Basic Electricity, AC And DC Current, Hot, Neutral, and Ground

AC And DC Current

Electricity can be AC, alternating current, or DC, direct current. Alternating current (AC) is current that cycles or oscillates back and forth rather than traveling in only one direction. House current in the U.S. oscillates 60 times in one second (60 hertz), changing polarity from +110V to –110V, and causing current to flow in different directions depending on whether it’s positive or negative in the cycle.

AC current is the most economical way to transmit electricity to our homes and workplaces. Alternating current can be forced to travel great distances by decreasing current and increasing voltage (high pressure and low volume). When it reaches its destination, the voltage can be decreased and the current increased (low pressure and high volume) to make it more suitable for driving our electrical devices. Direct current (DC) travels in only one direction and is the type of current required by most electronic devices, including a computer. A rectifier is a device that converts alternating current to direct current. A transformer is a device that changes the ratio of current to voltage. Large transformers are used to reduce the high voltage on power lines coming to your neighborhood to a lower voltage before entering your home. Because the transformer does not change the amount of power in this closed system, if it decreases voltage, then it increases current. The overall power stays constant, but the ratio of voltage to current changes.

A computer power supply changes and conditions the house electrical current in several ways, functioning as both a transformer and a rectifier (see the figure below). It steps down the voltage from the 110- volt house current to 3.3, 5, and 12 volts or to 5 and 12 volts, and changes incoming alternating current to direct current, which the computer and its peripherals require. The monitor, however, receives the full 110 volts of AC current, converting that current to DC. Direct current flows in only one direction, from hot to ground. For a PC, a line may be either +5 or –5 volts in one circuit, or +12 or –12 volts in another circuit, depending on whether the circuit is on the far or near end of the power output (the hot point). Several circuits coming from the power supply accommodate different devices with different power requirements.

Hot, Neutral, and Ground

When AC current comes from the power station to your house, it travels from the power source at the power station to your house on a hot line and completes the circuit from your house back to the power source on a neutral line.

When the two lines get to your house and enter an electrical device, such as a lamp or radio, the electricity flows through the device to complete the circuit between the hot line and the neutral line. The device contains resistors and other electrical components that control the flow of electricity between the hot and neutral lines. The hot source is seeking ground and finds it by returning to the power station on the neutral line.

A short circuit, or a short, occurs when the electricity is allowed to flow uncontrolled from the hot line to the neutral line or from the hot line to ground. Electricity naturally finds the easiest route to ground. Normally that path is through some device that is controlling the current flow and then back through the neutral line. If an easier path (one with less resistance) is available, the electricity follows that path. This can cause a short, a sudden

increase in flow that can create a sudden increase in temperature, which can start a fire and injure both people and equipment. Never put yourself in a position where you are the path of least resistance between the hot line and ground!

A fuse is a component included in a circuit and designed to prevent too much current from flowing through the circuit. A fuse is commonly a wire inside a protective case, which is rated in amps. If too much current begins to flow, the wire gets hot and eventually melts, breaking the circuit and stopping the current flow. Many devices have fuses, which can be easily replaced when damaged. To determine if a fuse is good, you actually measure continuity. To prevent the uncontrolled flow of electricity from continuing indefinitely, which can happen because of a short, the neutral line is grounded. Grounding a line means that the line is connected directly to the earth so that, in the event of a short, the electricity flows into the earth and not back to the power station. The grounding serves as an escape route for out-of-control electricity. The earth is considered to be at no particular state of charge and always capable of accepting a flow of current.