Power Supplies

The power supply takes the AC (alternating current) voltage from your wall outlet and converts it to various DC (direct current) voltages needed to operate the motherboard, optical drive, hard drive, and other internal components within the PC frame. Usually, printers, scanners, and other external devices have their own power supplies and plug into a wall outlet or power strip separately.

 Introduction to Power Supplies

Individual components in a computer system are very particular about the kind of electricity they use. They don’t use regular household alternating current (AC). Instead, they use +3.3, +5, +12, –5, or –12 volts of direct current (VDC). To change AC to DC and to control voltage levels, manufacturers install a converting device called the power supply in each computer.

AT Power Supply

ATX Power Supply

 A power supply provides electrical power to the components in the computer that require power. A power supply also prevents a computer from starting up or operating if the correct power levels aren’t present. If the correct power levels aren’t available from the power supply, the components in the computer may be damaged.

Electrical and Electronic Safety

One of the most important lessons you can learn from this study unit is to work safely around all electrical devices. Electrical devices and circuits can be dangerous. It’s up to you to take the precautions necessary to prevent electrical shock, fires, explosions, equipment damage, and injuries resulting from the improper use of tools and test equipment.

Perhaps the greatest hazard is electrical shock. Electricity affects the body by overriding brain impulses and contracting muscles. A current through the human body in excess of 10 milliamperes "a unit of electric current equal to one thousandth of an ampere. Abbreviation: mA. Origin of milliampere". (0.01 amp) can paralyze a victim and make it impossible to let go of a “live” conductor.

Your skin can have approximately 1,000 times more resistance to the flow of electricity when dry as compared to being damp or wet. When dry, your skin’s resistance is in the vicinity of several hundred thousand ohms. When moist or cut, the skin’s resistance may become as low as several hundred ohms. In this circumstance, even so-called safe voltages as low as 30 or 40 volts might produce a fatal shock. Naturally, the danger of harmful or fatal shock increases directly as the voltage increases. Be very cautious, even with low voltages. Never assume a circuit is dead, even though the switch is in the Off position.

Working safely will protect you and those around you. Here are some rules to follow when working with electrical equipment.

1.      Don’t work when you’re tired or taking medicine that makes you drowsy.

2.      Don’t work in poor light.

3.      Don’t work in damp areas.

4.      Don’t work if you or your clothes are wet.

5.      Remove all rings, bracelets, and similar metal items.

6.      Never assume that a circuit is off. Check it with a device or piece of equipment that you’re sure is operating properly.

7.      Don’t tamper with safety devices. Never defeat an interlock switch. Verify that all interlocks operate properly.

Hertz is a unit of measure describing the frequency of an event in cycles per second. Often used to describe the speed of a computer. The greater the hertz, the faster the computer.

8.      Verify that capacitors have been discharged. Some capacitors may store a lethal charge for a long time. Note: It’s not recommended that you use a screwdriver to discharge stored electricity. Use a capacitor discharge tool, preferably with an indicator circuit.

9.      Don’t remove equipment grounds. Verify that all grounds are intact.

10.      Don’t use adapters that defeat ground connections.

11.      Follow directions when using solvents and other chemicals. They may explode, ignite, or damage electrical  circuits.

12.      Certain electronic components affect the safe performance of equipment. Always use the correct replacement parts.

13.      Use protective clothing and safety glasses when handling high-vacuum devices such as CRTs.

14.      Don’t attempt to work on complex circuits or devices such as power supplies without proper training. There may be many hidden dangers from a stored charge. This charge may remain for hours or sometimes days.

15.      When possible, keep one hand in your pocket while working with electricity. This reduces the possibility of your body providing an electrical path through the heart.

16.      Some of the best safety information for electrical and electronic equipment is the literature prepared by the manufacturer. Find it, read it, and obey it!

BASIC ELECTRICITY

Electricity is a form of energy where free electrons flow from atom to atom in a conductor. The conductor connects the power source to the device that consumes the energy. This concept is the same whether we’re talking about a power plant supplying electricity to a large city or a battery supplying electricity to a single light bulb. There are two forms that electrical energy can take: alternating current (AC) or direct current (DC). AC electricity doesn’t have constant polarity or constant voltage; its direction of flow and voltage change rate are measured in cycles per second, or hertz. In the United States, AC electricity alternates between +110 V and –110 V at 60 hertz. On the other hand, DC electricity has fixed polarity, constant voltage, and a fixed current flow direction within a circuit.

