Download How Computers Work – Course Information

EMMA HS1 Outline
Week #7
Review Homework- Computer Cases & ESD
Amps, Watts, Volts, and Ohms
Voltage = Volts
Current = Amps
Resistance= Ohms
Power
= Watts
Plumbing Analogy
Voltage = Water Pressure
Current = Flow Rate
Resistance= Pipe Size
Power
= Water Wheel Work
Power Supplies
Typical Input Voltage
115-120 VAC (230-240 VAC in Europe)
Typical Output Voltages 3.3 VDC, 5 VDC used by digital circuits
12 VDC used for fan and disk drive motors
5 VDC Standby Voltage (tell PSU to turn on)
ATX Power Supply fits inside ATX case, works with ATX Motherboard
APM – Advanced Power Management
Ready
Stand-by
Suspended
Hibernation
Off
Power Supply problems
Power Supply Improvements
Power Supply Wattage
Online
–
Homework -
What are amps, watts, volts, and ohms?
Description of Advanced Power Management
How PC Power Supplies Work
Online Quizzes for Power, Power Supplies, APM
What are amps, watts, volts and ohms?
The three most basic units in electricity are voltage (V), current (I, uppercase "i") and resistance (r). Voltage is measured
in volts, current is measured in amps and resistance is measured in ohms.
A neat analogy to help understand these terms is a system of plumbing pipes. The voltage is equivalent to the water
pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.
There is a basic equation in electrical engineering that states how the three terms relate. It says that the current is equal
to the voltage divided by the resistance.
I = V/r
Let's see how this relation applies to the plumbing system. Let's say you have a tank of pressurized water connected to a
hose that you are using to water the garden.
What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of
the hose. The same is true of an electrical system: Increasing the voltage will make more current flow.
Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also
makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases
the current flow.
Electrical power is measured in watts. In an electrical system power (P) is equal to the voltage multiplied by the current.
P = VI
The water analogy still applies. Take a hose and point it at a waterwheel like the ones that were used to turn grinding
stones in watermills. You can increase the power generated by the waterwheel in two ways. If you increase the pressure
of the water coming out of the hose, it hits the waterwheel with a lot more force and the wheel turns faster, generating
more power. If you increase the flow rate, the waterwheel turns faster because of the weight of the extra water hitting it.
Electrical Efficiency
In an electrical system, increasing either the current or the voltage will result in higher power. Let's say you have a system
with a 6-volt light bulb hooked up to a 6-volt battery. The power output of the light bulb is 100 watts. Using the equation
above, we can calculate how much current in amps would be required to get 100 watts out of this 6-volt bulb.
You know that P = 100 W, and V = 6 V. So you can rearrange the equation to solve
for I and substitute in the numbers.
I = P/V = 100 W / 6 V = 16.66 amps
What would happen if you use a 12-volt battery and a 12-volt light bulb to get 100 watts of power?
100 W / 12 V = 8.33 amps
So this system produces the same power, but with half the current. There is an advantage that comes from using less
current to make the same amount of power. The resistance in electrical wires consumes power, and the power consumed
increases as the current going through the wires increases. You can see how this happens by doing a little rearranging of
the two equations. What you need is an equation for power in terms of resistance and current. Let's rearrange the first
equation:
I = V / R can be restated as V = I R
Now you can substitute the equation for V into the other equation:
P = V I substituting for V we get P = IR I, or P = I2R
What this equation tells you is that the power consumed by the wires increases if the resistance of the wires increases (for
instance, if the wires get smaller or are made of a less conductive material). But it increases dramatically if the current
going through the wires increases. So using a higher voltage to reduce the current can make electrical systems more
efficient. The efficiency of electric motors also improves at higher voltages.
Description of the Different Advanced Power Management States
The Advanced Power Management (APM) Specification defines the following power states:
Ready, Stand-by, Suspended, Hibernation, Off
Three of these states apply both to individual computer components and to your computer as a whole. The
suspended state is a special low power condition that applies to your computer as a whole, and not the
individual components.
Ready
In the ready state, your computer or device is fully powered up and ready for use. The APM definition of Ready
only indicates that your computer or device is fully powered on, it does not differentiate between active and idle
conditions.
Stand-by
Stand-by is an intermediate system-dependent state which attempts to conserve power. Stand-by is entered
when the central processing unit (CPU) is idle and no device activity is known to have occurred within a
specific period of time. Your computer will not return to ready until one of the following events occur:
• A device raises a hardware interrupt
• Any controlled device is accessed
All data and operational parameters are preserved when your computer is in the Stand-by state.
Suspended
The Suspended state is a computer state which is defined to be the lowest level of power consumption available
that preserves operational data and parameters. The suspend state can be initiated by either the system Basic
Input Output System (BIOS) or the software above the BIOS. The system BIOS may place your computer into
the suspended state without notification if it detects a situation which requires an immediate response such as
the battery entering a critically low power state. When your computer is in the Suspended state, computation
will not be performed until normal activity is resumed. Resumption of activity does not occur until signaled by
an external event such as a button press, timer alarm, and so on.
Hibernation
Windows XP has built-in support for hibernation. Hibernation saves the complete state of the computer and
turns off the power. The computer appears to be off. This is the lowest power sleeping state available and is
secure from power outages. When you resume from a hibernated sleep state, the BIOS performs the normal
POST, and then reads the hiberfile that was created to save the computer state. The computer returns to the last
state it was in before the computer entered hibernation mode. Hibernate mode reduces start time.
Note that when you service the computer, make sure you shut down the computer instead of using hibernate
mode.
