Transcript Power Supplies

Power Supplies
Computer Service and Repair
Chapter 5
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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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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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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 voltampere (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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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!)
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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
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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:
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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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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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Using a Multimeter
Analog Multimeter
Digital Multimeter
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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
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Simpson 260 Display Area
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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 jacks
2. Turn selector to voltage AC (some meters 2 switches)
3. Touch leads to test locations
4. Read the display and record votage
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Using a Multimeter
Procedures for Reading Resistance:
• MAKE SURE SYSTEM IS DE-ENERGIZED!!!
• Voltage can destroy a multimeter set on resistance!
1. Insert leads into proper jacks
2. Be absolutely sure system is de-energized (check
voltage with your meter)
3. Set selector to highest value of resistance
4. 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)
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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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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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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
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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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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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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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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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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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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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The PC Power Supply
+ 12V Connector (2 X 2)
Floppy Drive Connector (1x4)
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The PC Power Supply
Peripheral Connector
(1 X 4) MOLEX
Voltage of typical connectors
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The PC Power Supply
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PC Power Supply
Power Input
Power Output
INPUT
Min.
VAC
Nominal Max
VAC
VAC
Output
VDC
RANGE
MIN.
MAX.
115 Vac
90
115
135
+12
+/- 5%
+11.40
+ 12.60
230 Vac
180
230
265
+5
+/- 5%
+ 4.75
+ 5.25
Hz
47
50/60
63
+3.3
+/- 5%
+ 3.14
+ 3.47
-12
+/- 10%
- 10.80
- 13.20
Most countries use : 230 VAC @ 50 Hz
Some countries use : 115VAC @ 50Hz
US is one of a few use: 115VAC @ 60 Hz &
230 VAC @ 60 Hz
Need adaptors to make plugs from one
country fit those of another
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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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The PC Power Supply
Troubleshooting the Power Supply:
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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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The PC Power Supply
Replacing a Power Supply:
1. Be Sure power is off (no wrist strap)
2. Sketch all connections
3. Remove power cord and all power connections carefully
4. Remove retaining / mounting screws
5. Install the new power supply into the case
6. Reconnect power connections
7. Place any extra power connections in a neat bundle away
from motherboard
8. Double check everything
9. With case cover removed turn on computer
10. If all is well replace case cover and power on PC once more
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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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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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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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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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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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