Power Supply
By
Mohd Zuhaimi Zolkifli
Inside PC Power Supply
Connector
Introduction
• A power supply unit (PSU) is the
component that supplies power to the
other components in a computer.
• Convert general-purpose alternating
current (AC) electric power from the mains
(220-240V) to usable low-voltage DC
power for the internal components of the
computer.
PC Power Supply
• In a personal computer (PC), the power supply is
the metal box usually found in a corner of the
case.
• The power supply is visible from the back of many
systems because it contains the power-cord
receptacle and the cooling fan.
• Power supplies, often referred to as "switching
power supplies", use switcher technology to
convert the AC input to lower DC voltages. The
typical voltages supplied are: 3.3V, 5V and 12V.
• The 3.3V and 5V are typically used by digital
circuits, while the 12V is used to run motors in
disk drives and fans.
Types of Power Supply
• The first generation of computers power
supplies were linear devices, but as cost
became a driving factor, and weight
became important, switched mode
supplies are almost universal.
• So that, two(2) types of PC Power Supply
are:
– Linear Power Supply
– Switching Power Supply
Overview
Linear Power Switching Power
Supply Supply
Linear Power Supply
• An AC powered linear power supply
usually uses a transformer to convert the
voltage from the wall outlet (mains) to a
different, usually a lower voltage.
• A rectifier is used to produce DC.
• Capacitor is used to smooth the pulsating
current from the rectifier.
• Some small periodic deviations from
smooth direct current will remain, which is
known as ripple.
• These pulsations occur at a frequency
related to the AC power frequency.
Further information,
please refer to E1002.
You had learned it.
Switching Power Supply
• Prior to 1980 or so, power supplies tended
to be heavy and bulky. They used large,
heavy transformers and huge capacitors
(some as large as soda cans) to convert
line voltage at 120 volts and 60 hertz into
5 volts and 12 volts DC.
How it work?
• The AC input is rectified and filtered to
give a steady DC voltage.
• This DC input to the voltage switching
block is either blocked or allowed to pass
through depending on the state of the
switch.
• The resultant waveform which is a square
wave is rectified and filtered (This
waveform is sampled).
• If the current required at the load is high,
then the switching control ensures that the
voltage switch is on for longer periods.
• The sampling is done by a comparator and
DC amplifier along with a DC reference.
• The output voltage is compared with a DC
reference.
• If the output voltage reduces due to
increase in load current, the comparator
sends a positive signal to the pulse width
modulator and increase the on time of the
pulse.
Fault Testing
• We need correct load resistor to test the power
supply.
• Also need isolation and auto transformer.
– Isolation transformer : for safety
– Auto transformer : provides adjustable AC output.
Power Supply Form Factors
• Technically, the power supply of PC’s
described as a constant voltage
switching power supply.
– Constant Voltage: the power supply puts out
the same voltage to the computer’s internal
components, no matter what the voltage of
AC current running it or the capacity of the
power supply.
– Switching: the design and power regulation
technique that most supplier use.
Power Supply Form Factors
AT-style vs. ATX-style
AT-Style vs. ATX-Style
• There are two basic differences between
AT and ATX power supplies:
– The connectors
– The soft switch.
• On older AT power supplies, the Power-on
switch wire from the front of the computer
is connected directly to the power supply.
• On newer ATX power supplies, the power
switch on the front of the computer goes to
the motherboard over a connector labeled
something like; PS ON, Power SW, SW
Power, etc
AT Power Connector
AT Power Connector
• Two main power connectors (P8 and P9, also
called P1 and P2), each with six pins that
attach the power supply to the
motherboard.
• These are rated at 5A per pin, at up to
250V.
• P8 and P9 connectors have them installed
end to end so that the two black wires on
both power cables are next to each other.
ATX Main Power Connector
ATX Main Power Connector
• The industry-standard ATX power supply-
to-motherboard main connector is the
Molex Mini-Fit, Jr. connector number 39-
29-9202 (20 pins ATX connector).
• Molex rates each pin to handle 6A, at up
to 600V.
ATX Auxiliary Power Connector
ATX Auxiliary Power Connector
• In particular, chipsets and DIMMs were
designed to run on 3.3V, increasing the current
demand at that voltage.
• This type of connector are rated for 5A per pin
at up to 250V.
• The connector is normally keyed to prevent a
misaligned connection.
• The additional +5V wire allows a total of 29A of
+5V to be available to the motherboard, and
the additional two +3.3V wires allow a total of
28A of +3.3V power to be available to the
motherboard.
Peripheral Power Connector
Floppy Power Connector
RED +5V
BLACK GND
BLACK GND
YELLOW +12V
Y-Adapter Power Cable
Common Power Problem
• Power failure: total loss of input voltage.
• Surge: momentary or sustained increase in the
mains voltage.
• Sag: momentary or sustained reduction in input
voltage.
• Spikes; brief high voltage excursion.
