– Shelly –

This article presents a basic explanation of the considerations involved in powering and protecting your personal computer. To minimize complexity, it is deliberately limited in scope to typical home or small office computing situations. It begins with a simplified explanation of electrical units and terminology that we often see misused.

Electricity 101

This section provides a basic understanding of common electrical terms and how they relate to the devices we use daily. The terms voltage, current, wattage, capacitance and resistance are frequently used but not completely understood by most who use them. What is electricity anyway?

Electricity is made up of electrons, a fundamental part of the atom. They are negatively charged particles that orbit about the atomic nucleus. Certain materials have atoms in which some of the electrons are able to break away and travel from atom to atom, these are the materials that can conduct electricity. When you gather a lot of roaming electrons (6.24 X 1018) it is called a coulomb, a term you may quickly forget after I tell you that when one coulomb of electrons passes through a point in an electrical circuit in one second, it is called a current of one ampere of electricity.

What makes the electrons move through a circuit in the first place? Because electrons carry a negative charge they will be attracted to a positively charged point in a circuit, and repelled by a negatively charged point. The amount of difference between the positive and negative points is known as the potential or electromotive force and is measured in volts.

Electrical power refers to how much energy is expended performing work, and is equal to the current (amperes) multiplied by the force (volts) , and is measured in watts, for example: 12 volts X 5 amperes = 60 watts.

A common analogy to make this all more understandable in familiar terms is by comparing it to water in a hose. If we connect a hose to a faucet and turn the faucet on, water will fill the hose and eventually come out the other end. Let the water represent the electrical current, the pressure that pushes the water when we turn on the faucet represents the voltage. If we eliminate the pressure by turning off the faucet, the flow stops. The amount of water passing through the hose multiplied by the pressure pushing it determines how much work the flow can do. A ½ inch garden hose at 50 pounds pressure can sweep pebbles from your driveway, a 4 inch fire hose at the same pressure can knock a person over.

We often talk about Ground, but what is Ground? True ground refers to Earth itself, but just driving a metal rod into the earth is not necessarily a true ground point. Dry, sandy soil can be well above true ground electrically. The same rod driven deep enough to be into the water table will be very close to a true ground.

In an electronic device ground has an entirely different meaning. In this case ground is just a common point where power supplies and metal supports such as a case or chassis are connected together, a common reference point. The term Ground potential however means there is no difference in potential (voltage) between a circuit point and the earth.

Electricity comes in two flavors, Alternating Current (AC), and Direct Current (DC). DC maintains a constant value above (+), or below (-) a zero reference level. DC always travels in just one direction. AC current alternates between a positive and a negative value over a time period. AC is constantly reversing direction.

AC is almost universally used to send electricity over distances because transformers can be used to boost it to very high voltages at relatively low amperages. This reduces the losses caused by the heating of the high voltage transmission wires and results in more of the power arriving at its destination. Once the power reaches the end of the trip, other transformers reduce it to whatever voltage is required.

Most electronic equipment operates on DC power, at relatively low voltages. The job of conditioning the electricity into the type and values needed is the work of the power supply, which we will take a closer look at next.

There is a lot more we could go into such as electrical resistance, measured in ohms, and electrical storage, or capacitance measured in farads, but for the most part they are beyond the needs of our discussion here, and I can sense your eyes starting to glaze over. I’ll finish this section by giving one relationship: it requires a force of one volt for a current of one ampere to pass through a resistance of one ohm.