Hardware

Build Your Skills: How electricity powers your systems

Understand the basics of how electricity works in computer systems, including how it is converted and the effects of surges and brownouts.


If you’re mystified by electricity, then you're not alone. Many IT professionals today know little more about electricity beyond what it takes to properly plug a computer into the wall socket. However, to successfully keep your systems running, you need a basic grasp of a few electrical concepts. Here’s what you need to know—from what happens inside the computer to the factors that impact its reliability.

Converting power
Electronics don't directly run off power coming out of the wall. The power coming out of the wall socket must first be converted from alternating current (AC) into direct current (DC). AC reverses its direction at a fixed interval. This makes power distribution from the generators much easier. However, it also makes AC incompatible with electronic circuitry that expects the same current flowing from the same direction at all times.

The power flowing out of the wall must undergo another change before the computer can use it: The voltage and amperage must be changed into something more palatable for the digital circuitry. Voltage and amperage are related to each other. The product of voltage and amperage can tell you the total energy involved.

One way to understand these electrical terms is to think of plumbing. Wire is like the pipes bringing water into your home. Water pressure is like voltage, with high voltage and high pressure providing more forceful streams. And amperage—the rate at which electrons flow through the wire—can be likened to the rate of water flow. A common measure of the rate of water flow is gallons per minute, while the flow of electrons is given in amperes (amps).

The electricity coming out of your wall is relatively high in voltage: 120 Volts (V). "High," is of course a relative term. Although 120 volts may not kill you, it can wipe out your computer if it's delivered raw. The power coming out of the wall can also be relatively high in amperage. Most computers, however, use only a very small amount of amperage. A typical computer uses less than 2.0 amps.

In contrast to the voltage coming out of the sockets on the wall, internally a computer uses only 12V of electricity or less. In fact, the 12V feed is used only for the motors on hard drives. Most of the other add-in components are driven by a 5V feed or a 3.3V feed. The processor may be running on 2.8V—or less. However, these components are running at several amps of current.

The power supply in the computer is designed to take the electricity coming out of the wall and convert it into something that the computer can use. In principle this is a simple task, but in practice, it's rarely simple. The problem is that the incoming electricity is in a constant state of flux. It’s filled with power problems such as spikes, surges, sags, brown outs, and complete outages. Digital electronics, in contrast, must have a very clean and undisrupted power flow. If the flow of electricity to digital electronics is disrupted even slightly, the results are usually a locked up computer or one that reboots at random.

Power problems
The power problems in the electricity coming from your wall socket can be caused by a variety of factors both internal and external to your home. We’ve all seen the lights dim briefly as the dryer is started or as the air conditioner fires up. We’ve also all seen the lights flicker in a thunderstorm. So how are these power problems created?

The way that power lines work causes them to react in some less-than-perfect ways. If you're consuming only a few amps of current in your home and suddenly a large appliance such as a dryer or an air conditioner turns on, there is an increase in the demand, and a momentary dip and surge of power occurs. It’s much like the ripple of a rock dropped into a pond. Initially the voltage is pulled down because there is not enough power flowing through the line. Next, the current comes rushing in to fill the void—at a higher rate than is needed, just like water rushes in after the rock. The result is a set of increasingly smaller dips and surges of power after a large drain starts.

External factors, such as your neighbors creating and stopping large demands on the electrical network, can also cause sags and surges. Occasionally, the squirrel that barbeques itself on a transformer or the near miss by a lightning bolt can cause havoc.

Although they stem from a variety of causes, power problems break down into two major categories. The first category is not enough power, and the second is too much power.

Not enough power
In the category of not enough power you have sags, brownouts, and outages. Power supplies generally handle sags and brownouts relatively well. Because there is more than enough amperage flowing through the line, the power supply accepts the lower voltage and increases the amperage as needed. In addition, power supplies typically use capacitors to “ride through” brief interruptions in power. In this case, the power supply continues to provide power to the computer even when there is little or no power coming into it from the outlet.

This ride-through is useful to allow other devices, such as an uninterruptible power supply (UPS), to engage, but it isn't typically enough to carry the load for any substantial length of time. The ride-through in most power supplies is substantially less than one second.

The design of the power supply itself will determine how low a voltage it will tolerate and the overall length of time for a ride-through. Generally, high-end power supplies will support voltages as low as 90V. However, when lower voltages are being used to drive the computer, the ability of the computer to survive a brief interruption is reduced.

If there’s a power outage for more than a few tenths of a second, it's necessary to have an external device to support the power needs. Typically, this backup power source is a UPS.

