Power Supplies Computer Service and Repair Chapter 5 1Ch 5.

Ch 5 1

Power Supplies

Computer Service and RepairChapter 5

Ch 5 2

What is Electrical Energy• Electrical Energy is best described as a flow of

electrons• Direct Current (DC): Electrical current flows in one

direction from negative to positive– Positive terminals are marked red, (+), or both– Negative terminals are marked black, (-) or both

• Alternating Current (AC): Electrical current flows first in one direction and then in the other. A complete sequence is called a cycle ( the frequency of cycles is measured in Hz. US frequency is 60Hz))– AC has no (+) or (-) because polarity is constantly changing

Ch 5 3

What is Electrical Energy (cont’d)Measuring Electricity:• Voltage (measured in volts or V): is electrical pressure. It is

akin to PSI in a water pipe• Current (measured in Amps or A) is volume of electron

flow. It is akin to gallons per minute in a water pipe – amount not speed!

• Resistance (measured in Ohms, expressed as R or omega): is the opposition to the flow of electrons– Continuity is the ability of a component to allow the

unobstructed flow of electrons (0 ohms)– An Open is the complete resistance to the flow of electrons

(infinity Ohms)

Ch 5 4

What is Electrical Energy (cont’d)Measuring Electrical Energy• Power (measured in Watts or W): the amount of

energy provided or used by equipment• DC watts = volts x amps• AC watts are harder to calculate and expensive to

measure (due to induction). So, we use volt-ampere (VA) as an approximate measure of AC watts (calc. like DC watts)– Wattage is considered true power– VA is considered apparent power

Ch 5 5

What is Electrical Energy (cont’d)Measuring Electrical Energy:– Wattage measures two things:

• When on a device that consumes power – how much electricity that device uses

• When on a device that produces power – how much that device produces (note: these device may produce more power but associated heat will damage them)

– Electrical limits: (real risk of fire!)• Power strip – NEVER exceed 1600 watts / never daisy chain• Wire Size: 12-20A, 14-15A, 16-7A, and 18-5A• Never use extension cord smaller that 16 gauge wire• Wire gauge smaller number = bigger wire

Ch 5 6

What is Electrical Energy (cont’d)• Clean Power: Commercial Power that is steady, at

the correct voltage, and does not contain voltage spikes.– Voltage spike and results from:• Lightning strikes• Cars hitting poles• Line maintenance• Electrical motors operating in same circuit

– Voltage spikes can be reduces or eliminated by line conditioning devices

Ch 5 7

Using a Multimeter• Multimeters are used to measure Current (Amps),

resistance (ohms), and Voltage (volts)– Can be digital or analog

• Probes and test leads (there are two)– Black lead is negative, ground or common is plugs into

socket marked (-), black, or “COM”– Red is plugged into socket marked voltage & resistance or

Amps (you’ll probably never need Amps socket)– Avoid touching leads while testing circuits

• Miss read • Dangerous

Ch 5 8

Using a Multimeter

Analog Multimeter Digital Multimeter

Ch 5 9

Using a Multimeter

• Range Selector– Choose appropriate value to be read Volts, Ohms

or amps– Select the lowest range that is higher than the

value to be measured• A meter with 10,50,500V options would be put on 50 ti

measure a 12 volt system

• Display Area– The area where reading is displayed

Ch 5 10

Simpson 260 Display Area

Ch 5 11

Using a Multimeter

Procedures for reading voltage:• Have an idea of what voltage to expect:– Common wall outlet 120 VAC– Power supply might be 12, 5 or 3.3 VDC

• Never wear anti-static ground strap!1. Insert probes into appropriate jacks2. Turn selector to voltage AC (some meters 2 switches)

3. Touch leads to test locations4. Read the display and record votage

Ch 5 12

Using a MultimeterProcedures for Reading Resistance: • MAKE SURE SYSTEM IS DE-ENERGIZED!!! • Voltage can destroy a multimeter set on resistance!1. Insert leads into proper jacks2. Be absolutely sure system is de-energized (check voltage

with your meter)3. Set selector to highest value of resistance4. Touch probes together to make sure meter is working

properly (reading should be 0 – if not check battery)5. Touch probes to test locations 6. Note reading (Adjust selector switch setting as needed)

Ch 5 13

Using a MultimeterChecking Fuses, Cables, and Switches• When Checking Fuses, cables, and switches there only two

values typically displayed by the meter: 0 Ohms or infinity.• When Checking device resistance remove it from system to

avoid Backfeed – reading backward through the system• Fuses are glass or ceramic cylinders with metal on each end

and a thin wire that is the fusible link. • To test fuse remove from holder and test resistance across

the fuse– 0 = good fuse – Infinity (open) = blown fuse

Ch 5 14

Using a Multimeter

Checking Fuses, Cables, and Switches• To test Cables use same rules as testing a fuse• To test a switch– Remove switch from system– Check resistance through switch when it is on• Resistance should be zero

– Check resistance when switch is off • Resistance should show open

Ch 5 15

Using a Multimeter

Checking Power Outlets:NEVER TOUCH THE TIPS OF THE LEADS WHEN TAKING READINGS !!!• Remember Wall outlets are AC so there is no polarity• Good voltage is considered +/- 10% of the 120 V rating• Time of day affects the voltage

– between 4 and 6 pm largest demand on power systems • Business are still open • People are coming home and starting dinner• Heating and cooling systems are turning up• Most brown outs occur at this time of day

• Weather– Extreme hot or cold weather

• Electrical heat or a/c

Ch 5 16

Branch Circuits• Many computer problems are generated by the home

or office electrical system.• Operating a vacuum cleaner or power tool can

generate voltage spikes that disrupt the computer process, damage the computer or lock up the computer– Surge protection a must have

• Branch circuit is the wiring from the electrical panel to the outlet the computer is plugged into– Must know what type of equipment is on that circuit– An office circuit may seem fine until you find it also serves a

welder on the other side of the wall!

