One main cause of data loss is the electrical power supplied to the PC. If power does not stay reasonably uniform the PC can suffer adverse affects. While it is rare (but certainly not impossible) that the wall would provide such a radical departure from the norm that the PC would be physically damaged, data loss from interruptions and surges is very possible under adverse power fluctuations.

None of the internal components of the PC use wall AC current. They are all small power consumption integrated circuits and motors within the drives that run on ±5VDC or ±12VDC. Therefore the primary design objective of the PC power supply is to convert 120 VAC wall current into 5 and 12 VDC for these components. The power supply of the PC (and any device that plugs into the wall and employs small DC voltages internally) therefore performs the following processes:

	Phase-shifter/Inverter: Provides a 180° phase shifted (or inverted AC waveform)
	Rectifier: Converts AC to DC by removing the opposite polarity half of the wave by chopping it off.
	Voltage reduction: This is done by a step down transformer. It simply divides the incoming wave's voltage by a specific value. So +150V wave heights can be divided by 30 yielding +5 volts which is one the voltages used by the PC. There is a another circuit that provides the +12 and another for the +3.3 and another for the –12V.
	Amplifier/Filter: This is done by a large capacitor or circuit that has a large capacitance. The capacitor charges up during the rise of the voltage wave. On the fall of the voltage wave the capacitor bleeds out voltage to compensate so the circuit provides a steady +5V.

Knowing how the power supply works we can clearly see that if the frequency gets altered (usually by RFI and EMI) the capacitor will provide too much power causing the voltages going to the motherboard and devices to rise causing the components to malfunction. If the wall current waves spike, the voltage reducer will pass this on (albeit divided by 30) and again the wrong voltages are delivered to the components.

Because of this and the fact that wall current itself has an extremely poor standard for what is considered (good service) and the fact that this standard is basically not enforced and it is certainly not good enough to power a computer the ATX specification includes failure thresholds. These thresholds are defined based on the wall current being provided to the power supply:

	80V (RMS) for 2 seconds
	70V (RMS) for 0.5 seconds
	143V (RMS) for 1 second

Obviously there are other wall current supply events that would be considered catastrophic to hardware and software. The ATX failure thresholds simply state that the power supply will fail if these events OR WORSE occur. The worse cases include:

	Spike: Very short duration (fraction of a second), very high voltage (no theoretical maximum voltage limit due to short duration)
	Surge: Short duration (can last over a second), high voltage condition (rises to any voltage above ATX failure threshold, with a theoretical maximum of 6000V at which wall wiring fails, then falls back to normal)
	Sag: Essentially the opposite of a spike, a very short duration, very low voltage transient, very capable of resetting DC-to-AC/AC-to-DC converter circuits which would effectively reboot the PC.
	Brown out: Essentially the opposite of a surge, short duration (can last over a second) low voltage condition (falls below the ATX failure threshold)
	Interruption: Total loss of service for a brief period of time generally between a fraction of a second (power grid switchovers) up to a minute.
	Black out: Total loss of service for an extended period of time generally longer than a few minutes.
	Severe transient: aberrant waves which includes height = high voltage and/or wavelength. These are generally caused by powerful sources of RFI or EMI interference such as large electric generators or motors and can severely affect AC-to-DC/DC-to-AC converter circuitry causing them to reset, effectively rebooting the PC.

Of these AC power source problems, two of them can damage even destroy computer equipment: spikes and surges. If no other precautions are taken, any computer system should be protected from at least these power source threats which are very real and can completely destroy a computer. In the course you have designed PC's that cost well over $20,000 to build, what is another $50 for a high quality surge protector to keep $20,000 worth of PC from going up in smoke? To protect the PC against these powerline events there are three main classes of power line devices that can be employed:

	Surge protectors: These eliminate voltage increases by shunting (diverting) the excess to ground. The component that does this is called an MOV - Metal Oxide Varistor. These have extremely high speed "clamping" or reaction times usually measured in nanoseconds. A surge protector that does not have an MOV should be considered worthless. Note: high quality surge protectors are purosely designed so that if the voltage exceeds the ability of the MOV to shunt it, then it will fry it and break the connection to the rest of the circuitry, still saving all downstream equipment.
	Line conditioners: These devices contain a AC current generator that creates a brand new "clean" AC current waveform. It is usually perfect in wave form so these eliminate wave form transients including single wave height (single wave voltage spikes) and width (RFI and EMI) extremely well but some cannot defend against large voltage spikes or reduced voltage input since they run off of the "dirty" wall AC current. Most UPS'es include a DC/AC conversion circuit which means that while running off of the batteries (only) a UPS is also a line conditioner by definition (a circuit generating a near perfect AC waveform).
	UPS - Uninterruptable Power Supply: Most of these are actually battery backup power supplies. These come in four different major categories: 1) Standby UPS: also called an offline or switching backup power supply. 2) Online UPS: This is the "true" Uninterruptible Power Supply, by definition 3) Hybrid UPS: Mixes inputs from both the batteries and the wall AC effectively providing power like both the switching and the online UPS simultaneously. 4) Ferromagnetic Core UPS: Also a

Battery Backup systems or UPS'es switche to the backup batteries until stable service is detected again. The online UPS has the PC running off of the batteries at all times and simply charges the batteries continuously from the wall AC supply. When power fails the PC continues to run off of the batteries as usual so no switching is necessary. The hybrids come in two main varieties: the dual power in which both the wall AC supply and the battery power are mixed by a circuit and this supplies the PC. When the wall AC service fails, the mixer simply begins drawing more power from the batteries to compensate so again no switching is needed. The other hybrid uses a large ferroresonant transformer in a switching backup power supply. As power fails its magnetic field collapses which is a moving magnetic field. This moving magnetic field is tapped by the design of the transformer to provide more energy. This is enough to bridge the gap while the power supply switches to battery power. Both hybrids are expensive but very effective. Most "uninterruptible" power supplies sold these days are switching or "off-line" types. This design is the least expensive and most commonly found in the low end systems intended for home use. The dual supply hybrids (which are better than true online systems because they are not so hard on the batteries) are usually much more expensive and not commonly seen in the home user oriented stores. True online systems are extremely expensive and usually special order equipment usually marketed by business-to-business types of computer supply companies.

Switching power supplies are usually more than acceptable for home use and usually include surge protection (MOV circuitry) built in. To test the device set it up and then pull its plug out of the wall while the PC is on (but not copying a file!). The PC should stay at the desktop as if nothing happened. Though most "U" PS 'es should sound an alarm at this point. Many UPS'es include drivers and software that will initiate an unattended shutdown at this point as well. This is a good solution and the system should be set up with the unattended shutdown feature enabled and this should be tested as well.

The purpose of a UPS is to give the user time to properly shutdown the PC and prevent data loss during power loss – not to continue working as if nothing were wrong. Most UPS's only provide 5 to 10 minutes of power at best.

Just as dangerous to the PC is the phone line to the modem. This line can carry as large and damaging voltage spikes as the wall AC line. For this reason a surge protecting device for the phone line should be provided as well.

If a line conditioner is used, the surge protector should be in front of it. As a matter of fact, surge protectors are so inexpensive compared to the type of protection that they offer that it is not unreasonable to put one in front of a UPS to protect it too, despite the fact that the UPS often has built in surge protection. If the voltage exceeds what the MOV can handle it will trip a circuit breaker, which will physically disconnect the wall AC power source from the circuitry on the computer's side of the breaker. The whole device could be damaged in the process defeating the UPS purpose of preventing power loss to the PC.

What are the four steps that the power supply does to the wall AC to convert it to DC:

What form of data loss do power line appliances help defend against?

List the ATX failure thresholds:

List the four catastrophic power line events that can cause hardware and software loss:

List the device and its main component that defends against high voltage spikes:

List the device that defends against power line transients or irregularities in the AC waveform.

List the three types of backup power supply and describe how each works:

You want to connect a UPS to a surge protector which is then attached to the wall. Why?

In an office you see that all of the machines have surge protectors but the modems plug straight into the wall jacks. Are they fully protected against damage from high voltage spikes? Why?

In the event of a total power failure a UPS is rated at 1000VW which means that if you know what Wattage the PC draws then you know how long the device will run before the battery dies. You determine that it will function for 10 minutes after power fails. When power fails what should the user do first on the PC? Why?

What type of defense against total power failure would a backup power supply be? Explain.

What type of defense against total power failure should be used for a "mission critical" server that can never go down?