Electrical Power

Lesson 1: Power

The energy or power to drive a computer is derived from electricity.

Whether it uses 110 volts alternating current (AC), the U.S. standard; 220 volts AC, the European standard; or direct current (DC) from a battery, a computer is useless without a steady, reliable source of power.

When we encounter problems with a computer, it is crucial to be able to test the entire power system.

This lesson covers the basics of power and electricity.


After this lesson, you will be able to

Explain the difference between electricity and electrical energy

Define the terms used to measure electrical energy

Identify basic electric and electronic components

Perform basic and advanced electrical energy tests

Understanding Electricity and Electrical Energy

What is electricity?

The meaning of the word varies with the user.

Electricity to physicists is the primal property of nature, and they call the power delivered at the wall socket and stored in batteries electrical energy.

Most people, including computer technicians, are less fussy, often using the term electricity to refer to both:

The form of energy associated with moving electrons.

The energy made available by the flow of electric charge through a conductor.

For our discussion, we talk mainly about the flow of energy used to run computers—electrical energy—and do not worry about the fine points of scientific philosophy.

Some Definitions

For our discussion we employ the following definitions:

Electricity. The form of energy associated with charged particles, usually electrons.

Electric charge. When charged particles move in tandem, they generate fields, producing energy.

Electrical circuit. The path taken by an electrical charge.

Electric current. When an electric charge is carried, or flows through a conductor (like wires), it is known as a current. A current-carrying wire is a form of electromagnet. Electric current is also known as electron flow.

Power. The rate at which an amount of energy is used to accomplish work. Electrical power is measured in watts, which is determined by multiplying voltage by current.

Conductors. Materials that can carry an electrical current. Most conductors are metals.

Resistance. A quality of some materials that allows them to slow the speed of an electrical current, producing heat, and sometimes light, in the process.

Insulators. Materials that prevent or retard the electrical current of electrons.

Ampere. A measurement of current strength, equal to 1 coulomb per second.

Coulomb's law. Two charges will exert equal and opposite forces on each other. Opposite charges attract and like charges repel.

Ohm. A unit of electrical resistance. Ohm's law states that voltage is equal to the product of the current times the resistance, or voltage = current × resistance.

Volt. The unit of electromotive force, or potential energy, that, when steadily applied against a resistance of 1 ohm, produces a current of 1 ampere.

Voltage. The potential energy of a circuit.

Ohm's Law

In addition to defining terms, we need to understand some basic principles that are applied in testing electrical devices.

One formula that all computer professionals should know is Ohm's law.

All basic power calculations can be performed from this formula or a derivation of it.

Ohm's law states that the current (electrons) flowing through a conductor, or resistance, is linearly proportional to the applied potential difference (volts).

A conductor is any medium, usually metal, that allows the flow of electrical current.

Resistance is any device or medium that resists the flow of electrons. In mathematical terms, this means:

Resistance: R = V/I

Current: I = V/R

Volts: V = IR

In these formulas, R = resistance in ohms (W), V = voltage, and I = current in amperes.


In some cases, you may find E used instead of V in formulas expressing Ohm's law. The E stands for electromotive force, and is often used by engineers as a more precise technical term for their measurements.

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) is equal to the amperage (the current or flow of electricity) multiplied by any resistance to that flow of electricity.

The more resistance there is in a circuit, the lower the current flow for a given voltage.

Personal Computers and Electrical Power

That personal computers (PCs) use electrical power to operate is no surprise, even to the casual user.

The technician must understand the different types of electrical energy and how they work inside the PC.

A PC's electrical power can come from a wall outlet, in the form of AC, or from a battery in the form of DC.

AC Power

AC power is what most people think of as electricity. It comes from the wall and powers most of our lights and household appliances.

AC power is man-made, using generators.

As the wire coil inside the generator rotates, it passes by each pole of unit magnet(s) producing an electric current.

When it passes the opposite pole, the current reverses, or alternates, the direction of flow.

The number of revolutions made by the generator per minute is called its frequency.

In the United States, power companies run their systems at 60 turns per second to produce a high-voltage, 60 Hz (cycles per second) AC as they rotate.

The power system drops the voltage in stages before it is connected to the consumer's home or business.


Flow of electrons

The power company delivers AC power to our homes or businesses with three wires.

Two of the wires are hot, meaning that they carry a charge.

One, the bare wire that runs from the breaker box to the power pole, is neutral.