• Noise; high frequency transient or oscillation,
usually injected into the line by nearby equipment.
• Frequency instability: temporary changes in the
mains frequency.
• Harmonic distortion: departure from the ideal
sinusoidal waveform expected on the line.
Power Protection System
• Sometimes power supply by AC outlet not
in stable condition (up to 280V).
• It called ‘Power Surge’.
• To overcome this situation, there is few
power supply supporting peripherals such
as:
– Uninterrupted Power Supply (UPS)
– Automatic Voltage Regulator (AVR)
– Power Line Conditioner (PLC)
UPS
A small free-standing UPS A large datacenter-scale UPS
UPS
• An electrical apparatus that provides emergency
power to a load when the input power source fails.
• It will provide instantaneous or near-instantaneous
protection from input power interruptions by means
of one or more attached batteries and associated
electronic circuitry for low power users.
AVR
AVR
• A device intended to regulate voltage
automatically: that is to take a varying
voltage level and turn it into a constant
voltage level.
• An automatic voltage regulator combined
with one or more other power-quality
capabilities such as:
– Surge suppression,
– Short circuit protection (circuit breaker),
– Line noise reduction,
– Phase-to-phase voltage balancing,
– Harmonic filtering, etc.
PLC
PLC
• A device intended to improve the quality of
the power that is delivered to electrical
load equipment.
• A device that acts in one or more ways to
deliver a voltage of the proper level and
characteristics to enable load equipment
to function properly.
Power Supply Specification
PC Item Watts
Accelerated Graphics Port
20 to 30W
(AGP) card
Peripheral Component
5W
Power
Interconnect (PCI) card
small computer system interface
20 to 25W
Supply
(SCSI) PCI card
floppy disk drive 5W
network interface card
50X CD-ROM drive
4W
10 to 25W
Wattage
RAM 10W per 128M
5200 RPM Integrated Drive
Electronics (IDE) hard disk 5 to 11W
drive
7200 RPM IDE hard disk drive 5 to 15W
Motherboard (without CPU or
20 to 30W
RAM)
550 MHz Pentium III 30W
733 MHz Pentium III 23.5W
300 MHz Celeron 18W
600 MHz Athlon 45W
Power Use Calculation
• One way to see whether your system is
capable of expansion is to calculate the
levels of power drain in the various
systems components and deduct the total
from the maximum power supplied by the
power supply.
• Example:>>>>>>>
• 200-watt power supply rated for 20A at
+5V and 8A at +12V.
• Power Consumption Calculation
– Available 5V Power : 20.0 A
• Motherboard - 5.0 A
• 4 slots filled at 2.0 - 8.0 A
• 3 ½” floppy drive logic - 0.5 A
• 3 ½” hard disk drive logic - 0.5 A
• CD-ROM/DVD drive logic - 1.0 A
– Remaining Power: 5.0 A
– Available 12V Power: 8.0 A
• 4 slots filled at 0.175 each - 0.7 A
• 3 ½” hard disk drive motor - 1.0 A
• 3 ½” floppy drive motor - 1.0 A
• Cooling fan motor - 0.1 A
• CD ROM/DVD drive motor - 1.0 A
– Remaining Power: 4.2 A
• With half of slot filled, a floppy drive, and
one hard disk, the system still has room
for more.
Maximum Power Consumption Per
Bus Slot (Amps)
Bus type +5V Power +12V +3.3V
Power Power
ISA 2.0 0.175 n/a
EISA 4.5 1.5 n/a
VL-bus 2.0 n/a n/a
16-bit MCA 1.6 0.175 n/a
32-bit MCA 2.0 0.175 n/a
PCI 5 0.5 7.6
Power Supply Troubleshooting
• List of PC problems that often are related to the
power supply:
– Any power-on startup failure
– Spontaneous rebooting
– Intermittent parity check
– Hard disk and fan failing to spin (no +12V)
– Overheating due to fan failure.
– Electric shocks felt on the system case
– Slight static discharges that disrupt system operation
– Erratic recognition of bus-powered USB peripherals.
Power Supply Troubleshooting
• Fairly obvious symptoms point right to the
power supply as a possible cause:
– System that is completely dead (no fan, no
cursor)
– Smoke
– Blown circuit breakers
Zero in on common power supply-
related problem
1. Check the AC power input. Make sure the cord is firmly
seated in the wall socket and in the power supply socket.
Try a difference cord.
2. Check the DC power connections. Make sure the
motherboard and disk drive power connectors are firmly
seated and making good contact. Check for loose screw.
3. Check the DC power output. Use digital multimeter to
check for proper voltages. If it’s below spec, replace the
power supply.
4. Check the installed peripherals. Remove all boards and
drive and retest the system. If it works, add items back in
one at a time until the system fails again. The last item
added before the failure returns is likely defective
Repairing the Power Supply
• Simply replacing the supply with a new
one is usually cheaper. Even high-quality
power supplies are not that expensive
when compared to the labor required to
repair them.
