AC Sources

Why it Matters:

Whenever you turn on a TV, plug in a computer or cell phone, or use any household appliance, you are tapping in to alternating current (AC) sources. AC sources provide power for our homes and work places, and have enabled the great technological advancements of the 20th century. Devices that use direct current (DC) power need to use batteries or circuits that convert AC to DC power. Even for battery powered devices, many use rechargeable batteries that are recharged by a AC sources.

DC vs. AC Sources

We saw in the last lesson that alternating current (AC) changes in direction and magnitude, whereas direct current (DC) stays the same. Let’s look at some common DC and AC sources.

Direct Current (DC): Batteries, solar power cells, fuel cells, DC power supplies.

Alternating Current (AC): Turbine and generator power plants (fossil fuels, nuclear, wind, hydroelectric, geothermal).

Transmission Power Loss and Transformers

Since AC is more complex than DC, why use AC sources at all? The answer is that electrical power is lost even in conductors, due to resistance (in DC circuits) or impedance (in AC circuits). Using Ohm’s Law to solve for electrical power in terms of current, we find that power losses are proportional to the square of the current:

P = IV = I(IR)=I^2R

So if we can find a way to divide the current by a factor of 2, we will reduce power losses by 4 times! This result is the critical reason that we use AC power today. To understand why, we’ll need a short history lesson.

In the late 19th century, Thomas Edison was selling his famous lightbulbs along with an electrical system to power them, a DC system. His main problem was that he had no way to change the current in the transmission lines. Since his DC generators were only capable of producing a high current (low voltage), and he could not reduce the current, his system was inefficient. Edison’s DC systems would lose a lot of energy even over small distances, but Edison believed DC was still the ideal electrical power source.

Meanwhile George Westinghouse was promoting his own alternative, AC power, which Edison believed was more dangerous than DC.

But AC power had an undefeatable weapon: the transformer.

A transformer is a device that allows you to take an AC signal and change its voltage and current easily. Unlike Edison, Westinghouse could take the power produced by his AC generators and use transformers to reduce the current significantly. This meant that he could generate power in one location, and transmit it a long distance away. A viable technology to transmit power long distance had been found. Unfortunately for Edison, transformers don’t work on DC- at all. Edison scrambled to find ways to increase the efficiency of his DC system, but it was a losing battle. To this day, the most common way to scale DC power is to convert it to AC, use a transformer, and convert it back into DC.

Westinghouse quickly overtook Edison, but eventually Edison’s company embraced AC power as well. Over time, AC power expanded to become the dominant source of electrical power around the world. All thanks to transformers and their ability to reduce transmission losses.

AC Sources: Different Shapes and Sizes?

Until now, we have discussed sinusoidal AC waveforms; AC signals that look like a sine wave. However, other signal shapes are also possible. Square, triangle, and even saw-tooth patterns may be used depending on the system.

These names describe the shape of the waveform over time. Square waves are particularly important because they are often encountered in electronics. For instance, MOSFETs, the most common type of transistor used in electronics, produce a square wave function.