1. Computer Service and Repair Chapter 5
Power Supplies
Computer Service and Repair
Chapter 5
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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
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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)
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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
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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: A, 14-15A, 16-7A, and 18-5A
Never use extension cord smaller that 16 gauge wire
Wire gauge smaller number = bigger wire
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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
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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
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8. Using a Multimeter
Analog Multimeter
Digital Multimeter
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9. Using a Multimeter Range Selector Display Area
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
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10. Simpson 260 Display Area
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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!
Insert probes into appropriate jacks
Turn selector to voltage AC (some meters 2 switches)
Touch leads to test locations
Read the display and record votage
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12. Using a Multimeter Procedures for Reading Resistance:
MAKE SURE SYSTEM IS DE-ENERGIZED!!!
Voltage can destroy a multimeter set on resistance!
Insert leads into proper jacks
Be absolutely sure system is de-energized (check voltage with your meter)
Set selector to highest value of resistance
Touch probes together to make sure meter is working properly (reading should be 0 – if not check battery)
Touch probes to test locations
Note reading (Adjust selector switch setting as needed)
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13. Using a Multimeter Checking 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
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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
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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
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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!
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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 !
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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
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19. The PC Power Supply Main 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”
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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
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21. The PC Power Supply + 12V Connector (2 X 2)
Floppy Drive Connector (1x4)
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22. The PC Power Supply Peripheral Connector (1 X 4) MOLEX
Voltage of typical connectors
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23. The PC Power Supply
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24. PC Power Supply Power Input Power Output INPUT Min. VAC Nominal VAC
Max VAC
115 Vac 90
115
135
230 Vac
180
230
265 Hz 47
50/60 63
Output
VDC
RANGE
MIN.
MAX.
+12
+/- 5%
+11.40 +5
+3.3
-12
+/- 10%
Most countries use : Hz
Some countries use : 50Hz
US is one of a few use: 60 Hz & Hz
Need 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 .
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25. The PC Power Supply Troubleshooting 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)
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26. The PC Power Supply Replacing a Power Supply:
Be Sure power is off (no wrist strap)
Sketch all connections
Remove power cord and all power connections carefully
Remove retaining / mounting screws
Install the new power supply into the case
Reconnect power connections
Place any extra power connections in a neat bundle away from motherboard
Double check everything
With case cover removed turn on computer
If all is well replace case cover and power on PC once more
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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
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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.
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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
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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!
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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
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