Ch 5 17

Branch Circuits

• Dedicated Circuits are electrical distribution systems that are designed to only serve computer equipment.– Isolation transformers– Special grounding– Special colored outlets– No additional equipment should ever be plugged

into dedicated outlets• * remember to warn Cleaning People !

Ch 5 18

The PC Power Supply

• The power supply is responsible for converting standard 120 VAC to +12, +5, +3.3, -12 and -5 VDC

• Power supplies have form factors as do motherboards they are named after the motherboard type they are typically installed with: AT, Baby AT, LPX, ATX, ATX12V, and NLX

• Must know if a tower or a desktop• Warning: Wire color coding not a true standard

Ch 5 19

The PC Power SupplyMain Power Connectors• Power plugs have special shapes that match

voltage level• PWR_OK is a special connection that sends the

power good signal to the motherboard during POST (verifies power supply is working properly)

• +5VSB (standby) provides voltage when power switch is set to off. Use to “wake up” computer on event only way to disable is to unplug system– Keystroke or “wake on LAN”

Ch 5 20

The PC Power Supply

• Standby Power connection: (soft power) provides power to keyboard when computer is in sleep mode

• Power on and Standby power connection is used by Windows O/S to turn computer off using software commands

• Following are common power connectors found on PCs

Ch 5 21

The PC Power Supply

+ 12V Connector (2 X 2) Floppy Drive Connector (1x4)

Ch 5 22

The PC Power SupplyPeripheral Connector

(1 X 4) MOLEX Voltage of typical connectors

Ch 5 23

The PC Power Supply

Ch 5 24

PC Power Supply

Power InputINPUT Min.

VACNominal VAC

Max VAC

115 Vac 90 115 135

230 Vac 180 230 265

Hz 47 50/60 63

Power OutputOutput VDC

RANGE MIN. MAX.

+12 +/- 5% +11.40 + 12.60

+5 +/- 5% + 4.75 + 5.25

+3.3 +/- 5% + 3.14 + 3.47

-12 +/- 10% - 10.80 - 13.20Most countries use : 230 VAC @ 50 HzSome countries use : 115VAC @ 50HzUS is one of a few use: 115VAC @ 60 Hz &

230 VAC @ 60 HzNeed adaptors to make plugs from one country fit those of another

See power supply label for output voltage and wattage the unit can deliver. Replacements must be same rating or higher rating .

Ch 5 25

The PC Power SupplyTroubleshooting the Power Supply:• Inoperable cooling fans: (cooling fans get power directly from power supply)

• No indicator lights• Smoke coming from power supply• Burnt Smell• Circuit breaker tripping• Automatic rebooting • Electrical Shock• Excessive* heat (normal Power supplies create heat)

Ch 5 26

The PC Power SupplyReplacing a Power Supply:1. Be Sure power is off (no wrist strap)2. Sketch all connections3. Remove power cord and all power connections carefully4. Remove retaining / mounting screws5. Install the new power supply into the case6. Reconnect power connections7. Place any extra power connections in a neat bundle away from

motherboard8. Double check everything9. With case cover removed turn on computer10. If all is well replace case cover and power on PC once more

Ch 5 27

Surge Protection Devices

• Surge: when a higher than desired voltage is present in electrical system

• Brownout: low voltage is present• Blackout: no voltage is present– Can happen anytime– Only need the absence of one electrical cycle or

less to cause a crash or lockup– Can happen and not be noticeable by the human

eye

Ch 5 28

Surge Protection Devices

• Uninterruptible Power System (UPS) ensure a constant supply of quality power to a computer system

• Quality Power no surges, brownouts, or blackouts

• Monitor power input, maintain fully charged batteries, switch to battery if commercial power fails. Some even shut computer down before their batteries fail.

Ch 5 29

Surge Protection Devices

Power Strips• Not all power strips protect against surges!• Power strips that protect against surges have a

metal oxide varistor (MOV) that shorts the power strip during a spike, thus providing alternate current path and trips the breaker to protect computer equipment

• Not fool proof some spikes are too severe for the MOV or happen to fast fior breaker to trip

Ch 5 30

Batteries• UPS batteries tend to be lead acid batteries or Gel

type– Should be inspected on a regular basis– Check for :

• Corrosion• State of charge

• CMOS and laptop batteries tend to be Lithium Ion batteries

• Environmental Protection Agency (EPA) regulates how to dispose of batteries properly often illegal to just throw away!

Ch 5 31

Power Management Standards

• Controlling Power consumption is critical for battery life in mobile devices.

• Advance Configuration and Power Interface (ACPI) is an open industry power managment standard for desktops, laptops, and servers.– Sets amount of time before monitors, harddrives

shutdown