1. Power Supplies
Chapter 7

2. Overview In this chapter, you will learn to
Explain the basics of electricity
Provide proper power and cooling to the PC
Troubleshoot electrical problems
Instructor Tip
When gaining attention and establishing common ground, ask questions of the class such as, “Who here knows the difference between an AT power supply and an ATX power supply?”
For a positive statement, tell the class, “In this lesson, we are going to learn about power supplies and how to test them, and how to protect the computer from electrical sags and spikes.”

3. Understanding Electricity

4. Parts of an Atom
Bohr’s model says that an atom is made of a nucleus of protons and neutrons
Protons are positively charged
Neutrons are neutral in charge
Electrons revolve around the nucleus of the atom similar to the way in which the planets revolve around the sun
Electrons are negatively charged

5. Flowing Electrons Electrons are negatively charged
Protons are positively charged
Opposite charges attract (Coulomb’s Law)
Velocity of electrons keep them in orbit around nucleus
Electrons pulled free from the atom is what we call electricity!

6. “Dynamic” Electricity
Electricity can be viewed as a dynamic process
Dynamic means changing
Electrons are changing—moving from one atom to another
This flowing of electrons is called an electrical current

7. Static Electricity Static means stationary or unchanging
Electrons have been “loosened” from the atom and stay in one place
The electrons have voltage but lack a current
A conductor supplies the current—or path—for static electricity to discharge

8. Electrostatic Discharge
Electrostatic Discharge (ESD) is the process of static electrons jumping to a conductor
Rub your shoes on a carpet (this will cause a voltage to build up around your body)
Touch a metal door knob (the metal is a conductor providing a path for the flow of electrons—high voltage electricity!)

9. Conductors
Conductors have a large number of loosely attached electrons
These electrons can easily be freed from the nucleus of the atom when voltage is applied

10. Examples of Conductors
Metals
Gold
Silver
Copper
Water
Humans

11. Insulators
Insulators are materials with a high resistance to electrical current
Electron orbits are very close to the nucleus
Examples
Plastic
Glass
Wood
Air and other gases

12. Semiconductors
With semiconductor materials, the flow of electrons can be precisely controlled
Examples:
Carbon
Germanium
And Silicon
Because silicon is widely available (sand), it is the material we use for computer chips

13. Measuring Electricity
Voltage—force or pressure caused by the separation of electrons and protons
Unit of measurement: Volts (V)
Current—the free flow of electrons in an electrical circuit
Unit of measurement: Ampere (amp)
When voltage (electrical pressure) is applied and there is a path, electrons flow producing current
Resistance—impedance or opposition to the flow of electrons: conductor=low resistance insulators=high resistance
Unit of measurement: ohms (Ω)

14. Two Types of Current
Alternating Current (AC)—electrical current flows in both directions; positive and negative terminals continuously trade places (polarity)
Example: Electricity provided by AmerenUE
Frequency at which AC electricity alternates is measured in cycles per second, or hertz (Hz)
Direct Current (DC)—electrical current flows in one direction; negative to positive
Example: Electricity provided by batteries

15. Powering the PC

16. Type of Power PCs use DC voltage but power companies supply AC voltage
The power supply in a computer converts high-voltage AC power to low-voltage DC power
Some questions on the A+ Core Hardware exam could refer to a power supply as a PSU, for power supply unit. A power supply also falls into the category of field replaceable unit (FRU), which refers to the typical parts a tech should carry, like RAM and a floppy disk drive.

17. Safety Ground Wire
Safety Ground Wire prevents electrons from energizing metal parts of the computer
Without grounding, severe shock and fires can occur
Safety grounds are connected to the exposed metal parts of the computer’s chassis
Do not use ungrounded plugs in PCs
Discussion Point
Electrical Ground
Remember the days when electrical plugs with a ground plug were “optional”? It was around the same time polarized plugs with one blade wider than the other were introduced in the market. It was common to ground down that wider blade or simply snip off the ground plug because the wall outlets or the extension cords were not yet compatible. However, this is very unacceptable for computers! Electronics are very and susceptible to damage. You must have a properly grounded source, including any extension cords.

