As we’ve talked about before, electrical current flows in a wire similar to the way water flows in pipes. Similar to the purpose of any plumbing system, which is to help water do the work it needs to do like washing clothes or heating homes, wires conduct an electrical current that flows along them to where it can be used — to power a light or a computer processor, for example.
However, the water analogy for the flow of electrical current isn’t perfect. While it’s a great way to describe direct current (DC) flow, it’s not as good at describing alternating current (AC). With AC electricity being very common around the world, this can be a big problem.
What are Direct Current (DC) and Alternative Current (AC)?
DC electricity is likely the easier of the two to understand: it flows from a higher (more positive) voltage to a lower (less positive) voltage, just like water flows downhill. DC power provides a constant voltage and current that flows in one direction. Microprocessors and USB devices are powered from DC supplies, for example.
Alternating current (AC) electricity is a bit more complex, so understanding a few different analogies can help. Think back to the days before the television remote control was invented. Perhaps you’ve seen this represented in an old movie — an extended broomstick used to turn the TV on and off from a great distance (i.e., the couch).
This work, done at a distance, was not accomplished by the wood “flowing” from the hand of the viewer to the television. Rather, the small movements at the couch end of the broomstick, forward and back, were translated to small movements forward and back at the TV end, accomplishing the task required. It was the oscillation, or movement, that did the work.
Another analogy to consider is ocean waves. It is easy to witness and comprehend the great power of ocean waves as they beat against the cliffs, constantly reshaping the shoreline. But the waves are not comprised of a constant flow of water molecules from the center of the ocean onto the shore. Instead, the movement of water molecules are translated one to
another, and it’s the back and forth motion that accomplishes the “work” of erosion where the waves meet the shore.
AC power provides a constantly oscillating voltage, positive to negative, and back, much like ocean waves. Most power is transmitted in power lines as AC, and consequently, the wall power outlets you see everywhere are almost all AC. In the US, the outlets in your home provide 120 VAC, or a signal that oscillates between +120 V and -120 V.
Why Do We Have Both AC and DC?
Scientifically and economically, AC power can be stepped up and down in voltage using simple and relatively inexpensive transformers. This enables AC voltage to be increased to hundreds of thousands of volts on power transmission lines, so that a lot of power can be transmitted at a lower current (because power equals voltage times current). Lowering the
current reduces the amount of power lost during transmission, and wires heat up more with higher currents than with higher voltages.
A large reason we have both AC and DC power comes from The War of the Currents. In the 1880s and 1890s, Thomas Edison was marketing an indoor electrical lighting systems based on DC voltage, while George Westinghouse was introducing a new indoor electrical lighting system based on AC voltage, building off his company’s successful AC power generation and transmission. Edison promoted exaggerated reports and news articles claiming AC power was dangerous in order to prevent Westinghouse’s electric lighting from being accepted. Edison even conspired to make sure that the first ever electric chair used for executions was powered by AC in an attempt to tie death by electrocution to Westinghouse. From this, increased media attention on the dangers of AC power led to many testing and safety improvements, and AC power distribution became the standard.
AC power is typically used for high power and long distance transmission, while DC power is used for lower power items like computers and other devices. This is because the transistor — the basic building block of integrated circuits — requires a DC voltage. Conveniently for all our portable devices these days, batteries naturally provide DC voltage and current.
How AC and DC Work Together
AC power can be rectified and regulated to convert it into DC power. Rectification does not permit voltage to go negative, and regulation smooths out the voltage. DC power can also be inverted into AC power using switching circuits. These conversions, however, come at a cost — power is always lost to inefficiencies. A power brick or wall charger plugged into the wall is likely such a converter, taking the AC power from the outlet and converting it to a DC power for charging a devices or powering many different electronics. Some larger electrical devices, like TVs, take AC power from the outlet and convert it to DC power inside the device. Still, other devices like lamps or toasters use AC power directly.
KEMET offers key components for AC or DC circuit applications. If you found this blog interesting, learn other Circuit Basics by heading to our YouTube channel to watch the episodes.