How A Computer Works
Part 1 – The Power Supply




It all starts with electricity. Electricity is made by spinning a magnet inside a coil of wire which directs free electrons to flow through that wire, making them available to do productive work with specially designed electronic devices.

This happens because the magnet not only draws steel and iron towards it, but it also draw small atomic particles we call “electrons.”

You can do this process of generating electricity yourself if you are interested in a home science experiment. Go get a bar magnet, attach it to a spindle. Wrap some coils of wire around it. Put a crank on the spindle. Attach a small flash light bulb to the wire ends out of the coil. Turn the crank and the light bulb should begin to glow.

If you’re a bike rider you’ve probably seen the “generator” head lights. The generator attaches to the wheel which makes it spin and produce enough electricity to power the headlights. The fan belt in your car turns a generator or alternator which makes enough electricity to recharge your car battery which gets used when you start the engine.

A TV set and a computer monitor that uses a picture tube (as opposed to the new LCD screens and plasma tube sets that will eventually replace the picture tube, which is becoming obsolete) works because a bean of electronics is made from the back of the set towards a phosphorous screen. This beam of electrons is controlled by magnets and magnetic coils which alter the direction of that stream of electrons.

Electricity is made by directing a flow of electrons not across empty space in a picture tube, but along highly conductive metal wire. The amount of electrons flowing through the wire (also known as electrical current) determines the power or jolt of electricity. Your little hand-crack generator can’t make more than a few volts (v) of current at a few milliamps (ma). Enough to make a small flash light bulb glow.

Computers need anywhere from 3 to 20 volts of electricity with closer to an amp of current. Computers also need direct current (DC) not alternating current (AC) that comes from the wall.

Alternating current was adopted in the 19th century long before modern electronics. Were electricity to be re-invented today, most homes would probably get no more than 25 volts DC at an amp (a) of current with a little AC supplied for things like refrigerators and air conditioners (we may be able to drop the massive electrical amounts required for these devices down in the coming years). Currently our homes get 110-125 volts (v) in the U.S. and 220-250 volts (v) in Europe. This current is not direct current (DC) which always moves in a single direction, but is alternating current (AC) that is generated in a push-pull or phase shifted manner from positive to negative at a fixed frequency of cycles per second (CPS), rated in Hertz (named after a famous scientist and abbreviated Hz). In the US this rate is 60 Hz in Europe it is 50 Hz.

Since US and European electricity is not interchangeable a computer – indeed most electronics that plugs into a wall – must be designed for the location where it will see use. You can’t take even an American laptop to England, without first obtaining a recharger that works on their current. But, when all is said and done, all of the inside components run on the same amount and intensity of DC (direct current) power! To provide this a converter is necessary and it is called a power supply. This is one of the primary components in the computer and it needs to be replaced every year or two as it gets a lot of wear and tear changing AC wall power into a lesser amount of DC power for the small electronics. It also generates a lot of heat in the process of converting AC wall power into DC computer power.

When electricity flows through a wire it generates friction based upon the speed, intensity and also the size of the wire. This is why you need big thick wires to jump start a car or run a refrigerator and a little skinny wire for a clock or table lamp. The amount of electricity flowing through a wire also explains why using a little skinny extension cord to run a refrigerator or air conditioner makes that cord get warm and even causes electrical fires to occur. The same thing happens inside the computer power supply, which is why they put a fan in there. That fan cools down components called rectifiers which filter the AC current and help to make it DC.





The steps of changing AC power (top) into DC power (bottom) by filtering out half the cycle and then storing it in a capacitor to help fill in the gaps taken out between cycles...


A rectifier passes electrons only in one direction. But there is a gap in AC current using this method so a temporary storage bin or reservoir (or cache or buffer) is used to hold an electric charge and release it at a constant level (like a dam in a river controls flooding). This is called a capacitor. It holds a charge on one side up to a certain point then releases it. This capacitor is designed to release power at a lower current flow (amperage) going out then coming in (several amps can come in but less go out and at lower voltages). The process of rectifying (turning AC into DC), building up a reserve and controlling the flow (capacitance) generates a lot of heat, which the fan removes. Should the fan stop working these components will eventually melt down and stop working.



Top shows how a diode or rectifier filters out only one half of the sine wave that comes out of the wall at 120 v and how the capacitor stores this power on one side until it reach capacity and then the power jumps across the plates and goes out the other side in a steady stream. The lower picture shows how a transformer, which is nothing but two coils of wire, reduces (or increases, depending on the direction of application) the 120 v DC power to any other volatage because of the difference in the number of coil wires on either side. It is the fact that electrons have to flow through more or less wire that transforms the levels of power or electron flow.


In a lap top computer a rechargeable battery is used to store rectified power. This battery acts a little like a capacitor, except discharge is far slower and storage is far greater. A capacitor inside your computer power supply (or the recharging unit rectifier) is a simple piece of metal that is capable of holding a small charge and another piece of metal separated from it that accepts and passes overflow of electricity to another piece of metal a small distance away.

Batteries are metallic devices that have an overflow of electrons on one surface (type of metal that can hold a charge) and another surface some distance away that attracts the overage of electrons.

A capacitor only holds an electronic charge for a short while (under an hour) while a rechargeable batter holds a charge for a month or more.

There are, however, fundamental similarities between the concept of a capacitor as used in rectifying AC current into DC current and a rechargeable battery function in supplying current to an electronic system.

Some power supplies work for both US and European current by the flick of a switch, allowing the actual “box” to be used in either location. The wall current powered electronic monitor, however, is not universally compatible and you must obtain a special monitor designed for either U.S. or foreign current.

Take note that some power supplies have a power socket that allows you to connect the monitor or some other similar device. The power supply is not rectifying or altering the power for this socket, but is only tapping into the same current that comes out of the wall and making it conveniently available for a monitor. In a situation where you have a power supply with a universal current switch this will not automatically provide the proper power to the monitor. It will only provide what comes out of the wall to any device connected. If the monitor was designed for 117 volts AC but you go to Europe where there is 240 volt AC this will burn your monitor up and out! You must leave the 117 volt monitor at home and buy one designed for 240 in Europe.

Without power the rest of the computer will not function or do any work for the end user...




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