The measured voltage between the two hot wires is between 220 and 240 volts AC (VAC), and the measured voltage between either of the hot wires and the neutral wire is between 110 and 120 VAC.

These voltages, which are called nominal voltages, can vary by plus or minus (±) 10 percent.

AC volts

Typical electrical outlets are connected between one of the hot wires and the neutral wire.

These outlets are usually three-prong connections.

The smaller rectangular hole is the hot, the larger rectangular hole is the neutral, and the small round hole is called the ground.

The ground wire is used as a safety wire.

In the event of a short circuit, a large flow of current (amps) is discharged all at one time.

This short, strong flow of current will burn out circuits unless it can be safely sent somewhere else.

Electricity will always seek the path of least resistance to ground.

By providing this wire, a short circuit will cause less damage by providing a path for safe dissipation of the current.

To provide a safe working environment for the computer and yourself, make sure that the ground wire is properly installed.

Older structures might have two-wire electrical outlets without the ground wire.

An electrical outlet without grounded plugs and the third ground wire is unacceptable for use with a computer.

An extension cord without a ground wire is also unacceptable.

The proper type of outlet includes a ground.


A short circuit can cause physical damage to equipment and personnel.

It can cause a fire, component damage, permanent disability, or even death.

The ground plug provides a direct connection to ground, giving the electricity an alternate path away from equipment and people.

DC Power

AC is used for transporting low-cost power to end users.

However, a computer's electronic components won't run on AC power—they need a steady stream of DC.

The PC's power supply performs several tasks, but the main function is to convert AC into DC.

A computer's power supply combines two components to handle this job: a step-down transformer and an AC/DC converter.

The AC adapters used for laptop computers, many low-cost ink-jet printers, and many other consumer electronics do the same thing—turn AC into lower voltage DC.

As we have seen, DC is electrical energy that travels in a single direction within a circuit. (The electrical energy in a thunderstorm is another example, but not very practical to electronic applications.)

DC current flows from one pole to another, hence it is said to have polarity.

The polarity indicates the direction of the flow of the current and is signified by the + and – signs.

Measuring Electricity

A computer professional should know how to use a multimeter—sometimes called a volt-ohm meter (VOM) or a digital volt-ohm meter (DVOM).

An electrical test meter is probably the best (and most practical) tool for troubleshooting electrical problems.

It is not necessary to be an electronic technician to use this tool effectively.

The Multimeter

A multimeter is used to measure several aspects of electricity.

All multimeters are designed to provide at least four major measurements:

AC voltage
DC voltage

A multimeter consists of two probes, an analog or digital meter, and a multiposition switch to select the type of test you wish to perform.

Testing AC Power

In any new building installation, failure to properly test AC outlets can result in damaged or destroyed equipment, as well as possible injury and electrocution.

In the event a wiring error was made that causes the voltage to be outside of the specifications (either two high or too low), problems are sure to arise.

Don't take for granted that the building power supply provides the correct voltage or that all of the other inputs are wired correctly.

When testing an AC power source, check these three things:

Is the hot wire sending the correct voltage, and is it wired to the correct pin?

Is the neutral wire connected to ground and to the correct pin?

Is the ground wire connected to ground and to the correct pin?

Testing AC Outlets with a Multimeter

The first step when testing an AC outlet is setting up the multimeter.

Then you need to know how to read the meter.

You can also use special equipment if the multimeter does not provide enough information.

Setting Up the Meter

Basic multimeter usage with AC circuits is quite straightforward:

Attach the black test lead to the negative (–) marked hole. In some low-cost meters, leads are permanently attached to the meter.

Attach the red test lead to the volts (+) hole.

Be careful—if this lead is placed in the wrong hole (ohm or amp) it can cause permanent damage to the meter.

Set the selector switch to AC volts (this choice is often denoted by red lettering).

If there are multiple selections, use the highest setting possible (if voltage is unknown), or select one level higher than the estimated voltage.

For standard household outlets, 200 VAC is a good selection.

Some digital meters use "auto-range" and don't need any selection except AC volts.

Reading the Meter

After the meter is set up, you are ready to test a wall outlet.

There are three tests to perform.

With AC voltage, it does not matter which lead is placed in which connector.

Hot to neutral.

Place one lead in hot (smaller of the two vertical slots) and the other in neutral (larger of the vertical slots).

The reading should be between 110 and 120 VAC.

Hot to ground.

Place one lead in hot (smaller of the two vertical slots) and the other in ground.

The reading should be between 110 and 120 VAC.