When discussing electrical measurement, you’ll need to be familiar with volts (V), amperes (A), watts (W), and ohms.

	Table 1
	ELECTRICAL UNITS OF MEASURE
Electrical Unit	Definition	As Related to a Power Supply
Volts (V)	The electrical potential difference between two points in a conducting wire carrying a constant current	A typical power supply outputs five different voltages: +3.3V, +5.0V, –5.0V, +12V, and –12V.
Amps or Amperes (A)	Amount of electrical energy flowing through a conductor or device over a given time at a specified voltage	A power supply's current rating will be different for each voltage output (e.g., 22A for +5V, 14A for +3.3V, and 6A for +12V.
Ohms ()	Measured resistance to current flow through a conductor or an electrical device	Measuring resistance allows you to check for continuity of a wire or the condition of a fuse. Low resistance indicates continuity (current will flow), while high resistance indicates a broken wire or that a fuse is bad (current won't flow).
Watts (W)	One watt is the power produced by a current of one ampere across a potential difference of one volt.	To calculate watts, multiply the current times the voltage.

Voltage

Voltage is the electrical potential difference that exists within a circuit and is measured in volts. To measure the voltage in a circuit, you’ll use a voltmeter or multi-meter.  Voltage represents the pressure that pushes the electrons through the circuit. When measuring volts, power must be turned on and the leads of the voltmeter must be placed on either side of the device being tested.

Measuring the Voltage of a Circuit

Amps

amps measure the current, or the volume of electricity flowing through a circuit at any given moment. To measure the current in a circuit, you’ll use a device called an ammeter. The ammeter is connected in series with the device or circuit being tested, as shown in Figure 4. Remember that power must be turned on to measure current. You can also measure current with a multi-meter.

Measuring Current with an Ammeter

CAUTION

Since the ammeter is part of the circuit, ensure that it can handle the anticipated current of the circuit being tested. Exceeding the current limit of the ammeter may damage the meter or, at a minimum, blow its fuse. Also, the ammeter may affect the current reading slightly.

Ohms

An ohm is the measure of resistance to current flow within a circuit. To measure resistance of a circuit or electrical component, you’ll use a device called an ohmmeter (Figure 5) or a multi-meter that measures ohms. To get an accurate resistance reading, you must first disconnect the device from the circuit. The ohmmeter is connected in parallel with the device being tested.

Measuring Circuit resistance with an Ohmmeter

Ohm’s Law

To better understand the properties of an electrical circuit, you need to be able to apply Ohm’s law. In Ohm’s law, voltage is represented by the letter E, current is represented by the letter I, and resistance by the letter R. Therefore, Ohm’s law states that E is equal to I times R (E = I  R). This equation, if restated to solve for current, becomes I = E/R (current equals voltage divided by resistance).

Similarly, the formula can also be restated to solve for resistance, which is R = E/I (resistance equals voltage divided by current). Although we’re discussing only simple circuits in this study unit, you should be aware that Ohm’s law applies to all electrical circuits, including complex circuits having multiple paths and components. 

Watts

Watts is the measure of electrical power. Power is calculated using the formula P = E  I (power equals voltage times current).

  

 Suppose we want to know how much power the power supply is providing to that device. We see that the voltage is 12 volts and we calculated the current as 0.5 amp. Using the formula P = E  I and substituting these values, the power equals 12 (volts) times 0.5 (amp), or 6 watts.

Power supplies are rated by the amount of total power that they can provide, such as 250, 300, 400, 500 watts. By adding together the power amounts required by the CPU, fans, and peripheral devices, we can determine how much power the power supply is delivering relative to its rated capacity. 

If another device, such as a hard drive, is added to a PC, it will require a 5-volt and a 12-volt input from the power supply. The power supply must have enough reserve capacity to provide the power required by the new drive. 

