duty cycle and average voltage are calculated in the same manner as for a square wave.

A repeating triangular wave has equal positive and negative ramps that have linear rates of change with time. Note the period-time measurement. The slope is computed as a voltage change, which occurs at the same rate as the time and is therefore linear.

With a sawtooth wave the ramp is linear but it returns immediately to the starting point without a reverse ramp. Generation may be either positive or negative in direction.

QUALITY CONTROL

• Electronic equipment manages the flow of information, while electrical equipment manages the flow of power.
• Pulsating dc rises from zero to maximum, and falls from maximum to zero, and then repeats. It remains dc, however, since current flows in only one direction.
• Alternating current (ac) flows first in one direction and then in the opposite direction in response to related changes in voltage.
• Alternating current, or sine wave, is produced by an alternating voltage source that reaches a maximum in one direction (+), decreases to zero, reverses itself, and continues in the opposite direction until a maximum is reached. Then the cycle repeats.
• The sine wave is the most common type of waveform. It is so named because it changes in value at the ramp rate, as the trigonometric function known as the sine. Waveform amplitude is its magnitude, and the constantly changing current gives it its direction. A peak amplitude occurs on both the positive and negative alternations. Peak-to-peak value, therefore, is between the positive and negative maximums. The root mean square (rms) or effective value is equal to 0.707 of the peak value. RMS is the dc equivalent. The average value of a complete sine-wave cycle is zero; however, with a positive or negative alternation it is equal to 0.637 of that peak value.
• Homes and industry utilize ac, since both ac and dc are needed. It is easier to produce dc from ac than the reverse situation. Ac generators have fewer internal complexities and are less expensive than dc generators.
• Ac power transfer is obtained through the use of transformers. Commercial voltages may be stepped up for transmission, and then reduced for reception and use.
• Communication is the transfer of information between two points with information being the property of a signal that conveys something meaningful to the receiver.
• The wavelength of either an electromagnetic or sound wave is dependent on the frequency and velocity of the transmission.
• The frequency of a periodic wave is equal to the reciprocal of its period, or F = 1/t. Its phase is relative to another periodic wave of the same frequency. If out of phase, one waveform will lead or lag the other by some number of degrees.
• The duty cycle of a waveform is the ratio of pulse width to the overall waveform period. Results are in percentage.
• Time domain analysis is a method of representing a waveform by plotting its amplitude versus time, whereas frequency domain analysis is a method of representing a waveform by plotting its amplitude versus frequency.

TOOL BOX ITEMS

Measuring and Generating AC Signals

The AC Meter

Multimeters are used to measure either dc or ac. To measure ac, most have either a mode switch for current selection purposes, or a test lead to be inserted in a different jack on the meter.

Because you have studied analog meter movements, you know that dc is required for their operation. Inside an ac meter is a rectifier circuit that changes the ac signal to a dc equivalent. It is this pulsating dc signal that the meter senses and interprets. For a digital multimeter, which displays the reading in numerics, pulsating signals below 10 Hz cause the digits on the meter display to change back and forth.

Difficulty can also be encountered with an analog meter. For signals under 2 kilohertz, it reads average, rather than effective voltage (RMS). Upper frequency limits are also encountered. Above 8 kHz, inaccurate readings may be encountered, due to capacitive reactance in the rectifier filter circuit and, in analog types, the meter coil inductive reactance.

Signal information other than effective voltage is scarce when using a multimeter; the oscilloscope on the other hand provides a technician with the ability to see at every point what is happening to the signal, or signals being generated by that circuit.

Oscilloscopes may not look like identical, but the control names will be similar and their purpose the same.

When the oscilloscope is first turned on, a "trace" will be seen. The CRT "line" appears after warm-up. The trace is moved through the use of the horizontal and vertical positioning controls. Prior to turn-on, ensure these controls are in their center position. Also, all switches or controls that are marked "calibrate" should be in that position. Most push button controls should be "out," and all magnification (5x, 10x) controls should be in the non-magnifying mode. Now, with a little luck, the trace should appear!

A single-trace oscilloscope allows the viewing of one signal. A dual-trace, or dual-beam scope allows multiple signals to be observed. In either case, your understanding of what is being viewed depends on your use of the cross-hatched lines, or graticule inserted on the face of the CRT. Notice that the face is 8 x 10 cm. The center lines, both vertical and horizontal, have four minor marks in between each major one. For peak, or peak-to-peak signal control, display voltages may be increased or decreased using the vertical input VOLTS/cm control. For frequency control, one or more complete waveforms may be displayed through the use of the TIME/cm control.

Since dc is in one direction, the trace will simply move up for a positive dc voltage, or down for a negative dc voltage. The VOLTS/cm setting times the number of graticule lines above or below the center zero reference determines the value.

Remember, only time is displayed on the oscilloscope. Frequency must be calculated after the waveform period is determined. Practice in reading amplitude voltage and determining signal frequencies may best be accomplished with an oscilloscope and some type of signal generating equipment. If your lab offers this, make good use of it—practice.

The Function Generator

This item of test equipment is used to select either a sine wave, square wave, rectangular wave, triangular wave, or a sawtooth generated signal. Switch selection allows a choice of waveform outputs.

QUALITY CONTROL

• Test equipment usage may be either to sense a signal or to generate one to monitor a circuit's response.
• A function generator can produce a sine wave, square wave, rectangular wave, triangular wave, or sawtooth wave.
• Audio and radio frequency generators produce signals within those spectrums for testing purposes.
• A frequency counter measures and displays the number of cycles per second (hertz) on a digital display.
• An analog or digital multimeter can be used to measure either ac or dc. The analog meter, unlike the DMM, can be used to measure low-frequency ac.
• Multimeters, when constructed to measure ac, contain an internal rectifier to convert the ac input to dc meter voltages. Ac multimeters are normally calibrated to indicate rms values.
• Multimeter accessories assist the technician. A high-voltage probe measures voltages in the kilovolt range. A current clamp allows ac current measurement without opening a current path. An RF probe more accurately measures high frequencies above 2 kHz.
• An oscilloscope may be used to display input signals. Signal characteristics are determined by measuring dc and ac voltage, waveform duration, and waveshape.
• The oscilloscope-displayed waveform shapes allow calculations of period time and amplitude characteristics. A dual-trace or dual-beam oscilloscope displays two waveforms on its screen, allowing waveshape, amplitude, phase, and timing comparisons. A good technician has a thorough knowledge of electronics, test equipment, troubleshooting techniques, and equipment and system repair.

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