Basic Electricity Boot Camp

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This crash course in electricity will help you with some of the terms and technology.

The circuitry in computer systems is powered by direct current (DC), the type of electricity produced by batteries. A battery produces a voltage at a constant level. Electric utilities, however, supply power with alternating current (AC), in which voltage constantly varies. To power a computer system, the computer's circuitry converts AC into DC of the proper voltage.

Generator X

When an electrical current flows through a conductor, a magnetic field (or "flux") develops around the conductor. The highest flux density occurs when the conductor is formed into a coil having many turns. In electronics and electricity, a coil is usually known as an inductor . If a steady DC current is run through the coil, you would have an electromagnet-a device with the properties of a conventional magnet , except you can turn it on or off by placing a switch in the circuit.

There's reciprocity in the interaction between electron flow and magnetism . If you sweep one pole of a magnet quickly past an electrical conductor (at a right angle to it), a voltage will be momentarily "induced" in the conductor. The polarity of the voltage will depend upon which pole of the magnet you're using, and in which direction it sweeps past the conductor.

This phenomenon becomes more apparent when the conductor is formed into a coil of many turns. Figure 1 shows a coil mounted close to a magnet that is spinning on a shaft. As the north pole of the magnet sweeps past the coil, a voltage is induced in the coil, and, if there is a "complete" circuit, current will flow. As the south pole of the magnet sweeps past, a voltage of opposite polarity is induced, and current flows in the opposite direction.


Figure 1: To generate electrical power, a coil is mounted close to a magnet that is spinning on a shaft. As the poles of the magnet sweep past the coil, voltages of alternating polarity are induced in the coil.

This relationship is the fundamental operating principle of a generator. The output, known as alternating current, is the type of power that electric utility companies supply to businesses and homes . A practical generator would likely have two coils mounted on opposite sides of the spinning magnet and wired together in a series connection. Because the coils are in a series, the voltages combine, and the voltage output of the generator will be twice that of each coil.

Figure 2 is a graph of the voltage produced by such a generator as a function of time. Let's assume that this happens to be a 120-volt, 60-Hz generator. The voltage at one point in the cycle momentarily passes through 0 volts, but it's headed for a maximum of 169.7 volts. After that point, the voltage declines, passing through 0 volts, then reverses its polarity, and has a negative "peak" of -169.7 volts .


Figure 2: A 120-volt, 60-Hz generator produces power output that cyclically varies from 169.7V to -169.7V.

This curve is known as a sine wave since the voltage at any point is proportional to the sine of the angle of rotation. The magnet is rotating 60 times a second, so the sine wave repeats at the same frequency, making the period of a single cycle one-sixtieth of a second.

Current Events

In a direct-current system, it's easy to determine voltage because it is nonvarying or varies slowly over time. You can simply make a measurement with a DC voltmeter. But in an AC circuit, the voltage is constantly changing.

Electrical engineers state the voltage of an AC sine wave as the RMS (root-mean-square), a value equal to the peak value of the sine wave divided by the square root of two, which is approximately 1.414. If you know the RMS voltage, you can multiply it by the square root of two to calculate the peak voltage of the curve. If you were to power a light bulb from 120V(RMS) AC, you would get the same amount of light from the bulb as you would by powering it from 120V DC. Yet another device uses electromagnetic induction: the transformer.

Remember that a coil (inductor) develops a magnetic field when current flows through it. If alternating current is sent through the coil, it will produce an undulating magnetic field that reverses its polarity whenever the current reverses direction. If a second coil is wound around the first (but the two are electrically insulated from each other), the magnetic field of the first coil will induce a voltage in the second coil. In effect, the first coil sets up the same type of alternating magnetic field that is produced by the spinning magnet of a generator.

Just as an iron core improves the inductance of a coil, it has the same positive effect in a transformer, and most power transformers are wound on iron cores. As Figure 3 shows, a transformer is made up of two coils (usually referred to as windings) that are electrically insulated from each other. The two parallel lines separating the windings indicate that this is an iron- core transformer. The winding on the left, termed the primary, is connected to the electric utility's AC power grid. On the right is the secondary winding, in which the magnetic flux induces an AC voltage.


Figure 3: In a transformer, two coils (usually referred to as windings) are electrically insulated from each other. The left-hand winding, which is connected to the utility AC power grid, is called the primary winding. The secondary winding, on the right, is connected to a load.

The "turns ratio" of the transformer describes how many turns are on the primary and secondary winding relative to each other. A turns ratio of 5-to-1 means that there are five times as many turns in the primary winding as there are in the secondary winding.

A transformer "transforms" AC voltages in direct proportion to the turns ratio. With a 5-to-1 turns ratio, the voltage on the secondary will be one-fifth that on the primary. This would be a "step-down" transformer, since it steps down ( reduces ) the voltage. A transformer with a 1-to-3 turns ratio, on the other hand, would be a "step-up" transformer, with the voltage on the secondary being three times that on the primary. The current, however, will be reduced by the same factor; the current on the secondary will be one-third that on the primary.

Step-up transformers at utility power plants increase the voltage for transmission over long distances. Then, nearer the delivery points, utility substations use step-down transformers to bring the voltage down to a lower level for distribution through neighborhoods. Finally, numerous transformers on utility poles step the power down again before it goes to commercial and residential customers. Energy losses in power distribution are caused by the resistance of the conductors, and forcing large currents through a resistance produces high power losses. By stepping up the voltage in long distance transmission lines, then, utilities are able to minimize the current, and thereby minimize transmission losses.

The world's earliest commercial electric power systems were DC, but AC eventually won out, primarily because transformers, which only work with AC, could easily step voltages up or down, as needed.

Well-Grounded

One further point we need to address: How is voltage measured? What is the frame of reference? On virtually all computers (and many other devices), the chassis is connected to "ground." This means that the chassis is at the earth's potential, and that is the reference point for measuring the voltages in the system's various circuits.

The block diagram in Figure 4 shows how grounding works. The symbol of a sine wave in a circle represents the power source. For our purposes, this is the building's main switch panel. Below the power source, the stacked series of lines represents ground. This normally consists of a heavy-gauge wire or cable that connects the switch panel to a thick copper ground rod driven into the earth. (In large, steel-framed buildings , one of the building's steel columns will serve as the ground point.)


Figure 4: Power-using devices such as computers are connected to "ground" through the facility wiring.

Note that one side of the AC power wires is also connected to ground at the switch panel. This line is known as the "neutral" line, because it is at essentially earth potential. The other power-carrying wire is usually referred to as the "hot" line. It is this line that will rise and fall in voltage over the 360-degree cycle of a complete sine wave. (The neutral line can't vary significantly from 0 volts, because it is grounded at the main switch panel.)

On the right side of the figure is a block diagram of the computer, including its power supply. The computer is connected to the main switch panel via the building's wiring, with its three-wire power cord plugged into a wall outlet. Of the three wires, the two power-carrying wires (hot and neutral) go into the power supply-and nowhere else. The computer's chassis is connected to the grounding wire. The grounding wire serves two purposes: safety and signal reference. Grounding the chassis ensures that it will be at the same potential as the earth, which eliminates any possibility of electrocution if someone should happen to touch the chassis. The second function performed by grounding the chassis is to set up a reference (of 0 volts) to which all other voltages in the system can be compared.

This tutorial, number 74, written by Alan Frank, was originally published in the October 1994 issue of LAN Magazine/Network Magazine.

 
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Network Tutorial
Lan Tutorial With Glossary of Terms: A Complete Introduction to Local Area Networks (Lan Networking Library)
ISBN: 0879303794
EAN: 2147483647
Year: 2003
Pages: 193

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