18. AC Power In the U.S. 115 V and 60 Hz
PCs may have a small switch on the back to choose 115 or 230 V (used in other countries)
Hot and neutral provide the path for AC
Four wires to the fuse box:
Bare wire that goes to ground and not the pole
Two 115-volt hot wires (black) from the pole to the fuse box
Neutral wire from the pole (black or striped)
House gets 230 V AC from the pole
Safety Alert
Do not flip the switch if you are in an area serviced by 220 volts. In a 110-volt power grid, switching the switch to 220 will simply make the PSU appear dead, without any real harm. If you are in a 220-volt area, flipping the switch to 110 will likely burn up the power supply, and it could cause a fire!

19. Multimeter Basics
A multimeter (or Volt-Ohm meter: VOM) is used to measure:
Voltage
Resistance
Continuity (level of resistance)
When using a multimeter, you must properly set it to either AC or DC, depending on the voltage you’re trying to measure
Discussion Point
Testing Equipment
A decent multimeter (or Volt-Ohm Meter) can be purchased inexpensively at most electronics or hardware stores; it can test for AC and DC voltage, amperage, continuity, and resistance. Also available is a less-expensive small module tester to test your AC outlet. This simply plugs into the AC outlet and checks for proper voltage, proper wiring, and proper grounding. A series of LED lights on the module show the results.

20. Testing AC Voltage Test all of the wall outlets that the PC uses
Include any peripheral devices plugged into a wall outlet
Include the PC and the monitor plugs
Circuit testers are available
The ‘hot’ should output approximately 115 V, the ‘neutral’ should output approximately 0 V, and the ‘ground’ should connect to ground (also approximately 0 V)

21. Uninterruptible Power Supply
An uninterruptible power supply (UPS) provides protection against a power dip or power outage
Contains a battery that provides AC power to the computer
Online is true protection and power conditioning – battery is constantly being charged and system is running off the battery at all times
Stand-by uses AC until the voltage drops enough to switch over to the battery – no power conditioning and a fail-over time required
All uninterrupted power supplies are measured in watts

22. Typical UPS

23. Surge Suppressors
Surge suppressors provide protection against power fluctuations
Insert between the power supply and the outlet
Joule is a unit of electrical energy, and the joule rating of a surge suppressor needs to be checked before purchasing one
Surge suppressors with modem protection are also available
Important: No surge suppressor in the world can handle the ultimate surge, the ESD of a lightning strike. If your electrical system takes such a hit, you can kiss your PC good bye if it was plugged in at the time. Always unplug electronics during electrical storms!
Recommendations
UL 1449 for 330V rating
Minimum of 800 joules
UL 497A for modem protection

24. The Power Supply
The power supply acts as a step-down transformer converting high voltage AC into 5, 12, and 3.3 V DC
PCs use a 12V current to power motors on devices such as the hard drives, and CD-ROM drives
PCs use a 5-volt/3.3-volt current to support onboard electronics
Discussion Topic
Power Supply
All PCs run on DC. All utilities companies provide AC, either as 110–120VAC or 220–230VAC, depending on the country you are in. Therefore, we need a method of taking that AC and safely converting, or “transforming,” it to the 12, 5, and 3.3 volts of DC. All power supplies, regardless of form factor, size, internal, or external, act as a “step-down” transformer to provide that conversion from household AC to the DC that the PC can use.

25. The Power Supply
AT and ATX are the two types of power supplies that can be installed in any PC
The ATX form factor motherboard, with its unique ATX power supply, dominates today’s systems
All power supplies share a number of common features such as the power connection, motherboard power, power switch, peripheral connections, and the fan

26. Power Supply

27. Power Connections
The power supply connects to the power cord through a standard IEC-320 connector
Each power supply must have standard AC power from the company

28. DC Power
DC power comes out of the computer’s power supply, and provides electricity to all the components in the PC
Flows in one direction, from negative to positive
All PC power supplies provide both positive and negative voltages

29. AT Power Connectors
P8 and P9 are a pair of connectors that link the AT power supply to the AT motherboard
As they are ‘faced’, they cannot be installed backwards
Note:
When connecting P8 and P9 to the motherboard the black ground wires should be kept next to each other.

30. ATX Power Connector Uses a single P1 power connector
The P1 has a notched connector to ensure proper installation
Both AT and ATX motherboard connectors provide negative voltages as well to the motherboard, such as –12 V and –5 V. Nobody messes with them and they don’t show up on the exam, so most techs treat them like an appendix, a holdover of older technology no longer used.

31. ATX Power Connector

32. P4 Connector
Many motherboards require a second, four-wire connector to push more 12-volt power into the board

33. P1 and P4 Voltages
Many power supplies have a 6-pin auxiliary power connector that supplies 3.3- and 5-volt DC current. Some also have a three-wire fan sensor wire.

