Computer Power Supplies

All computers need a source of electrical power. Our portable computers rely on battery power when an electrical outlet is not handy. Our desktop machines use a power supply, which really doesn’t supply the power, but converts the power the electric company delivers to our homes and businesses into the form our computers use. The electricity delivered to most of our homes in the United States is alternating current (AC) at a cycle rate of 60Hz, and at 240 volts. Except for appliances that have heavy power requirements like air conditioners, electric stoves, etc., this is split into separate circuits of 120 volts each. Most other countries use between 220 to 240 volts at 50Hz.

Our computer power supplies change the AC we get at the wall outlet to the 12 volt and 5 volt direct current (DC) that our computers run on. They are a type of supply known as a switching power supply, which is relatively inexpensive to make and very efficient in the amount of power available in a fairly small, light weight unit.

Our power supplies are rated in watts of power capacity. The early PC’s only had 65 watt supplies, but we have come a long way from those days. Today we have machines with CPU’s that contain tens of millions of transistors, video processors that have huge current needs Floppy, hard, and CD drives filling every available space, and enough memory to give an elephant an inferiority complex. To top it off we need a bunch of fans to keep this whole mess from melting down. It all adds up to a lot of watts! Even a modest computer these days should have at least a 250 watt supply. A fairly loaded, high speed machine will choke on less than a 300 to 400 watt supply. A switching power supply will deliver only as much power as the load requires. A 400 watt supply in a computer that only requires 250 watts will run at 250 watts. This means your electric bills will not increase because you installed a more powerful supply, and the supply will run cooler and last longer because it is operating well below its design level.

There is a big difference in quality and price in power supplies. A top of the line supply can cost five times as much as a supply of the same wattage in a generic brand. Is it worth the price difference? Yes, that expensive supply will be made of better quality components, run quieter, and cooler, and have more precisely regulated output voltages. Do you need a top of the line supply? Probably not, as long as the one you buy is decent quality, and approved for the CPU you intend to use, but buy the best you can justify and you will not be likely to regret it. Remember, a failing power supply can take a lot of the equipment you’ve invested in with it.

There is more than one type of computer power supply. The original AT supply connects to an AT motherboard with two six pin connector plugs, and has a two position power switch with an off or on position. The newer ATX style power supply connects to the ATX motherboard with a twenty pin plug, and usually has a momentary push button type switch.

The ATX supply is designed to always deliver a voltage to the motherboard, even when the computer is turned off. It is therefore important to disconnect an ATX machine from the wall outlet whenever any work is done inside the case to prevent damage to internal components.

Calculating how big a power supply you actually need is a bit messy since you have to know just how much power each component in your computer needs.

Here are some typical values for common components:

Motherboard without CPU or RAM 30 Watts
Floppy Drive 5 Watts
IDE Drive 5400RPM 10 Watts
IDE Drive 7200RPM 15 Watts
SCSI drive 7200RPM 25 Watts
SCSI drive 10000RPM 40 Watts
128MB SDRAM 10 Watts
IDE 50X CD or 10X DVD Drive 20 Watts
High Performance AGP Card 30 Watts
Network Card 10/100 5 Watts
Pentium III 700MHz 25 Watts
Athlon 600MHz 45 Watts

To determine the size of the power supply you need, add up the total power demand of your computer, multiply that total wattage by 1.5 so that you do not exceed 70% of the supply’s capacity.


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