The document discusses various components of computer power supplies. It describes how power supplies convert AC power to DC power required by computer components. Power supplies have cables that supply different voltages to parts like the motherboard. Power supply wattage requirements depend on the system configuration, with 500W sufficient for most average systems. Graphics cards may use additional 6-pin or 8-pin power connectors. The document also discusses switch mode power supplies, universal power cables, and uninterruptted power supplies (UPS).

1. POWER SUPPLIES
PART-A
The power supply (power supply unit or PSU) is a rectangular box with several cables that fits in
the top back of the case. Its primary purpose is converting AC power from an outlet to DC power
the computer can use. The cables have connectors on the ends which supply different voltages to
various internal components with the main connector plugging into the motherboard. However
some, called modular power supplies, contain built-in connectors so you can use only the cables
that are necessary.
Computer Power Supply
Power supplies conform to the ATX factor and come in a range of different wattages - from less
than 300W to more than a 1000W. For the average system with one or two hard drives, a DVD
drive, dual core CPU, and a low-end graphics card, need at least a 500W PSU. Very powerful
machines such as gaming systems can use power supplies that are 1000W or more.
Motherboards designed for SLI or Crossfire has PSUs specifically made for them. After
purchasing, make sure the voltage is set to the correct number by using the voltage selector (100-
120V for N. America. & 220-240V for Europe).
Universal Power Cable:
Most personal computers (their power supply) and computer monitors use a universal power
cable. To be more specific it's a IEC 60320 C13 power cable. Those supplied with most
computer equipment are 16/3 (16 gauge, 3 conductors) but some of the larger computer power

2. supplies (especially those rated for more than 900 watts) will use a 14/3 cable which is good for
15 amps of current.
Computer Power Supply Cables
4-Pin Molex Power Connector
There are many different types of connectors on the power supply wiring harnesses. The most
common are the flat 4 pin molex and the 20/24 pin power connector. The flat molex generally
supplies power to older hard drives and optical drives but can also be used to feed power to the
motherboard. When the 4 pin molex is used to feed power to the motherboard, it's typically used
to supply power to the PCI-E sockets. This can supply power to low end graphics cards that use
the PCI-E slots.

3. 20-Pin AT/24-Pin ATX (Advanced Technology Extended) Power Connector
Most new motherboards use 24 pin connectors. Older boards used 20 pin connectors. The 4
additional pins provide 1 extra terminal for the 12v, 5v, 3.3v and ground. Most new supplies
have a 20+4 pin configuration that makes them backwards-compatible with older boards. The
following shows a 20 pin power supply being used with a 24 pin motherboard. This computer
had very limited power requirements so this wasn't a problem here but for high-end computers,
you should use the proper power supply.

5. PCI – E Connector
High-end graphics cards need more power than can be supplied through the PCI-E slot so they
use other types of power connections. Typically, the high-end graphics cards use either a 6 pin or
an 8 pin connector. In the photo below, you can see that the card uses a 6 pin connector but the
power supply has provided a second connector that provides the extra 2 pins needed for the cards
that use the 8 pin connector.

7. PART-B
Old AT Power Supplies
The following is an old AT (not ATX) power supply. Newer ATX supplies are switched on by
grounding the green wire of the supply. It is grounded by the motherboard after the motherboard
receives a momentary pulse from the power switch. AT supplies had a mechanical switch that
made/broke the mains power connection to the supply. This was used for some of the earlier
Intel computers. It's no longer used.
SWITCH MODE POWER SUPPLY
A typical power supply serves the following main functions:
• Changing the form of electric power. For example, electricity from the grid is transmitted
in the form of AC, while electronic circuits need low-level DC;
• Regulation. The nominal mains voltage varies worldwide from 100 to 240VAC and is
usually poorly regulated, while the circuits normally require well stabilized fixed
voltages;
• Safety isolation. In most applications the outputs have to be isolated from the input.
Practically every piece of electronic equipment needs some form of power conversion. Power
supply unit (PSU), technically speaking, is a device that transfers electric energy from a source

8. to a load and in the process changes its characteristics to meet specific requirements. Of course,
this term is not the most adequate. A PSU does not really supply power, it only converts it. Its
typical application is to convert a utility's AC into required regulated DC rail(s). Depending on
the mode of operation of the semiconductors, the converters can be linear or switching.
SMPS stands for switch mode PSU. In such a device, power handling electronic components are
continuously switching "on" and "off" with high frequency in order to provide the transfer of
electric energy via energy storage components (inductors and capacitors). By varying duty cycle,
frequency or a relative phase of these transitions an average value of output voltage or current is
controlled. The operating frequency range of commercial SMPS units varies typically from 50
kHz to several MHz. Below is a conceptual circuit diagram of a typical off-line SMPS.
AC power first passes through fuses and a line filter. Then it is rectified by a full-wave bridge
rectifier. The rectified voltage is next applied to the power factor correction (PFC) pre-regulator
followed by the downstream DC-DC converter(s).
Note that except for some industries, such as PCs and Compact PCI, PSU output connectors and
pinouts in general are not standardized and are left up to the manufacturers. F1 and F2 shown on
the left of the circuit diagram are fuses. Everybody knows about them, but some people are under
impression that a fuse blows immediately once applied current exceeds its rating.
Uninterrupted Power Supply (UPS)
If you're using a computer and there is a power outage, physical damage can occur as well as
losing your data. Many users have surge protectors that guard against a sudden increase of
electricity (above the standard 120 volts) that could possibly harm components, but they do not

