Lesson #13 - Computer Power Systems

The A+ exam doesn't put much emphasis on the actual power supply inside a computer.  Instead, it asks about the voltage properties of equipment inside a computer.  All you really need to know about the power supply itself is that it supplies 12 volts, 5 volts,  and 3.3 volts DC in both positive and negative polarities, and that it runs on electricity between 110 to 115 volts AC provided by your home or business electrical supply.  Pretty much everything else you see on the A+ exam about power will be about spikes, surges, and protecting your computer.

External Power

There are two types of power that a power supply recognizes.  The first type is external power, which is the 110-115 Alternating Current (AC) power that comes from the electrical sockets.  This is the power before it enters the computer, and is more or less thought to be free of troubles.  This is not the case.  Power surges, blackouts, and lightning strikes are all problems that can damage your computer through the external power systems.  You should know what kind of problems can plaque PC's from the external power systems, and how to avoid them.

Surge Protectors/Suppressors

The primary method of avoiding damage through external power systems is the surge suppressor.  Surge suppressors take the voltage changes that come down a power line and absorb them into a Metal Oxide Varistor.  This shields the computer from damage as long as the varistor is working.

Unfortunately, many people buy cheap surge suppressors thinking they are getting adequate protection from line damage.  The truth is that most cheap surge suppressors are nothing more than fancy extension cords, and provide no real protection.  Here are the characteristics you should be looking for in a surge suppressor;

Clamping Speed - This is the time it takes for the suppressor to stop a surge, and is measured in nanoseconds(ns).  The best surge suppressors have instantaneous clamping speed, or 0ns.  The longer it takes for a suppressor to clamp the voltage, the more of the spike it will let through.

Clamping Voltage - This is the voltage that the spike must pass for the suppressor to begin working.  If a suppressor has a 150 Clamping Voltage  rating, it will act faster than a 200-rated suppressor.   

Energy Absorption - This is the amount of power the surge suppressor can absorb, rated in Joules(J).  Most suppressors will suppress up to about 1200 Joules, but for the test know that the higher the number, the better the suppression.

Line Conditioning - This is the ability of the suppressor to eliminate line noise, measured in decibels.  Many suppressors don't have line conditioning, so be careful to make sure you check the box.  The higher the decibel rating, the better the conditioning the suppressor can perform.

Protection Levels - All surge suppressors are rated on their levels of protection in Watts.  This is the number of watts a suppressor will allow to pass through before restricting power.  They are generally rates at 330, 400, and 500, with the lower number being the better number.

Uninterruptible Power Supplies (UPS)

Where a surge suppressor works to suppress voltage spikes and line noise, a UPS works to avoid blackout and brownout problems by providing a continuous supply of power to the computer.  It does this by working on a battery system that gives a time-limited supply of power to allow back-ups and files to be saved.  Most UPS's also contain some form of surge suppression.

UPS's used to come in two formats, but generally are all in-line power supplies these days.  In-line power supplies continually work from the battery, while the AC circuit continually charges this battery.  The other format is a standby UPS, where it waits for the voltage to drop before switching over to the batteries.  Standby UPS's are uncommon because in-line UPS's have surge suppression and line conditioning built in to them.  (Because you always run off the battery, you never use the AC from the wall outlet.)

UPS's are rated by their back-up time.  Basically this rating states how long you have before the batteries run out if the power stops.  Some allow 5 minutes, which is enough to give time to do file saves and turn the computer off.  Large-scale business UPS's have generators that provide constant power even in the event of a long-term blackout.  The longer the up-time, the more expensive the UPS will be.

Internal Power

Internal power is the power that exists inside your computer.  Computers run on Direct Current, or DC.  Wall sockets provide Alternating Current, or AC.  It is the job of the power supply that is built into your computer to convert the AC current into DC.

The power supply is the silver-colored box with the fan that shows out the back of the computer.  It generates a fair amount of heat, and requires that the fan run full-time in order to cool it's components.  I can tell you first hand what a power supply will do when the fan stops working.  (It's a rather cool flash followed by a burning smell...  I've seen it a few times and once was a little too close when it happened.  If you see the power supply fan stop turning, turn off the computer right away.)

Another fact you should be aware of is that the power supply fan also produces all the air flow through the rest of the computer.  Except for the heat sink and fan on the processor, there is no other method of cooling used.  Therefore if a power supply fan stops working, internal components on the motherboard can overheat and stop functioning.

One major difference that you should be aware of is the difference between AT and ATX power supplies.  AT power supplies pull air out of the computer by blowing air out the fan.  ATX power supplies suck air in.  AT cases are prone to dust problems because they suck in all the outside air through the grill  holes in the side of the case.  ATX pressurizes the inside of the case, but tend to collect dust inside the power supply itself.  Both types require that you use pressurized air to blow them out regularly.

NEVER EVER EVER open up a power supply.  If you've ever been lucky enough to accidentally touch a 115 volt line, you know why.  You can do some real damage to yourself and your computer by playing with a live power supply.  And it also voids your warranty.  If you have to open one up, unplug it from the wall outlet and don't touch any of the large capacitors inside.  They don't hold enough power to kill you, but a couple of them will sting you pretty good.  There is a fuse soldered on the circuit board that you can replace if you notice it's blown, but most times if the fuse is blown it's a sign of more serious problems.

One other suggestion I have from personal experience.  Be careful when dealing with AT cases and their power switches.  Most AT power switches run a cable from the power supply through the computer to the front of the case.  Therefore, if you have to remove the power supply, you have to remove the cable from the switch.  Remember which color wire goes where, as placing the wrong connector in the wrong spot will short out the entire electrical circuit you're plugged into and burn out the power supply, switch, and possibly even mother board.  

Converting Voltages

The power supply is required to convert the electrical power from 110 volts AC to a level the computer can use.  The internal components of a computer use 12 volts, 5 volts, and 3.3 volts, as well as -5volts and -12 volts.  For the exam you should know why there are different voltage levels, and what color of wire corresponds to each level.

+5 Volts - Red - Mostly used by older processors below 100MHz and some peripherals.

+12 Volts - Yellow - Used by disk drives and other motorized equipment, as well as some ISA expansion slots.

-5 and -12 Volts - White and Blue - Used for compatibility with older systems.  Generally the power is provided by the power supply but not used by the computer.

+3.3 Volts - Purple - This power is to run the processor.  All 486 and above processors run at +3.3 volts or below.  Processors using voltage less than 3.3 volts have voltage regulators built on to the motherboard to reduce the voltage to the desired level.

The brown and grey wires in an ATX case are used to signal between the power supply and case when to turn power on and off.  We will explain this in the next section.

Form Factors

There are differences between AT and ATX power supplies beyond the air flow differences we noted earlier.  AT power supplies require a switch to turn power on and off in the computer.  ATX has these switches as well, but also have several more power wires.  These wires carry signals that allow the power supply to be turned on and off from software.

One of the questions you will probably see is about whether to unplug or leave plugged in a system you are working on.  Because an ATX motherboard uses electronic signals to control power, there is always power to the motherboard.  This means you have to unplug it to stop power flow.  AT cases should be plugged in when being worked on because the power cord provides positive ground.

Connectors

There are four different types of power connectors inside a computer.