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Macworld Expo was buzzing Wednesday about the new thin notebook computer introduced by Apple at Steve Jobs’ keynote earlier this week.

The MacBook Air, which Jobs billed as the world’s thinnest notebook, includes a 13.3-inch LED display and a full-sized keyboard. It’s scheduled to ship in two weeks, priced starting at $1,799.

“I got excited because of the weight,” said Debra West-Maciaszek, manager of the tech pubs department for Nikon Precision. “It’s only three pounds. If you travel a lot, that’s cool.”

Her husband, Mac Maciaszek, who is retired, was a little less enthusiastic. “It’s a nice machine — don’t get me wrong,” he said. But he was concerned that the lack of a built-in optical drive would make it hard to install applications.

The system will sell with an optional $99 external detachable optical drive or the MacBook Air can borrow an optical drive from another computer. MacBook Air users can use software installed on another Mac or Windows PC to wirelessly connect to and address the remote computer’s optical drive as if it were local to the MacBook Air. Users can then use the remote computer to install software to the MacBook Air — the external software will even let Windows run Mac installer files.

West-Maciaszek was also impressed with Apple’s plans to rent movies over iTunes. “I travel a lot, and I always take my DVDs,” she said. “This will let me leave the DVDs behind.”

And Maciaszek was impressed with the Time Capsule, a wireless networked storage appliance optimized for use with the Time Machine backup software built into Mac OS X Leopard. The Time Capsule is an Airport Extreme Wi-Fi base station with a built-in 500-Gbyte or 1-Tbyte hard disk. “I just bought an Airport Extreme without a hard drive,” Maciaszek said. “I ought to learn to wait for January before I buy anything.”

John Gilmore, president of Gilmore Technology Services, an IT consultancy in San Ramon, Calif., was also impressed with the MacBook Air. “I think it’s a great product. If I had the money, I would buy one,” he said. “It’s gorgeous, it’s very light, and it has the advantage of instant-on.”

Support for 802.11n Wi-Fi will make networking throughput adequate for most jobs, Gilmore said. Earlier Wi-Fi standards were too slow for large amounts of data.

He said the iTunes movie rental announcements and upgrades to Apple TV aren’t useful to him, personally. But he added, “Apple is going to make a lot of money on that.”

Gabe Langhout, general manager of the Dutch Republic, Web designers in Holland, said he likes the light weight for the notebook, but thinks it’s expensive.

“Solid state memory is expensive,” he said. The notebook’s standard 80-Gbyte hard drive is conventional spinning storage, but it comes with an optional 65-Gbyte solid state drive for $999. Even Jobs said the solid state drive is expensive.

“It’s a little pricey, but it’s super-fast,” he said at the keynote.

Russell Holmes, executive vice president for Praxis Integrated Communications, said he was “a little disappointed” by the announcements. “I was expecting something a little better” — a blockbuster announcement, he said.

But, still, he liked what Apple did announce, specifically the MacBook Air. “It’s great. It’s a nice little computer. It’s lightweight and thin,” he said.

However, he said he was concerned that the notebook would require a lot of peripherals and attachments to make it usable.

“They didn’t say what market they were selling the MacBook Air to — is it a personal computer, business computer?” said Ismail Naheed, a technical engineer for Bell Microprocessors in Minneapolis.

He said he also finds movie rentals from iTunes appealing. “I don’t want to rent movies from Netflix and wait for the mail, where I can get it from iTunes and it’s instant,” he said.

The Time Capsule is also impressive, he said, but he’s concerned it will be complicated to configure. At the keynote, Jobs said it would require no configuration at all — just turn it on and it starts backing up — but Naheed was skeptical. If that were true, he said, the Time Capsule would automatically back up data from every Wi-Fi enabled Mac running Leopard within networking range, whether or not the owners of the machines wanted them to back up over Time Capsule.

As was heavily predicted before its unveiling, Apple’s new laptop, called the MacBook Air, is not quite an ultraportable but is still very small.
art.jobs.air.ap.jpg

Steve Jobs shows off the impressively thin MacBook Air.

Mimicking the 13-inch silhouette of the current MacBook line, it’s .76-inch thick at its thickest part. Apple calls it the “world’s thinnest notebook.”

Though the MacBook Air is not quite the thinnest laptop ever, it is among the thinnest we’ve seen (the Fujistu LifeBook Q2010 and the Toshiba Portege R500 both measure 0.8 inch thick, but neither tapers to 0.16 inch as the Air does).

