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Course Notes:

Power Supply

(Note the difference in handout voltages shown – USA=110v Aust=220 200/240V

Why is Power important

Power to the computer is like petrol to a car – without it. It doesn’t go

Main functions of power supply

• Conversion of mains power (240v AC) to a stable DC power that the computer uses
• More than one voltage
• +5v used by most cards on the computer
• +12v mainly used to power the motors in the computer i.e. fan, drive motors
• -5v and –12v used on old machines
• Older processors used +5v
• Newer processors use +3.3v to 2.8v – the motherboard does the stepping down of the voltage on the motherboard with the AT power supplies – the ATX power supply has a 3.3v lead.

Power Good Signal (PGS)

• The computer needs to get up to speed (1/2 second) and for the power to stabilize – the power good signal is sent out to the motherboard when the power is ready.
• When there is a malfunction the PGS turns off the computer and shuts it down. When the power is restored the PGS signals the mother to reconnect to normal
• When there is a brownout (low voltage the PGS signal goes low shutting off the motherboard.

Power On and 5v Standby Signal

ATX power supplies as well as having a direct 3.3v lead to supply the CPU have a power on and 5v standby signal lead.

• Allows software to control the ‘power on’ signal to the motherboard.
• The 5v standby lead is a special 5v source to the motherboard that remains on even when the rest of the system is powered down. This can be used to allow a software restart of the computer.

Power Supply Loadings

Some power supplies are designed to have some sort of loading on the +12v leads – This is usually some sort of drive (eg. Hard drive) Some power supplies won’t work without the lead connected. With others, damage can occur to power supply if they are switched on without this load on.

Parts of the power supply system

Power cords

Single cord to the power unit – older computers have a ‘pass through’ connector that was used by the monitor.

Switch

On the older machines the on/off switch was on the actual power supply box. On later machines (AT form factor ) this was moved to the front of the computer and was a mechanical connections or the 240v power leads were extended to a switch on the case. ATX form factor power supplies use an electronic switch, which connects to the motherboard and acts like a reset switch. When the button is pressed it signals the motherboard (requests) the on/off action that in turn either turns the power supply on or off.

Drive Connectors

The following are pin numbering for the two types of connectors

Disk Drive Power Connector Pinout:

Motherboard Connectors

Typical PC/XT and AT Power Supply Connections:

ATX Power Supply Connections:

POWER SUPPLY FORM FACTORS

Currently in the industry there are eight power supply form factors. Each of these form factors can have various amounts of configurations and power output levels.

The form factor is the shape and general physical layout of a component.   When systems are designed the designers can pick an existing power supply form or create a custom supply.  The form factor of a system is based on what type of case is used.  There are six popular cases and power supply types:

• PC/XT Style
• AT/Desk Style
• AT/Tower Style
• Baby AT Style
• Slim Style
• ATX Style

Each of these supplies are available in numerous different configurations and power output levels.

PC/XT Style

When IBM introduced the XT, it used the same basic power supply shape as the original PC, except that the new XT supply had more than double the power output capability.   Because they were identical in both external appearance and the type of connectors used, you could easily install the better XT supply as an upgrade for a PC system.  psxt.gif

AT/Desk Style

When IBM later introduced the AT desktop system, it created a larger power supply that had a form factor different from the original PC/XT.  This system was rapidly cloned as well, and to this day still represents the basis for most IBM compatible designs.   The power supply used in these systems is called the AT/Desktop style power supply.   psatdesktop.gif

AT/Tower Style

This form factor is identical to the AT/Desktop except for the power switch location.   Most AT/Desktop systems required that the power switch be located right on the power supply itself, while most AT/Tower systems use an external switch attached to the power supply through a short 4-wire cable.  psattower.gif

Baby-AT Style

This form factor is also based on the full AT style except that it is shortened on one dimension.  This supply can be used in the larger AT style chassis, however, the full six AT/Tower power supply will not fit into the Baby-AT chassis.  psbabyat.gif