34. Power to Peripherals
Motherboards require power from the power supply as we’ve already seen
Peripherals like hard drives, floppy drives, CD-ROM drives, and fans also require power from the power supply
Different types of connectors are used

35. Molex Connectors
Primarily used for devices that require 12V and 5V of power
A Molex connector uses ‘chamfers’ (notches) for easy installation
Installing a Molex backwards can destroy the device into which the Molex is connected

36. Mini Connectors Primarily for 3.5-inch floppy drives
Care should be taken when installing mini connectors
As with any power connector, plugging a mini connector into a device the wrong way will almost certainly destroy the device. Check twice before you plug one in!

37. Splitters and Adapters
Occasionally, there are not enough connectors to power all the devices inside a PC
Splitters are used to create more connections
Tech Tip
P4 Converters
If you have an older power supply that lacks a P4 12V 4-pin connector, you can try an adapter for a Molex connector—they do exist, but you’re almost always better off getting a newer power supply. If you don’t provide proper power to a higher-end system, you can create a very buggy machine! Check the section on wattage in this chapter for more details.
The A+ Certification Core Hardware exam does not require you to figure precise wattage needs for a particular system. When building a PC for a client, however, you need to know this stuff!

38. Wattage Power supplies are rated in watts
A PC requires sufficient wattage to run properly
An average desktop with two hard drives and a CD-ROM drive requires about watts while running, and up to 200 watts when booting up
Buy 230 to 250-watt power supplies
Power requirements may be calculated by adding up the power required by each peripheral and the motherboard and CPU
Calculating Power
If the wattage isn’t indicated in the documentation or on the device, it may be computed by multiplying the number of amps by the number of volts.
Wattage used by devices can vary a lot among manufacturers, by the way, so don’t take the numbers above as gospel for all CPUs or all hard drives! A quick glance at a Western Digital hard drive in my system shows it draws .66 × 5 V, which puts it on par with the Seagate, but only .24 × 12 V. Total power consumption of the Western Digital drive is ~2 watts lower than the comparable Seagate.
Tech Tip
Build in Aging
Don’t cut the specifications too tightly for the power supplies. All power supplies produce less wattage over time simply because of the wear and tear on the internal components. If you build a system that runs with only a few watts of power extra available from the power supply initially, that system will most likely start causing problems within a year or less! Do yourself or your clients a favor and get a power supply that has a little more wattage than you need.

39. Sizes
Power supplies are available in a variety of shapes and sizes depending on the form factor
Most desktop and mini-tower PCs use the standard ATX power supply
Take the defective power supply with you when getting a replacement

40. Power Supply Issues
A bad power supply causes intermittent lockups and reboots, as well as intermittent bootup difficulties
Bad power supplies erase CMOS information and sometimes even erase data on mass storage devices

41. Power Supply Test Put the black lead onto any black wire connection
Put the red lead onto a yellow +12V connection
11-13 V is good
V indicates a new power supply is needed
Less than 10.5 V and your PC won’t boot

42. Power Supply Test The computer must be turned on
Do not touch any chips or circuit boards to prevent damage
Do not touch a probe to the hot circuit and to ground at the same time!
Do not allow both probes to touch each other while one is touching hot and one is touching ground
Make sure a probe only touches one metal object or pin at a time!
Discussion Point
Danger in opening power supply
Although it is possible to replace the power supply cooling fan, be very careful. The power supply has capacitors that store power for a long time. It is dangerous to touch any electrical components inside the power supply. It is not a good idea to open the power supply at all, even to replace a fuse.

43. Power Supply Notes
If you don’t detect any power, disconnect all devices except the motherboard – check for power to the motherboard by itself
If the motherboard gets power, then one of the devices is causing the problem
If the motherboard still does not have power, check the power coming from the power supply without the motherboard plugged in – if you get power then the motherboard has an issue
Most PCs today come with a 230-watt power supply – if you add too many devices the power supply may stop working due to too much wattage being required

44. AT Power Switches Rocker and plunger types
Used to turn the system on and off
Each of these switches has four tab connectors that attach to four color-coded wires leading from the power supply
Black and brown wires are 115V – make sure the power supply is unplugged!
Discussion Point
Power Switches
The AT power switch is either on or off. It may be either a plunger or a rocker style that runs directly from the power supply itself to the on/off switch. Sometimes you can replace an inexpensive switch rather than the entire power supply, but you must be extremely cautious since it switches 120 V AC. Be sure to turn off the power before you work on it. Never work on an energized switch. When replacing the switch, it is a good idea to replace one wire at a time to make sure that you set all four wires correctly. Most AT switches will have black and white wires on one side and brown and blue on the other. If you do not connect them correctly, you may harm either yourself or the computer (or both). Instead of coming directly from the power supply itself, the on/off switch for an ATX power supply runs through the motherboard and is low-voltage (only 5 VDC).