Neutral to ground.

Place one lead in neutral (larger of the two vertical slots) and the other in ground.

The reading should be 0 volts.

Using AC Testers

An alternate method for testing electrical outlets is to use an AC tester.

These small devices are made especially for testing outlets and can be purchased at any home improvement or electronics outlet store.

By simply inserting the tester into an outlet, all voltages for all combinations can be tested at the same time.

Many testers provide several light-emitting diodes (LEDs) that tell whether or not each function passes the test.

This device is not as accurate as a multimeter, but it is more convenient.

It will provide a pass–fail indication rather than an accurate voltage reading.

Testing AC Ripple

The function of the power supply is to convert AC to DC voltage.

When it is working properly, a pure DC signal will be produced.

However, sometimes, as the power supply ages, its ability to produce pure DC falters.

A power supply uses electrolytic capacitors (discussed later in this lesson) to filter or smooth the voltage after it has been converted from AC to DC.

These capacitors are second only to fuses as the part of a power supply most likely to fail.

When an electrolytic capacitor begins to fail, it allows more and more AC voltage to pass through.

This small amount of AC voltage is superimposed on top of the DC voltage and called noise or ripple.

To test for ripple, set a meter to read AC.

Then connect a .1µfd (microfarad) capacitor to the red lead.

With the power turned on, measure the DC voltage.

Any ripple present will be displayed as AC voltage.

Testing Resistance

Resistance is an opposition to the flow of current through a conductor and is measured in ohms (W).

To find out the level of resistance, place one lead of the meter on each side of the circuit or component to be measured.

Keep in mind that measuring a component while it is still soldered in its circuit can lead to inaccurate readings—other components connected to the circuit can affect the total resistance.

Unlike voltage checks, you should test resistance with the power off.

If a meter is set up to read resistance, you will damage it if you connect it to an electrical outlet.


Be careful when measuring resistance.

If the meter is set too high or the resistance is too high for the meter, you will get an inaccurate reading.

Also, before taking a measurement, be sure that any charge stored in a capacitor is properly discharged.

Refer to the applicable product manual for details.

Testing Continuity

Continuity is a term used to indicate whether or not a connection exists between one point in a circuit and another.

It is used to determine the presence of breaks in wires and electrical circuits.

If no continuity setting is available, use the resistance setting (see the previous section).

If the multimeter measures infinite resistance, there is no continuity. This indicates a break in the line.

If the multimeter shows little or no resistance, there is continuity and the circuit is complete.

Testing DC Voltage

Testing for DC voltage is the same as testing for AC voltage, with one important difference: DC voltage is sensitive to polarity.

As mentioned earlier, DC voltage has a positive pole (+) and a negative pole (–).

When measuring DC voltage, it is important to place the positive (red) lead on the positive side and the negative (black) lead on the negative side of the circuit.

If the leads are positioned backward, the polarity of the reading will be the opposite of what it should be.


When using an analog meter (one with a dial and needle), connecting the leads backward will cause the needle to move in the opposite direction, possibly damaging the meter.

Testing a Power Supply

Many computer problems blamed on the operating system or hardware component are really power problems.

In some cases, it is the power produced and transmitted by the electric utility that will require line conditioning.

The quickest way to resolve this is by adding a quality UPS (uninterruptible power supply) with line conditioning circuits.

Before adding one, test the power supply to make sure it is functioning properly.


Find out if the client is having any problems with flickering lights or intermittent problems with other appliances, or if he or she is using a power strip with too many connections for the rated use.

Improper loading of the circuit, not the PC itself, can be the problem.

A bad power supply can cause intermittent lockups and unexpected computer reboots.

Erratic problems encountered during booting and changed or erased CMOS (complementary metal-oxide semiconductor) information can also be traced to a failing power supply.

Bad power supplies have been known to destroy data on mass-storage devices (such as hard drives, tape drives, and so on).

There are two types of tests for power supplies: a basic test used to verify voltages and an advanced test for checking its internal components.

Basic Voltage Test

The only purpose of this test is to verify the existence and value of voltages.

With time, most power supplies show their age by a reduction in voltage.

This voltage drop will show itself in both the 5-volt and the 12-volt outputs, but is more pronounced on the 12-volt side.

Prepare the Meter for the Test

Again, meter preparation is quite simple:

Connect the black lead to the common (–) connector and the red lead to the voltage (+) connector.

Turn the test selector to DC volts.

If the meter has an AC/DC switch, be sure it is set to DC.

