My
Piece de Resistance! Understanding AC/DC, Series/Parallel, Resistance and Impedance by Eddie Ciletti |
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We
interface equipment every day, assuming and hoping that the relationship
between source and destination will be a happy one. In any electrical
circuit, impedance is a major part of that relationship, a rather deep
subject that had me pouring through old textbooks and wishing I could
still do the math. Here is my “Piece de Resistance,” an overview
of basic electronic information that will prepare you for next month’s
further adventures. AC/DC: The Juice Electricity can be described as an Electromotive Force (EMF). You know AC—alternating current—as sound and power (the giant hum that comes from wall outlets). Batteries deliver direct current (DC). Both are expressed in volts. AC can be transformed, rectified and filtered into DC—power supplies do this most of the time.
AC implies time by way of repetition. The frequency of AC is stated in Hertz—Hz—formerly known as “Cycles Per Second,” or CPS. It is easiest to imagine one complete cycle of a sine wave starting at zero volts—going positive, then crossing zero, going negative and returning to zero. Before vacuum tubes, transistors or IC op amps can amplify AC, they must first be “turned on,” biased with DC using resistors! STORAGE AC cannot be contained like DC, which can be “stored” in an electrochemical form as a battery or in an electrostatic form as a capacitor. Imagine filling a bathtub with water or parking a car at the top of a very steep hill. With gravity as “the force,” throwing the switch is equivalent to pulling the drain plug or releasing the brake. Unlike the popular water analogy, DC does not leak from the battery terminals, although, sometimes, the chemicals do. When AC and DC are put to work, however, magnetic energy is radiated into the air from the cabling, the principle behind inductors and transformers. Radiated AC, in the form of hum and buzz, finds its way into vulnerable “appliances” such as electric guitar and bass—another topic! DC might imply a constant polarity and voltage, but time cannot be frozen. Even a battery—disposable or rechargeable—holds a charge over a defined period of time that is primarily determined by use. Within time as we know it, a rechargeable battery has a charge and recharge “cycle,” still technically DC, yet one could argue it being of subsonic frequency. SERIES AND PARALLEL Two circuit configurations are shown in Fig. 1a and Fig. 1b using resistors for each example, along with the respective formulae for calculating total resistance. Figure 1a shows resistors in series configuration, and Fig. 1b demonstrates parallel. In series, resistor values are simply added together. The total resistance decreases in the parallel configuration; the formula as shown matches the example but can be continued ad infinitum. Add more resistors, and the end result will eventually approach zero ohms; in essence, a piece of wire. Note: Both “E” and “V” may be used to denote voltage. Test No. 1: It is common knowledge that two 8-ohm speakers connected in parallel becomes 4 ohms. What if the two speakers were 8 ohms and 4 ohms? (Answer No. 1: 2.6 ohms.) Tip: Most Windows PCs have a calculator with both Standard and Scientific modes. Test No. 2a: Ohm’s Law and the power formula are included in Fig. 1. These essential electronic tools are used to determine current flow and power consumption. Most cars have a 12-volt battery. Assuming that the sound system’s amplifier can swing 10 volts peak-to-peak, what is the peak power output of one channel into an 8-ohm load? (Answer No. 2a: 12.5-watts peak power.) Test No. 2b: (Bonus Question) What is the RMS power for the same sound system? Hint: See the February issue of this column. (Answer No. 2b: 1.56-watts RMS.) Reprinted with permission from Magazine, May, 2001 © 2000, Intertec Publishing, A Primedia Company All Rights Reserved |