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Electricity and power supplies
Unit objectives:
 Identify electrostatic discharge, and
follow ESD safe practices while
working with computer components
 Install a new power supply in a PC
 Troubleshoot faulty power supplies
Topic A
 Topic A: Electrical safety
 Topic B: Power supplies
 Topic C: Power supply troubleshooting
Characteristics of electricity
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Electricity — Flow of electrons
Conductor — Permits flow of electricity
Insulator — Inhibits flow of electricity
Voltage — Force of electricity caused by
difference in charge at two locations
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Measured in volts
Also called “potential” or “potential difference”
Officially designated as uppercase V
May see it as lowercase v
continued
Characteristics, continued
 Current — Measure of the flow of
electrons past a given point
– Measured in amps, or amperes
– Must be a complete circuit (closed
circuit)
– Direct current — Flows in one direction,
at constant voltage, through circuit
– Alternating current — Flows repeatedly
back and forth through the circuit, at
constantly varying voltage levels
continued
Characteristics, continued
 Resistance — Force that opposes the flow
of DC through a conductor
– Measured in ohms (Ω)
 Impedance — Like resistance, but applies
to AC
 Power
– Measured in watts
– Calculate by multiplying voltage by current
 Energy — Electrical power delivered over
time
Activity A-1
Examining the characteristics of electricity
Electricity
 Current can kill
 The 1–10–100 rule
 Calculating current
V = i * r
 Resistance of the human body is
about 500 KΩ (500,000 Ω)
Safety precautions
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Don’t touch exposed contacts
Touch only insulated handles of tools
Leave covers on equipment
Work one-handed
Don’t insert anything into wall outlets
Remove jewelry, watches, etc.
Keep hands clean and dry
Don’t work in wet surroundings
Activity A-2
Considering electrical safety
Electrostatic discharge (ESD)
 Buildup of charges
 Static dangers
 Discharge voltages
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3000+ volts to feel
8000+ volts to see spark
35,000 volts on a dry day on carpet
30 volts can damage electronics
Preventing static buildup
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Don’t shuffle your feet
Increase the humidity: 50-60%
Keep yourself grounded
Wear cotton, not synthetics
Remove carpeting
Use air ionization system
Preventing static discharge
 Equalize charge differences safely
– Unplug the equipment
– Touch the metal chassis or power supply
 Keep yourself and equipment
connected together
– Wrist straps and antistatic mats
– Static-safe bags
 Goal is to be at a charge potential
that’s equal with the device you’re
servicing (not with ground)
 Service on a properly grounded bench
Antistatic (ESD) wrist straps
Typical internal components
CD-ROM drive
Power supply
CPU/fan
Adapter card
Motherboard
Hard drive
Slide catches
Slide catch, move to open side of case
Determining which side to open
Open this side to access components
Ports and connectors attached to motherboard
Removable front cover
Alignment holes
Alignment posts
Spring catches
Activity A-3
Opening the computer case while following
proper ESD precautions
Topic B
 Topic A: Electrical safety
 Topic B: Power supplies
 Topic C: Power supply troubleshooting
PC power supply
 Converts AC wall voltage to DC voltages for
PC components
 Includes a fan
 Provides some conditioning functions
 Can maintain power during brief drops and
outages
Power supply specifications
 Rated by DC power output in watts
– Modern systems typically =>300 watts
– Older systems <200 watts
 Rating isn’t an indicator of power draw
– Draws only the power needed to supply
internal components
Typical power requirements
Motherboard
30 W
Memory
10 W per 2 GB
CPU
45–145 W or more
Hard drive
5–15 W
CD-ROM drive
5–20 W
Floppy drive
5–10 W
Adapter card
5–30 W
Standard outputs
 +/- 5 V
 +/- 12 V
 + 3.3 V
Power connectors
 Two standards
– Drive power connectors
– Motherboard power connectors
Peripheral power connector
 Molex connector
 Typically used to connect hard drives and
optical drives
Floppy power connector
 Berg connector
 Smaller than Molex
 Typically used to connect floppy drives
SATA power connector
 New, for Serial ATA drives