If the meter does "auto-range," set the range to 15–20 volts.

Testing the Voltages

The best place to check voltage is at the power supply's P8/P9 or ATX power connectors.

For P8/P9 systems, use the following instructions:

Place the meter's black (ground) lead on the black wire connection and its red (positive) lead on the yellow (+12 volt) connection.

Record the voltages. A good power supply will provide a voltage between 11 and 13 volts DC.

Replace the power supply if the voltage reading is less than 10.


Be sure to reverse the leads when using an analog meter to check negative voltages.

This is not necessary with a digital meter because it will simply show a negative sign with the reading.

When No Voltage Is Present

If you have completed the basic voltage test and no voltage is present, the problem may not be the power supply.

It might, instead, be caused by an excessive load on the system due to another piece of hardware.

To determine if that is the case, try the following procedure.

Isolating the Problem

First you should test the hardware:

Disconnect the Molex leads from the power supply.

Connect the meter leads, as described in the previous sections.

Turn off the AC power.

Disconnect all the Molex plugs from the devices.

Turn the power back on.

If power is present at the motherboard, one of the devices is bad and is causing a drain on the power supply.

Reconnect each Molex plug, one at a time, and test the power.

When the power drops out, you have located the offending device.

Advanced Testing

The basic test is designed to quickly isolate the power supply as a problem.

In most cases, if the test proves the power supply to be defective, it may be more cost-effective to replace the power supply than to try to repair it.

Advanced testing requires a working knowledge of power supplies and removal of the power supply and its cover.

There are three sections to a power supply: the switching network, the transformer, and the voltage regulator.

The Switching Network

AC power coming from the power company is imperfect.

It is not uncommon to have sudden increases in voltage called spikes or decreases in voltage called sags.

To smooth out the power sent to the electronic components, a PC has basic line conditioning capability—a switching network.

The better the power supply, the more sophisticated the network.

The main components found in a switching network are a fuse, capacitors, rectifiers, and switching transistors.

The switching network performs the following three tasks:

Filters electrical noise (spikes, sags)

Eliminates frequency changes (ensuring that current stays at a constant 60 cycles)

Converts AC sine wave signals to AC square wave signals


The Transformer

The transformer reduces the voltage of the square wave DC into separate 12-volt and 5-volt square wave AC circuits.

The Voltage Regulator

The voltage regulator receives the low-voltage AC outputs of the transformer and converts them to clean DC power.

The main components in this section are rectifiers, capacitors, and coils.

The voltage regulator performs three functions:

It uses rectifiers (diodes) to convert the square wave AC output of the transformer into DC output.

It regulates the voltage to a constant output level and uses capacitors to remove any ripples that are present.

It monitors the amount of current used by the computer circuits and adjusts the switching network using a special circuit called the feedback circuit.

This compensates for variations in load on the power supply.



Do not open the power supply while it is plugged in, and do not open the power supply until it has been discharged.

The power supply can carry dangerous levels of power even when disconnected.

Only a properly trained technician should ever open a PC power supply.

Given the cost of a power supply, there is no good reason to disassemble one; defective units should be replaced.

Electronic Components

As a computer professional, you should be familiar with the more common types of electronic components found in a computer design.

Before the advent of the circuit breaker, fuses were common in the home and office.

A fuse serves one purpose—to fail, and thus cut the flow of power in the event of a current load that has exceeded the safe capacity that the system components can absorb.

Fuses come in many shapes and sizes, but a PC fuse is almost always a small, clear, glass tube with metal caps on each end and a wire inside the tube to electrically connect the two caps.

In general, the thicker the wire, the more current a fuse can conduct before failing.

When a fuse fails, the wire will melt or be broken.

You can check for a "blown" fuse by determining if the wire is intact or broken.

The amperage (A) rating (stamped on the metal cap) indicates the maximum current the wire is rated to conduct.

Be sure not to exceed to the rated limits of the PC design for a fuse, because an excess power load can damage or destroy the system.

If a fuse in a specific location fails more than once or repeatedly, the system is being overloaded, and you need to isolate the problem causing the failure.

Fuses are often found on power supplies and many external components.

If a fuse fails, first try to replace it with another fuse of the same rating.

If the replacement also fails, the fault probably lies with the motherboard or another internal component.

A capacitor is an electrical component used to hold an electrical charge.

In photography, electronic flashes use capacitors to build up power before a picture is taken and to vary the amount of power used in a flash to control the exposure.