45. Problems with AT Power Management
Power management involves going into power-saving modes, where devices are put to sleep
Power management also involves bringing devices back to an active state
The AT form factor and the AT power supply do not mix well with any type of power management

46. ATX Soft Power and CMOS
The soft power feature on ATX motherboards handle all power management issues
ATX power supplies put a 5-volt charge on the motherboard at all times
The important settings for ATX soft power reside in the CMOS setup

47. ATX Soft Power and CMOS
Some ATX power supplies provide a real on/off switch on the back
An ATX power supply never turns off: it continues to supply a 5 V to the motherboard as long as it is connected to the power outlet
Always unplug an ATX system before working on it
Use a screw driver or car keys to short the two power jumpers to turn the system on or off – should only be used if you are working on a PC with a broken power switch

48. Cooling The power supply fan provides basic cooling for the PC
The fan keeps the voltage regulators cool and provides a constant flow of cool air through the computer’s interior
If an expansion card is removed from the PC, be sure to cover the hole with a slot cover
Without the airflow the CPU can overheat and destroy itself
The CPU usually has its own fan and sits very near to the power supply so that it will not overheat from a disruption

49. Removing the Power Supply Fan
Replacement fans are easy to find at any Radio Shack and are inexpensive
There are only 4 screws to remove
You may need to cut off the connector and solder it onto the new fan

50. Troubleshooting Power
Discussion Point
Faulty Power Supplies
Power supplies fail more often than almost any other component in a PC except for floppy drives. When you identify a bad power supply, it is generally cheaper to replace the power supply considering the time and effort it would take to repair it.

51. Diagnosing a Dead Power Supply
A failure of the internal electronics of the power supply can cause some of the most difficult to diagnose problems
The secret to discovering that a power supply is dying lies in one word: intermittent
A voltmeter can be used to verify if the power supply is working or not
A failed power supply should be replaced rather than attempting to repair it
Power supplies break more often than many other parts in a PC
Discussion Point
Troubleshooting the Power Supply
The power supply must be “under a load” for it to be tested. There must be something hooked up to it - either a drive or a motherboard - in order to properly test it. A faulty motherboard or faulty device can shut down the power supply. If you do not detect power at the Molex connectors, first turn off the power and disconnect all devices except the motherboard and power backup. If you get power, then methodically plug in one device at a time until you no longer get power. Cut power each time before you plug in a new device. When you discover the unit killing your power, you have discovered your problem. If you unplug all your devices (except the motherboard) and you still have no power, then try plugging in a few devices and unplugging the motherboard. If you get power, then the motherboard was the cause for not getting power. If a power supply stopped working after you added a new device to your system, check the wattage rating of the power supply. It may not be able to provide enough power for all your devices.

52. Power Supply Switches
Broken power switches are also a common source of problems
On an AT system, a multimeter can be used to check the switch
On an ATX system, try shorting the soft power jumpers – if that works, then you need a new switch
Missing slot covers can cause the PC to overheat!

53. When Power Supplies Die Slowly
A power supply may be dying if you are getting intermittant problems such as
PC fails locks up as it is booting several times but finally boots up okay
Error codes show up on boot up but go away
PC runs fine for an hour or so and then locks up

54. Fire Extinguishers Do not open power supplies
The inside of a power supply contains high-voltage capacitors
Every PC workbench needs the right kind of fire extinguisher
A Class C fire extinguisher should be used for live electrical equipment
Class A fire extinguishers are for wood and paper
Class B fire extinguishers are for flammable liquids

55. Beyond A+
Some power supplies come in fancy colors, light up, and have extra fans
EPS12V power supplies provide more power for servers
Converting AC to DC causes a 30-50% loss in power, so the power factor number is used on power supplies to define the amount of loss (0.7 means a 30% loss)
An Active Power Correction (APC) power supply is more efficient with a power factor closer to 0.95