Electrical Terms, Abbreviations, and Symbols

	Table 2
C	OMMON ELECTRICAL TERMS AND ABBREVIATIONS
Abbreviation	Term	Definition
A or amp	ampere	unit of current
AC	alternating current	current that alternately flows in both directions (e.g., household current)
ACA	alternating current amps
ACV	alternating current volts
DC	direct current	current that flows in only one direction (e.g., battery current)
DCA	direct current amps
DCV	direct current volts
Hz	hertz	cycles per second (for AC)
k, or Kohm	kilohm or kilohms	1,000 ohms
kv or KV	kilovolt or kilovolts	1,000 volts
kw or KW	kilowatt or kilowatts	1,000 watts
µA	microamp or microamps	1/1,000,000 amp
mA	milliamp or milliamps	1/1,000 amp
mV	millivolt or millivolts	1/1,000 volt
MHz	megahertz	1,000,000 cycles per second
M or meg	megohm or megohms	1,000,000 ohms
	ohm	unit of electrical resistance
RMS	root-mean-square	effective value of an alternating current
V	volt or volts	unit of electric potential
VAC	volts of alternating current
VDC	volts of direct current

Common Electrical Symbols

Electrostatic Discharge

An electrostatic discharge (ESD) is simply the transfer of an electric charge between two objects. We’ve all fallen victim to ESD at one time or another: you walk across a carpeted floor then touch a metal object—zap! We call this static electricity. The electric charge is a result of the two objects having different voltage potentials. Though there’s no current, the voltage generated could be thousands of volts. Since electronic devices are very sensitive to ESD, you’ll understand the importance of grounding both yourself and the device being worked on before beginning. With both you and the device grounded properly, there can be no difference in voltage potential and therefore no possibility of ESD. The exception to this rule is when working on high-voltage components (like a monitor or a power supply). If you were grounded and accidentally touched a high-voltage device, you would act as the path of least resistance. The result could be fatal.

Electromagnetic Interference

Electromagnetic interference (EMI) results from uncontrolled radiated and conducted magnetic fields generated by the flow of electricity. Devices that emit EMI include but aren’t limited to electric motors, computer monitors, television sets, audio speakers, and fluorescent lights. 

EMI can cause problems with computer operation or the monitor. If a computer system exhibits random or sporadic operating errors, try moving it to a different location that uses a different circuit to see if that cures the problem. Likewise, if the image on the monitor is distorted, suspect EMI as a possible cause.

Radio frequency interference (RFI) is a type of EMI that occurs in the radio frequency range. RFI can interfere with radio and television signals as well as corrupt data on transmission lines, diskettes, or even a hard drive.

DIAGNOSTIC TOOLS AND TECHNIQUES

The Multi-meter

A multi-meter consists of several individual test equipment devices that are combined into one compact package. Multi- meters are available in either digital or analog displays. The digital version displays values in digits and letters; the analog multi-meter indicates values using a needle that moves across a graduated scale. 

Using Your Multi-meter

While there’s no substitute for reading and understanding the instruction manual that came with your multi-meter, there are some general rules that you should be aware of. Knowing how to use the multi-meter is important for your safety and to prevent damage to the circuit under test or your multi-meter. 

Always be sure that the multi-meter is set for what you want to measure (e.g., voltage, current, or resistance) and that it’s set to the correct range. When measuring voltage, place the test leads across the component or terminals to be measured. The black lead goes to the negative terminal or ground, and the red lead goes to the positive terminal or hot point.

Digital Multi-meter

When measuring current, the circuit must be broken and the meter must be inserted as a part of the circuit. The black lead must be connected to the negative or ground side of the circuit, while the red lead must be connected to the positive or hot side of the circuit.

When measuring continuity or resistance of a component or circuit, there must be no electricity present in the circuit. The component or circuit that you’re measuring must also be isolated or disconnected from any other electrical paths. Your multi-meter operates by applying a small voltage across the component under test and determines the resistance by the amount of current that flows in the circuit.

Here are a few rules to remember when using a multi-meter.

1.   Never connect a multi-meter to measure more voltage or current than it was intended for.

2.   Never connect the meter to a source of voltage with the function switch in the OHMS () position.

3.   Never operate the multi-meter without the cover(s) being in place and fully closed.

4.   Always disconnect the test leads before servicing the multi-meter.

5.   When making current measurements, make certain the multimeter is connected in the circuit (in series) with the load. Never connect a meter that’s set to measure current across voltage terminals or across a component.