The MacBook Air includes the usual iSight camera, an LED backlit display, an ambient light sensor, and a big touchpad that works with multitouch gestures, such as rotating a photo by twisting your fingers on the touchpad.

As for what’s inside this slim laptop, we’re looking at a 1.6GHz or 1.8GHz Intel Core 2 Duo CPU, custom-made by Intel to fit into the slim chassis, 2GB of RAM, and a choice of either an 80GB standard 1.8-inch hard drive or a 64GB SSD drive (which really should be standard for something so forward-looking).

Moving up to the SSD drive and faster CPU drives the price up from $1,799 to a whopping $3,098.
Don’t Miss

* CNET: User opinions on MacBook Air
* CNET: MacBook Air photos
* CNET: Most popular laptops

Bluetooth and 802.11n were expected, but the lack of an optical drive is a surprise — it’s a smart space and power-saving move we expect to see in more ultraportable laptops. External drives will work, and the Air can connect wirelessly to an optical drive in another nearby computer.

Missing features we’re less happy about having to live without on include any kind of mobile broadband, an SD card slot, FireWire, an onboard Ethernet jack, and Express card slot.

Getting a chance to use a test system, we were extremely pleased with the new multitouch track pad, which incorporates a range of gesture controls that will be familiar to iPhone users. It’s a smart move on Apple’s part; not only are the gestures easy to learn, but they’re difficult to forget.

Writers and students will be pleased as well with the MacBook Air’s keyboard, which is full size and similar to that of the standard MacBook. In terms of interaction, the MacBook Air is probably the first three-pound notebook that hasn’t asked users to make some kind of compromise.

The MacBook Air is available for preorder now and should ship around the end of January.

The prerelease hype was already huge for Apple’s next laptop, and it’s hard to say if anything could really live up to it, but this seems at first glance like a solid addition to the MacBook lineup.

However, we’ll have to keep waiting for a true ultraportable, something that’s been missing from the Apple lineup for several years

Apple’s new MacBook Air may be the thinnest laptop on the market, but it isn’t the lightest.

The Air, which Apple announced at the Macworld conference earlier this week, is a super-thin three-pound laptop. It will be available by the end of the month for $1,799.

But Toshiba, Lenovo, Fujitsu and Sony are just some of the companies already making laptops that weigh less than the 3-pound Air. Toshiba’s Portege R500 starts at 1.72 pounds, while Lenovo’s ThinkPad X61 is 2.7 pounds.

The Air “is really similar to a product that we came out with four years ago,” says Sony Senior Vice President Mike Abary.

But many people aren’t familiar with these pint-size PCs, and the market for them is relatively small, says tech analyst Stephen Baker at researcher NPD. Of the 110.3 million laptops sold worldwide in 2007, only 7.5 million had a screen size of 12 inches or less, researcher IDC says. (The Air’s screen is larger than that, but screen size is the way analysts typically identify light, ultra-portable PCs.)

That’s because ultra-portables are a niche market and a tough sell, Baker says. Reasons include:
FIND MORE STORIES IN: Apple | Sony | Air | Toshiba | IDC | Macworld | Fujitsu | Shim | Stephen Baker | Michelle Kessler | Macbook Air

•Price. The smaller an internal computer component, the more it typically costs. Since a normal laptop weighs 5 or 6 pounds, it takes a lot of expensive specialty parts to build an ultra-portable. One of Sony’s least expensive ultra-portable is $2,100, and its priciest model sells for $3,700. Toshiba’s tiny machines start at around $2,150. In comparison, full-size Dell laptops start at $499.

• Feature compromises. To save space, computer makers cut features. The MacBook Air doesn’t have a CD/DVD drive or an easily swappable battery. Some models of Fujitsu’s Lifebook P1620 come with a battery half as powerful as is standard. The Sony Vaio TZ has an 11.1-inch screen, compared with 15.4 inches on larger Sony models.

•Marketing challenges. Most retailers bolt laptops to their shelves. That prevents theft, but also makes it tough for shoppers to tell how light a laptop really is, Baker says. A picture on a website or in a catalog does a poor job of showing how a small laptop differs from a large one, he says.

Because of these drawbacks, business travelers remain the core market for ultra-portables. More than 73% of small laptops are sold to businesses, says tech analyst Richard Shim at IDC.

“You’re looking at road warriors, people with disposable income and a discriminating eye,” says Craig Marking, senior product marketing manager with Toshiba.

That could change as laptops become smaller and more powerful, Shim says. “You have to balance price with sacrificing performance and features,” he says. Since that’s tough to do today, the consumer market for ultra-portables will remain small for now, Shim says. Even the consumer-friendly Air “is still pretty expensive,” he says.