Too much power
The second category of problem is too much power. While these problems are typically not as urgent as having little power, they're potentially more dangerous because too much power can cause long-term damage. Spikes, surges, and overvoltages are the culprits in this category. An external surge suppressor best handles spikes and surges, which are both characterized by extremely high voltages for a relatively short duration. A spike is typically very short and may last less than one full cycle. A surge may last only a few cycles of power. Given that power is cycling at 60 times per second in the United States and 50 times per second in most of the rest of the world, a few cycles is a very short period of time.

Surge suppressors work by means of a device called a metal-oxide varistor (MOV). MOVs are designed to have an extremely high resistance at low voltages, and an extremely low resistance at higher voltages. When a surge or spike exceeds the threshold of the MOV, power begins to flow across the MOV. Typically, the MOV diverts the excess power back to the neutral line or the ground line, or both. This helps to minimize the amount of energy that is received by the power supply and can substantially lengthen its lifespan.

The problem with MOVs is that they are damaged by the energy they divert. The more spikes and surges absorbed, the less effective they become. Eventually they become completely ineffective at diverting energy. This is why surge suppressors have lights to indicate if they are still functional or not.

One of the little known facts about surge suppressors is that they are parallel devices, so they protect more than the equipment directly plugged into them. A computer plugged into the same duplex outlet as a surge suppressor has effectively the same protection as one that's plugged into the surge suppressor itself.

Surge suppressors can handle spikes and surges, but not overvoltage conditions. A surge suppressor might clamp down a spike or surge at 330V, but an overvoltage condition can occur at 125V or less. Overvoltage conditions occur when there is a sustained excess of voltage on the input, as opposed to the short-duration high voltage events that make up spikes and surges.

In addition to handling undervoltage or brown-out conditions, most UPS units can also handle overvoltage conditions. Essentially, when the voltage exceeds a reasonable range of values, the UPS has to disconnect the utility power to prevent damage.

Although there are two main power-problem categories, one unusual type of problem occasionally occurs when the cycling of the power falls out of range. It's such a rare condition with utility power that it’s hardly even considered. Out-of-range cycling is an odd condition, since the power may appear fine by observing the behavior of lights and electric motors, but it may be unsuitable for the power supply in a computer to convert. UPS units are also tasked with solving this power problem: They detect that the frequency is out of range and switch to battery power.

Circuit breakers
One of the problems with power is that if you attempt to move too much of it through too small a wire, the wire will heat up. In fact, toasters work by running a large amount of power through relatively small wires to heat them up. A hair dryer works on the same principle. Wires are heated, and then a fan blows air across them.

Having a wire heat up is obviously a problem when the wire is in close proximity to something flammable. Around Christmas time each year, fire marshals throughout the country start a public campaign to prevent people from plugging too many Christmas light strands into each other. It is possible to overload the light strand and cause the wires to overheat and catch something on fire.

The wires used between the power distribution panel and the outlets in a business setting are specifically sized to prevent excessive heating from occurring. To prevent heating, only a certain amount of power should be pulled through the circuit. The wires in a business setting are sized to allow substantially more power to flow through them than most devices should take; however, it's possible to pull more through them than safe when multiple devices are connected.

To prevent too much power from flowing through a circuit, a circuit breaker must be used. A circuit breaker is connected between each circuit and the power distribution point in a home or business. Every circuit is then protected from having too much power pulled through. Circuit breakers are rated in amps. Their rating is the amount of power they will allow to flow through the circuit before disconnecting it. Once that power is reached, they disconnect the circuit until they are reset.

Surge suppressors typically have barrel-type circuit breakers located on one end. When the circuit breaker is tripped, the barrel sticks out and must be pressed in to reset it and allow power to flow again. Distribution panels have throw-type circuit breakers that fail to a middle position. These can be difficult to identify because the amount of movement from the on position to the middle position may be very small.

Although circuit breakers are designed primarily to prevent wires from overheating and causing a fire hazard, they're also useful in disconnecting direct shorts. These occur when power is transferred directly through the circuit without any intervening device. In most cases, a direct short is caused by a wiring problem, but might also be caused by water flowing over the circuit. By preventing direct shorts, circuit breakers can prevent personal injury or death by stopping a person from accidentally becoming a part of the circuit.

In computers, circuit breakers are an important power consideration. If you overload a circuit, a circuit breaker may engage and disconnect the power from your devices. But you must remember that circuit breakers are not fool proof! It's possible that you’ll exceed the maximum rating for the circuit breaker and it won’t disconnect the circuit. It's not safe to depend solely upon the circuit breaker to ensure that you don’t overload a circuit.

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