9. prevent data loss. Moreover, depending on how strong the surge, even a regular surge protector
may not protect from extensive physical damage.
An uninterruptible power supply (UPS) is an external box containing a battery that provides
power to a computer and other electronics in case there is a loss of power. There are three basic
design types, each offering more power protection than the proceeding.
• OFF-LINE (UPS), the lowest grade
• LINE-INTERACTIVE (UPS), the middle grade
• ON-LINE (UPS), the highest grade
The Off-line UPS (or) Standby UPS
The off-line UPS offers the bare bones power protection of basic surge protection and battery
backup. Through this type of UPS the equipment is connected directly to incoming utility power
with the same voltage transient clamping devices used in a common surge protected plug strip
connected across the power line. When the incoming utility voltage falls below a predetermined
level the UPS turns on its internal DC-AC inverter circuitry, which is powered from an internal
storage battery. The UPS then mechanically switches the connected equipment on to its DC-AC
inverter output. The switch over time is stated by most manufacturers as being less than 4
milliseconds, but typically can be as long as 25 milliseconds depending on the amount of time it
takes the UPS to detect the lost utility voltage.

10. When selecting this type of UPS, most off-line UPS products on the market today only provide a
sinewave output to the equipment when operating normally from the utility line. When they
switch to their internal DC-AC inverter they may only provide a square wave, modified square
wave or quasi-sinewave, not a pure sinewave. In many cases the equipment may appear to
operate normally on these waveforms, but over time may be damaged by them. If only minimal
protection is needed, it is always best to select an off-line UPS that states it has an inverter with a
true sinewave output. Most off-line UPS units will not be capable of accepting additional battery
packs for extended battery operation. To keep the cost down and prevent overheating, their
inverters are designed to only operate as long as the internal battery capacity allows. For
reference units of all three design types typically provide from 5 to 15 minutes of battery back-up
time when loaded to their full output capacity. Slightly longer backup times can be achieved by
overrating the UPS size.
The Line-Interactive UPS
The line-interactive UPS offers the same bare bones surge protection and battery back-up as the
off-line, except it has the added feature of minimal voltage regulation while the UPS is operating
from the utility source. This UPS design came about due to the off-line UPS’s inability to
provide an acceptable output voltage to the connected equipment during “brown-out” conditions.
A “brown-out” happens when the utility voltage remains excessively low for a sustained period.
Under these conditions the off-line UPS would go to battery operation and if the brown-out was
sustained long enough, the UPS battery would become fully discharged, turn the power off to the
connected equipment and not be able to be turned back on until the utility voltage returned to

11. normal. To prevent this from happening a voltage regulating transformer was added, hence the
term line-interactive was born. This feature really does help as low voltage utility conditions are
common.
Again when selecting a line-interactive UPS, it is always best to select a model with a true
sinewave output. Several manufacturers have models available that will accept extended battery
packs to provide additional battery runtime. This type of UPS typically costs more than the off-
line type, but is worth the additional cost.
The On-line UPS
The on-line UPS provides the highest level of power protection for the serious home office user.
It does typically cost more, but like all electronic equipment today the cost is coming down as
the technology advances. The true advantage to the on-line UPS is its ability to provide an
electrical firewall between the incoming utility power and your sensitive electronic equipment.
While the off-line and line-interactive designs leave your equipment connected directly to the
utility power with minimal surge protection, the on-line UPS provides an electronic layer of
insulation from power quality problems. This is accomplished inside the UPS in several tiers of
circuits.

12. First the incoming AC utility voltage is passed through surge protected rectifier stage where it is
converter to a Direct Current (DC) and is heavily filtered by large capacitors. This tier removes
line noise, high voltage transients, harmonic distortion and all 50/60 Hertz frequency related
problems. The capacitors also act as an energy storage reservoir giving the UPS the ability to
“ride-through” momentary power interruptions. The battery is also connected to this tier and
takes over as the energy source in the event of a utility loss. This makes the transition between
utility and battery power seamless, without an interruption.
Testing and Troubleshooting of SMPS
SMPS repair sometimes can be easy and sometimes are quite difficult to solve the problem. The
power supply fault could be an open start up resistor only or could be even more than ten
components burnt due to heavy lightning strike. Sometimes no power, power blink and low
output power symptoms are due to problems in the main circuit board. It is not necessary the
fault must be in the primary power section and shorted secondary output diodes.
The fault can be further down the secondary output lines which are in the main board. If you had
measured all the components in the power supply section and could not find the defective
component, then try troubleshooting components beyond the power supply section. A shorted
small ceramic capacitor in the main board can cause the whole power supply to malfunction.