Off
When in the Off state, your computer or device is powered down and inactive. Data and operational parameters
may or may not be preserved in the Off state.
How PC Power Supplies Work
by Gary Brown
If there is any one component that is absolutely vital to the operation of a computer, it is the power supply.
Without it, a computer is just an inert box full of plastic and metal. The power supply converts the alternating
current (AC) line from your home to the direct current (DC) needed by the personal computer. In this article,
we'll learn how PC power supplies work and what the wattage ratings mean.
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.
This is a power supply removed from its PC case. The small, red switch at
right, above the power-cord connector, is for changing line voltages in various
countries.
The interior of a power supply.
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:



3.3 volts
5 volts
12 volts
The 3.3- and 5-volts are typically used by digital circuits, while the 12-volt is used to run motors in disk drives
and fans. The main specification of a power supply is in watts. A watt is the product of the voltage in volts and
the current in amperes or amps. If you have been around PCs for many years, you probably remember that the
original PCs had large red toggle switches that had a good bit of heft to them. When you turned the PC on or
off, you knew you were doing it. These switches actually controlled the flow of 120 volt power to the power
supply.
Today you turn on the power with a little push button, and you turn off the machine with a menu option. These
capabilities were added to standard power supplies several years ago. The operating system can send a signal to
the power supply to tell it to turn off. The push button sends a 5-volt signal to the power supply to tell it when to
turn on. The power supply also has a circuit that supplies 5 volts, called VSB for "standby voltage" even when it
is officially "off", so that the button will work.
In this photo you can see three small transformers (yellow) in the
center. To the left are two cylindrical capacitors. The large finned
pieces of aluminum are heat sinks. The left heat sink has transistors
attached to it. These are the transistors in charge of doing the
switching -- they provide high-frequency power to the transformers.
Attached to the right heat sink are diodes that rectify AC signals and
turn them into DC signals.
Personal computer power supply label.
VSB is the standby voltage provided to the power
switch. The typical voltages and current provided
by a power supply are shown on the label on a
power supply.
Power Supply Standardization
Over time, there have been at least six different standard power supplies for personal computers. Recently, the
industry has settled on using ATX-based power supplies. ATX is an industry specification that means the power
supply has the physical characteristics to fit a standard ATX case and the electrical characteristics to work with
an ATX motherboard.
PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones.
Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to
easily obtain the 12 volts it needs. Color-coded wires and industry standard connectors make it possible for the
consumer to have many choices for a replacement power supply.
A PC power supply removed from its PC case. Cables and connectors
at right supply DC voltages.
Advanced Power Management
Advanced Power Management (APM) offers a set of five different states that your system can be in. It was
developed by Microsoft and Intel for PC users who wish to conserve power. Each system component, including
the operating system, basic input/output system (BIOS), motherboard and attached devices all need to be APMcompliant to be able to use this feature. Should you wish to disable APM because you suspect it is using up
system resources or causing a conflict, the best way to do this is in the BIOS. That way, the operating system
won't try to reinstall it, which could happen if it were disabled only in the software.
Power Supply Problems
The PC power supply is probably the most failure-prone item in a personal computer. It heats and cools each
time it is used and receives the first in-rush of AC current when the PC is switched on. Typically, a stalled
cooling fan is a predictor of a power supply failure due to subsequent overheated components. All devices in a
PC receive their DC power via the power supply.
A typical failure of a PC power supply is often noticed as a burning smell just before the computer shuts down.
Another problem could be the failure of the vital cooling fan, which allows components in the power supply to
overheat. Failure symptoms include random rebooting or failure in Windows for no apparent reason.
For any problems you suspect to be the fault of the power supply, use the documentation that came with your
computer. If you have ever removed the case from your personal computer to add an adapter card or memory,
you can change a power supply. Make sure you remove the power cord first, since voltages are present even
though your computer is off.
Power Supply Improvements
Recent motherboard and chipset improvements permit the user to monitor the revolutions per minute (RPM) of
the power supply fan via BIOS and a Windows application supplied by the motherboard manufacturer. New
designs offer fan control so that the fan only runs the speed needed, depending on cooling needs.
Recent designs in Web servers include power supplies that offer a spare supply that can be exchanged while the
other power supply is in use. Some new computers, particularly those designed for use as servers, provide
redundant power supplies. This means that there are two or more power supplies in the system, with one
providing power and the other acting as a backup. The backup supply immediately takes over in the event of a
failure by the primary supply. Then, the primary supply can be exchanged while the other power supply is in
use.
Power Supply Wattage
A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger
supply may be needed if you use every available slot on the motherboard or every available drive bay in the
personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices,
since the supply should not be loaded to 100 percent of its capacity.
According to PC Power & Cooling, Inc., some power consumption values (in watts) for common items in a
personal computer are:
PC Item
Watts
Accelerated Graphics Port (AGP) card
20 to 30W
Peripheral Component Interconnect (PCI) card
5W
small computer system interface (SCSI) PCI card
20 to 25W
floppy disk drive
5W
network interface card
4W
50X CD-ROM drive
10 to 25W
RAM
10W per
128M
5200 RPM Integrated Drive Electronics (IDE) hard disk
drive
5 to 11W
7200 RPM IDE hard disk drive
5 to 15W
Motherboard (without CPU or RAM)
20 to 30W
550 MHz Pentium III
30W
733 MHz Pentium III
23.5W
300 MHz Celeron
18W
600 MHz Athlon
45W
Power supplies of the same form factor ("form factor" refers to the actual shape of the motherboard) are
typically differentiated by the wattage they supply and the length of the warranty.