Wire colors
Wire color Molex Berg
SATA
Yellow
1
4
13, 14, 15 +12 V
Red
4
1 (optional)
7, 8, 9
Black
2 and 2 (optional)
3
and 3
Orange
Voltage
+5 V
4, 5, 6
Ground
10, 11,12
1, 2, 3
+3.3 v
Motherboard power connectors
Single power
connector
Dual power
connectors (P8/P9)
Power supply form factors
 Describes size and shape
 Must fit:
– Case
– Motherboard
– Other components
Activity B-1
Identifying your computer’s power supply
Installing a power supply
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Shut down computer
Unplug computer
Remove cover from computer
Disconnect all power wires from components,
including motherboard
5. Remove retaining screws
6. Remove power supply
7. Install new power supply and screw it into place
8. Connect power wires to components as needed,
including motherboard
9. Replace cover
10. Plug computer into outlet, and boot system to test
your work
Selecting the voltage
 Set power supply to run on 110-120
VAC or 220-240 VAC
 With computer off, slide switch
 Usually located near power cord port
continued
Selecting the voltage, continued
 Improperly set voltage, can cause
computer to:
– Fail to turn on
– Operates improperly
– Incur serious damage
 www.voltagevalet.com/elec_guide.html
Activity B-2
Installing a new power supply
Topic C
 Topic A: Electrical safety
 Topic B: Power supplies
 Topic C: Power supply troubleshooting
An AC signal
Power problems
 Blackout — Total loss of power
 Brownout — Brief decrease in voltage
level
 Noise — Disruption of smoothly
varying AC signal
 Spike — Very brief increase in voltage
 Surge — Brief increase in voltage
(longer than spike)
Power conditioning
 Surge protectors
– Filter out spikes and surges
 Battery backup devices
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Condition the power signal
Some protect against brownouts
Continuous UPS vs. standby UPS
UPS monitoring and management software
 Generators
– Provide power for extended periods
– Use a variety of fuels
– Computer-grade generators produce clean,
consistent AC signals
Activity C-1
Discussing power conditioning equipment
Electrical measurements
 Measure electrical values with a
multimeter
 Multimeters can be digital or analog
 Use to measure
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Resistance
Voltage
Current
Continuity
Measuring resistance
1. Turn off the device you’re measuring and
disconnect it from its power source
2. You might need to disconnect the device
from its circuit
3. Set the multimeter to read resistance
4. Touch the two leads of the multimeter
together
5. Touch the black and red probes to either
side of the circuit to be measured
6. Read the resistance from the meter’s
display
Measuring voltage
1. Verify the power supply is on
2. Set your multimeter to read either DC or
AC voltage
3. Touch the black probe to the ground, and
touch the red probe to the spot where you
want to measure the voltage
4. Read the voltage from the meter’s display
You must exercise care when taking
voltage readings as the computer is
powered up
Measuring current
 Break the circuit and insert the meter
in the break
 Ammeter doesn’t require break
 Clamp ammeters are often used to
measure current flow in 110 V and
higher circuits
Measuring continuity
 Set your multimeter to display
resistance (ohms) and look for circuits
with zero resistance
 Also can use continuity mode, if
available
Power supply tester
 Alternative to manual test
with multimeter
 Takes approximately 30
minutes
 Follow instructions for your
specific tester
 Advantages:
– Less susceptible to human
error
– Results are more conclusive
– Safer to perform
Activity C-2
Measuring electrical values
Power supply problems
 Computer fails to boot when powered
on, but boots after Ctrl+Alt+Del
 Computer intermittently stops working
or reboots
 You don’t have enough power
connectors for all the devices you
want to install
 Computer fails to boot at all (no lights
or beeps); fans don’t start
 Computer fails to boot, but fans start
The Power_Good signal
 Signals CPU that power is stable and
sufficient
 +5 V over specific wire
 Must arrive at correct time
Activity C-3
Troubleshooting power supply problems
Electricity and power supplies
 Identified electrostatic discharge, and
followed ESD safe practices while
working with computer components
 Installed a new power supply in a PC
 Resolved problems with faulty power
supplies