How Computers Work – Course Information



EMMA HS1 Outline Week #2

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 – What are amps, watts, volts, and ohms?

Description of Advanced Power Management

How PC Power Supplies Work

Homework - 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.

|[pic] |

|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. |

|[pic] |

|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.

| |

|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. |

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.

| |

|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.

|[pic] |

|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 APM-compliant 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.

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