Electricity Review

Electricity is what makes a computer run. Without it, a computer might as well be a doorstop. This may seem like a very fundamental statement, but it is true. This chapter begins with the study of electricity - the power that drives a computer. Since all components of a computer need power to run (whether plugged into a wall outlet or a battery), a computer professional must first understand the principles that govern electricity before understanding some of the problems encountered when hardware fails.

The energy or power to drive a computer is derived from electricity. Whether it uses 120 Volts AC (US standard), 220 Volts AC (European Standard) or a battery, without some source of this power, a computer is useless. Computers require a reliable source of power. As computer technicians, we do not have to be electricians or electronic experts. We do however need to be able to perform some basic tests to determine the reliability of the power provided to our computer and its components.

The best way to get started with an understanding of electricity is with a few definitions.

Electricity: A fundamental property of matter, associated with atomic particles whose movements, free or controlled, lead to the

development of fields of force and the generation of kinetic or potential energy.

Volts: The unit of electromotive force, or the potential energy that, when steadily applied against a resistance of one ohm, will produce a current of one ampere. Voltage is also considered the potential energy of a circuit.

Ampere: A measurement of current strength. High amperes can mean certain injury or death.

Ohm: A unit of electrical resistance to the flow of DC current.

Impedance: Resistance to the flow of AC current – measured in ohms.

Power: Strength or force actually put forth. Electrical power is measured in Watts which equals Voltage x Current.

All Computer Professionals should know one formula. From this formula, or a derivation of this formula, all basic power calculations can be performed. The name of this formula is “Ohm’s Law.”

Ohm’s law states that the current flowing through a conductor, or resistance, is linearly proportional to the applied potential difference (volts). In mathematical terms, this means:

By memorizing any one of these formulas, the other two can be easily derived using simple algebra. For example, the voltage (potential energy of the circuit) equals the amperage (the current or flow of electricity) times any resistance to that flow of electricity. From this, it can be seen that for a given voltage, the more resistance in a circuit, the lower the current flow.

Direct Current (DC)

The first sources of electricity were based on the concept of DC or direct current. When Ben Franklin was flying his kite, he was seeking DC electricity. DC is the same kind of power that we get from batteries. With DC, the current or electrons move in one direction. If you notice, a battery has distinct poles, one positive, and one negative. The current flows from the negative pole to the positive pole. The advantage of using DC power is that it is easy to control. DC is the power of choice for electronic equipment. Computers use low voltage DC power to operate their components.

AC or alternating current is the type of power that we get from the electrical outlets in our houses. The term AC power means that the flow of electrons in the wires moves back and forth or alternates. There are no positive or negative poles. The reason that we use AC power in our businesses and homes is that it is a very efficient way to transport power over long distances. AC power is measured in terms of voltage and frequency. Voltage represents the potential power and the frequency represents how many times per second the voltage alternates. Typical power is delivered to our homes as 110 to 120 Volts at 60 Hz (Hertz – cycles per second). In many nations (Europe for example), standard power is 220 Volts at 50Hz).

AC power is supplied to a computer via the electrical outlets or receptacles and then converted into DC power to run the components inside the computer. When connecting a computer or any other electronic equipment, you must be sure to use a properly grounded outlet. These outlets will have three slots and will require a three-prong plug. Most modern construction today will require all outlets to be of this type, however, there are still some older buildings and houses that do not meet these new codes. While you can still purchase an adapter to convert the three-prong plug that comes with a computer to a two-prong plug for these older receptacles, it is not advised and is dangerous. The reason

for the third connection (the small round one) is to provide a safety ground. This will protect you and you’re computer from short circuits. The grounded receptacle provides a direct connection to ground giving the electricity an alternate path should things get out of control. The following figure shows the proper outlet to use.

Measuring Electricity

A computer technician does not need to be an electronic technician to be successful. Electronic technicians learn how to test electronic equipment on a component level. As a computer technician, you will not need to test at this level, as you will be more concerned about whether or not the device works or not. Testing at the device level does not require the sophisticated equipment as testing at the component level, however, few electronic testing techniques will be helpful for the computer technician. The most important tool that you as a computer technician can use for device testing is the multimeter. Other names for this device are a VOM - Volt Ohm Meter or a DVOM -Digital Volt Ohm Meter. Beyond randomly replacing parts until finding the bad one, the electrical test meter is probably the best (and most practical) tool for troubleshooting electrical problems. In this section, we are going to look at the multimeter and how to use it to do some basic component testing.

The name multimeter was derived from its ability to measure several different parameters. You can test for AC and DC voltage, resistance, and continuity. Some meters will let you test for current as well, but only low amounts (less than 10 amps). With it, you can test various electronic components as well as the electrical power in the computer. Most will consist of a digital or analog meter or display, a pair of wires with probes (one black and one red) and a switch for adjusting the range of settings to be measured. The red wire is considered the positive probe and the black wire is considered the negative or ground probe.

Testing AC Power

Any time that you install a computer in a new location, you should confirm the power source at the outlet. This is especially true for new construction. In the event an error was made that causes the voltage to be outside of the specifications (either two high or too low), problems are sure to arise.

Testing AC Outlets with a Multimeter

Before testing an outlet, you will need to be sure that the meter is properly setup. Some of the newer and more expensive meters will have automatic settings for voltage range and will make adjustments accordingly. However, you may still have to tell it whether you are measuring AC or DC voltage. If the black and red wires are not permanently wired to the meter, make sure that the black wire is connected to the common (-) hole and the red wire is connected to the Volts (+) hole. Be careful; if this lead is placed in the wrong hole (Ohm or Amp), it can cause permanent damage. This should be the first thing you check when power is gone.

Make sure that your multimeter is set for AC voltage and that the range setting is at least one setting higher than what you anticipate the voltage to be. A setting of 200 volts or higher will work. With the meter properly configured, conduct these three tests.