This document provides an overview of computer maintenance and power supplies. It defines key electrical concepts like voltage, current, resistance, and Ohm's Law. It explains that a computer power supply converts high-voltage alternating current from a wall outlet to lower-voltage direct current needed by computer components. It stresses selecting a power supply with enough wattage for all components and warns to never open a power supply due to dangerous capacitors that can hold a charge.

1. Computer Maintenance I
Getting Started

2. What is Computer Maintenance?
Computer Maintenance is basically the same
as any other type of maintenance; it is a set of
services or procedures performed to keep
something (in this case, a computer) in good
working condition. A well-maintained
computer will run faster, more efficiently and
crash less than one that has not been
maintained.

3. Electricity and Ohm’s Law
 Ohm's law can be very difficult to understand
by anyone who has never had any basic
understanding or training in basic electricity.
We'll assume that you have some knowledge of
basic electricity. We'll explain it in terms of water
flow.
 Ohm's Law is made from 3 mathematical
equations that show the relationship between
electric voltage, current and resistance.

4.  There are four basic units of electricity:
 Voltage (V): is a measure of the force required to
push electrons through a circuit. Voltage is
measured in volts (V). A computer power supply
usually produces several different voltages.
 Current (I): is a measure of the amount of
electrons going through a circuit. Current is
measured in amperes, or amps (A)computer
power supply deliver different amperages for
each output voltage.

5. Again, everyone uses this term without really
knowing what it means. In very simple terms,
current is the flow rate of the electrons in the
circuit. How is that different from voltage? Let's
use a water tank and a pipe as an example:
In some neighbourhoods you'll see a water tank
raised higher above the ground on strong legs.
The water in this tank has been raised up there
to create pressure in the system.

6. A series of pipes carry the water down from the
tank, under ground, into your house, and then
to each sink, bathtub, and toilet. If all of the
faucets in your house are closed, no water flows
through the pipes. If you open one faucet, some
water flows. If you open all of the faucets, a lot
of water flows. This flow of water is similar to
electrical current.
Current is the flow rate of electrons through the
circuit.

7.  Power (P): is a measure of the pressure
required to push electrons through a circuit,
called voltage, multiplied by the number of
electrons going through that circuit, called
current. The measurement is called watts (W).
Computer power supplies are rated in watts.
 Resistance (R): is the opposition to the flow of
current in a circuit. Resistance is measured in
ohm’s. Lower resistance allows more current in
a circuit. A good fuse will have low resistance or a
measurement of almost 0 ohm's.

8. A basic equation expresses how three of the
terms relate to each other. It states that
voltage is equal to the current multiplied by
the resistance. This is known as Ohm’s Law:
V = IR
In an electrical system, power (P) is equal to
the voltage multiplied by the current:
P = VI
In electrical circuit, increasing the current or
the voltage will result in higher power.

9. As an example of how this work, imagine a
simple circuit that has a 9-Volt light bulb
hooked up to 9-Volt battery. The power
output of the light bulb is 100 Watt. Using the
preceding equation, we can calculate how
much current in amps would be required to
get 100 Watt out of this 9-Volt light bulb.

10. To solve this equation, we know the following
information:
P = 100W
V = 9V
Since P = VI
Therefore:
I = P/V
I = 100/9 = 11.11A

11. Computers normally use power supplies
ranging from 250-Watt to 650-Watt output
capacity. However, some computers may need
850-Watt and higher capacity power supplies.
When building a computer, select a power
supply with sufficient wattage to power all the
components. Each components inside the
computer use certain amount of power.
Obtain the wattage information from
manufacture’s documentation.

12. When deciding on a power supply, make sure
to choose a power supply that has more than
enough power for the current components.
A power supply with a higher wattage rating
has more capacity; therefore, it can handle
more devices.

13. Getting the most from your PC’s Power Supply
Within your home or office, the electricity
available from a wall outlet is 230-240-Volt
alternative current (AC).
Within the PC, the sensitive electronic devices
use a low-voltage (3V, 5V, or 12V) direct current
(DC).
Alternative current is so named because it
changes direction 60 times per second (in other
words, it changes direction at frequency of 60
hertz).

14. Direct current, in contrast flows only in one
direction. The PC power supply converts the
high-voltage alternative current to low-voltage
direct current.
The power supply provides cables that you
can connect to the motherboard and disk
drives within the system unit to provide their
source of power.

15. Depending on your system type, one or more of
the power supply cables will plug into the
motherboard to power the central processing
unit (CPU), random access memory (RAM), and
other chips as well as the cards that reside within
the expansion slots. You will connect other cables
to hard disk, CD-ROM or DVD-ROM drives.
Within each set of power cables are individual
cables that provide current at various voltages
(normally 3V, 5V, or 12V), as well as cables that
provide a ground.

16. Caution:
Within the power supply is a device called a
capacitor, the purpose of which is to hold a
charge. A capacitor can hold a charge long
after the power supply is unplugged. Never
open a power supply.
The electricity shock you may experience from
a capacitor could be fatal.