# AC and DC Power

## Highlights

•     In a DC circuit, electrons flow in one direction
•     In an AC circuit, electrions continuously change which direction they are flowing
•     Convert AC to DC by using a transformer then a rectifier
•     Convert DC to AC by using a converter then an inverter
•     Many circuit components are only compatible with AC OR DC power sources
•     Using too large a power source can destroy your circuit components and may cause fires

## Notes

The movement of electrons through a conductor (e.g. wire) is called “current”. Two different types of current are AC and DC; they are used for the transmission of electrical energy. DC moves continuously in one direction while AC alternates between positive and negative voltages. A graph of DC overtime is a horizontal line; an AC creates a sine graph.

Thomas Edison (developer of the Direct Current) and Nikola Tesla were involved in an encounter in 1880s now known as The Battle of Currents. In the Battle of Currents era, the main problem was that they couldn’t easily convert the direct current to higher or lower voltages. Tesla believed that alternating current was the solution for this problem. Since AC periodically reverses direction, it can be converted to different voltages using a transformer. On the other hand, Edison had patented DC and didn’t want to lose his patent earnings. Edison started giving false information saying that AC was more dangerous; to prove his point, he electrocuted animals using AC. Nowadays, AC and DC are both used. AC is primarily used in the power grid and it the current delivered through AC outlets in buildings. Batteries are DC sources, and are found in portable electronics and . Rectifiers and inverters allow AC power to be translated to DC and vice-versa.

#### Direct Current

In direct current (DC) the flow of electric charge is only in one direction at a constant voltage, distinguishing it from alternating current. In a direct current, electrons move around a closed loop. Direct current is produced by sources such as batteries. In a battery, there are two sides: one has an excess number of electrons and the other side has a shortage of electrons. Naturally, the electrons want to move from the excess side to the shortage side. In this arrangement, the electrons can only move in a single direction: from negative end to the positive end. While they are running along the wires, they transfer energy from the battery to the various components that are connected to the circuit.This creates a constant flow of electrons through the circuit, generating a direct current.

#### Alternating Current

In alternating current (AC) electric charge flows periodically and reverses direction; while in direct current (DC), the flow of electric charge is only in one direction. Alternating current does not use electrons that travel in a single direction but change direction in a periodic manner. So, instead of electrons flowing from an origin to a destination, they only wiggle back and forth; they get pushed forward and then get pulled backwards with some frequency.

Alternating current is generally graphed as a sine wave; in some applications it can be graphed as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current.

In order to understand AC, it is important to recognize that electrons themselves do not carry energy or power. They are not delivery trucks full of energy. In fact an electron coming out of a battery actually moves very slowly through a wire. In a copper wire with a diameter of 1mm subjected to a current of 1 Amperes, the electrons move roughly one feet in an hour. So it isn’t their kinetic energies that gets consumed by the circuit component. Instead, what happens is that they are being lowered from a higher potential to a lower one.

$V(t) = V_{peak} \sin(\omega t)$

#### Converting AC to DC

While some circuit components, like a light bulb, can operate with both AC and DC currents, many circuit components are not interchangeable between the two. Whenever working with a circuit, it is important to recognize which type of current is expected, and make sure that the correct type is used. However, there are devices that allow for conversion between AC and DC.

For example, you can think of the box on a laptop charger. The laptop has a battery (a DC source), that is charged from the wall (and AC source). The box on the charger takes the high voltage AC power and lowers its voltage and then changes it into DC power. A cell phone charger work the same way, or a TV, etc. The system that takes in an AC power and turns it into DC power is called an rectifier. The system that changes the voltage in an AC circuit is called a transformer. On the other side, a system that would go from DC to AC is called an inverter. The system that changes voltage in a DC circuit is called a converter.

It is also important to remember that the type of current is not the only thing that matters. It is also important to keep Voltage in mind. For example, plugging a 1.5V battery into a system that requires 9V will not work, and plugging a 9V battery into a system that requires 1.5V can cause a fire or explosion.