Slim Style

This type of form factor was designed to meet to needs of the slim line computers that was being made.  The slimline motherboard contained a riser card for all of the expansion slots that makes the system very low in height.  Some full sized AT Desktop and Tower cases today are designed to also accept the slimline form factor power supply.  psslimline.gif

ATX Style

This form factor is one of the newest on the market.  It resembles the slimline but has several major differences.  One difference is the fan location and the reverse air flow.  The fan is now mounted along the inner side of the supply and blows air across the motherboard and drawing it in from the outside at the rear.   This will also keep the computer much cleaner since air, dust, and dirt are not trying to be sucked in every hole in the case and drives.  Also, if the computer operates in harsh environments a simple air filter could be applied to the cooling fan to keep out more dirt.  Another new feature is the power connector that connects the power supply to the motherboard.  One of the major drawbacks to the AT style is the two connectors that connect the power supply to the motherboard.  These connectors could be swapped with each other and would destroy the motherboard.  The new ATX power supply has one 20 pin connector that is keyed and is virtually impossible to connect wrong to the motherboard.  And finally, there are three more signals added to the connector that connects the power supply and the motherboard.  These new signals are a 3.3 V supply which eliminates regulators on the motherboard, Power_On signal which can shut down the computer through software, and 5 V_Standby signal which is always active, giving the motherboard a limited source of power even when off.  psatx.gif

Fans

Cooling fans are used on all power supplies as well as extra fans now for both the cases and CPUs.

Normally on a AT power supply the fan blows outward and on ATX the fan blows inward

Power rating

Power supply ratings are usually supplied by the manufacturers in a technical reference manual or most of the time a sticker mounted on top or side of the power supply.  If the total wattage is not listed then you can calculate what it is by this formula:

Wattage = Voltage x Amperage

By multiplying the voltage by the amperage available at each output then adding them up, you can calculate the total capable output wattage of the supply.   The following table shows some data from various manufacturers that advertise the total wattage ratings but when you calculate them out and add them up it can be quite different especially in higher wattage ratings.

When selecting a power supply a minimum of 200W should be obtained.   Generally a 250W power supply will give the user enough extra power to allow for new hardware to be installed into the computer.

Typical Faults that Occur with Faulty Power Supplies

1. No lights on computer
2. Overheating
3. Nothing works
4. Fan not operating
5. Memory problems
6. Disk drive wont start
7. Computer freezes
8. Intermittent resets
9. Computer fails to boot
10. Smoke or burning smell
11. Electric shocks on touching
12. Weird intermittent faults
13. Erratic read/write hard drive faults
14. CMOS set-up lost
15. Chips damaged
16. Lost data on hard drive
17. Peripheral errors

Testing Power Supplies DC Voltages

Set the multimeter to the require DC voltage range (e.g. 0-20).

Connect one of the test probes of the multimeter to ground (earth) and the other to the voltage lead.

The readings should be within the following tolerance readings

• 5 volt – (+ or -) 5% = 4.75 to 5.25
• 12 volt – (+ or -) 5% = 11.4 to 12.6
• Power Good signal = 3 – 6 volts

*Note remember to load power supplies

Protection of Input Power

Surge Protectors – work like a fuse. They take the damage not the computer. There are two types, ones that can be reset and ones that once they blow must be discarded.

Power conditioners – include surge protection in their construction but are mainly used for line conditioning in that they filter out line noise and provide a steadier voltage.

UPS (Uninterruptible Power Supply) – these provide continuous no-break power supply to a computer. They rely on a battery backup to provide the power source while the mains power is off. The size of the battery usually determines the amount of time these units will provide power. Usually most range from 10 to 30 minutes.

SPS (Standby Power Supply) – These power supplies switch over to another source of power after a power break. There is usually some sort of break to the power supplied to computers while this system switches in.

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