In PCs, they are often used to regulate the flow of current to areas of the system circuits for a short period of time.

Some are fixed-capacity models, whereas others can absorb or hold variable amounts of power. The amount of electrical current a capacitor can control is called capacitance, measured in microfarads.

Most PC power supplies employ an electrolytic capacitor.

These devices are able to retain a significant charge for long periods.

You should work with such components only if you are properly trained to release any residual charge before disconnecting, testing, removing, or replacing one.

Failure to follow safe procedures can result in injury or death to you and damage to the system.

These capacitors have a distinct polarity (negative and positive) to their two leads.


Before you test a capacitor you must discharge the power supply.

Failure to discharge can create a serious hazard to you and your equipment.

Rectifiers and Diodes
Rectifiers are devices that convert AC power into a DC form (rectification).

A diode is a device that lets current flow in only one direction.

Two or more diodes connected to an AC supply will convert the AC voltage to DC voltage.

Single diodes are generally used to convert AC to pulsating DC.

Two diodes working in parallel produce half-wave rectification, resulting in a pulsating DC.

Four diodes produce full-wave rectification, with a continuous stream of pulses.

Normally, a computer technician does not test at this level; however, diodes can be tested with a multimeter.

With the power turned off, test for resistance across both leads of the diode.

Then reverse the leads of the multimeter and test again.

A good diode will exhibit low resistance in one direction and high resistance in the other.

The invention of the compact, power-efficient, and reliable transistor transformed the modern electronics industry, replacing bulky, power-hungry, and temperamental vacuum tubes.

Transistors are basically a pair of diodes connected in series with an on–off switch.

Varying the voltage sent to a transistor turns the switch on or off.

Early computers used vacuum tubes as switches and were so large that technicians could actually step inside the larger ones to plug in or remove the tubes to program them.

Transistors can be tested, although, this often requires special equipment.

Due to the reliability of transistors, a computer technician normally does not need to perform this level of testing.

The most common forms of electrical transformers are step-down or step-up devices.

A step-down transformer decreases the transformer's voltage on the output side; a step-up model increases it.

Both have a primary wire coil connected to other coils—secondary coils—joining two or more AC circuits.

Electronic transformers generally contain stacks of thin metal-alloy sheets, known as laminations, with coils of copper wire wound around them.

They are commonly employed in a circuit along with a rectifier that, as we have seen, supplies DC power to the equipment.

In the PC power supply, the transformer's secondary coils are used to provide 12-volt, 5-volt, and 3.3-volt outputs used by various components.

Testing a Transformer
As noted, a transformer is made up of several coils of wire.

Because each coil in the transformer is continuous, each can also be tested for continuity.

Follow these steps:

1. Disconnect the power.

2. Discharge all capacitors.

3. Check the bottom of the circuit board of the power supply to be sure all leads have been disconnected.

The primary connections, as well as the secondary connections, can be located below the transformer.

4. Configure the multimeter to measure continuity (or resistance).

Simultaneously touch each lead of the multimeter to one of the pairs of contacts.

A good transformer will show a reading of low resistance.

A very high reading could indicate that one of the coils is broken.

Inductors (Coils)
Inductors, commonly called coils because of their shape, are loops of conductive wire.

Current passing through the inductor sets up a magnetic field.

This field reduces any rapid change in current intensity.

Inductors can also be used to distinguish between rapidly and slowly changing signals in a circuit.

Testing a Coil
Because an inductor is simply a wire coil, it can be tested for continuity in much the same way a transformer is tested.

1. Visually inspect the wire for deterioration.

If it shows signs of breakage or burned areas, it should be replaced.

If the wire looks good, follow up with a conductivity test.

2. Turn the system power off.

3. Disconnect one lead to the coil (this might require a soldering iron) and connect one meter lead to each end of the coil.

A null or low reading indicates continuity.

A reading of high or infinite resistance indicates a lack of continuity.

4. Replace the coil.

Lesson Summary
The following points summarize the main elements of this lesson:

Ohm's law is the basic formula for power calculations.

There are two types of power: AC and DC.

A multimeter is used to measure volts, amps, continuity, and resistance.

A computer technician should be familiar with the various electronic components found on a power supply circuit board.

In a new installation, a computer technician should never assume that the power supplied in the wall outlets is correct. Always test and be sure.

Be careful with large electrolytic capacitors. They can store electrical charges and must be discharged before it is safe to work with them.

Quiz Basic Elecricity Techonology HTML