But ultra-portables can be incredibly appealing, says Michelle Thatcher, a product reviewer for tech website CNet. Awkward keyboards and slow processors — big problems on early models — have improved, she says.

Screen size is still an issue for many ultra portables. For that reason, they often work best as secondary laptops for handling small tasks on the go, Thatcher says. For heavy computing jobs or long typing sessions, a full-size laptop is still best, she says

u can get a much more capable—but larger and heavier—Mac Book Pro.
So who wants one?

Which raises the obvious question: Who, exactly, is this product for? Over the past couple days, I’ve heard quite a bit of criticism of the MacBook Air for its limitations. But I think many of these criticisms miss the larger goal of Apple’s latest laptop: Unlike the MacBook and MacBook Pro, the Air isn’t designed to be a general-purpose computer; it has, by design, limitations that will be unacceptable for many people.

But for a particular market—people who value light weight and are willing to give up other features to get it—it’s an interesting machine. And if you’ve already got another Mac at home, the MacBook Air may be an appealing on-the-go complement, with many of its limitations able to be overcome through the use of clever software features such as Remote Disk and Back To My Mac.

As for me, although it’s not exactly what I was hoping for, it’s close enough that I’m considering buying one to replace my aging—and heavy—original MacBook Pro. I’m also impressed by the MacBook Air for what it stands for: After years of Apple keeping its product line lean and tightly focused, the Air shows that the company feels its market is big enough to expand into niche products. In that respect, even if the MacBook Air isn’t for you—and I suspect that will be the case for most people—it’s something all Mac users should welcome

During his Macworld Expo keynote address on Tuesday morning, Apple CEO Steve Jobs introduced the MacBook Air, a computer that the company billed as the world’s thinnest notebook — small enough to fit inside an interoffice mailing envelope. It’s priced starting at $1,799 and will be available within two weeks.

Sporting a silvery finish, the MacBook Air features a 13.3-inch LED-backlit widescreen display that has a 1280 x 800 pixel resolution. The backlighting saves power and provides “instant on” response from the moment you turn it on, according to Jobs. The device has a slightly wedge-shaped profile. It weighs about 3 pounds, and sports a thickness of 0.16-0.76 inches. It’s 12.8 inches wide and 8.95 inches deep.

The MacBook Air also features a built-in iSight webcam and a full sized MacBook-style black keyboard. The keyboard is backlit, similar to MacBook Pros, and has an ambient light sensor that automatically adjusts brightness. The trackpad is also capable of recognizing multi-touch gestures, similar to using an iPhone or iPod touch. As a result, the MacBook Air’s trackpad is disproportionately large, compared to the size of trackpads found on the MacBook or MacBook Pro.

The MacBook Air features a 1.8-inch hard disk drive with 80GB of storage capacity standard. A 64GB solid-state disk (SSD) drive is an option. The hard drive is a Parallel ATA (PATA) model that operates at 4200 RPM.

The laptop is powered by an Intel Core 2 Duo chip running at 1.6GHz, with 1.8GHz available as an option. Jobs noted that Intel was willing to engineer a new version of the Core 2 Duo specifically to Apple’s specifications — it’s 60 percent smaller than others. The chip operates with 4MB of on-chip shared L2 cache running at full processor speed, and uses an 800MHz frontside bus. 2GB of 667MH DDR2 SDRAM is also included.

Like the MacBook and the MacBook Pro, the MacBook Air features a slimmed down MagSafe connector for power. It comes with a 45 watt power adapter. A flip-down door on one side reveals USB 2.0, Micro-DVI (to connect an external display) and a headphone jack. The MacBook Air also includes 802.11n-based wireless networking support and Bluetooth 2.1 + EDR.

Apple estimates that with wireless networking turned on, the MacBook Air can get about 5 hours of battery life.

No internal optical drive is included, but Apple will offer a $99 USB 2.0-based add-on SuperDrive for users who need it. For users that opt not to get the optical drive, Apple is offering a new software feature on this machine called Remote Disk; it enables you to “borrow” the optical drive of another Mac or PC on the same network as the MacBook Air, to use for installing software, for example.

Apple’s frequently been in the crosshairs of environmental group Greenpeace in recent years. Jobs offered information about the environmental goals behind the MacBook Air — it has a fully recyclable aluminum case, and is “the first” to have a mercury-free display with arsenic-free glass. All the circuit boards are BFR-free and PVC-free, and the retail packaging uses 56 percent less material than the MacBook packaging.

Resistance Temperature Detectors: Theory and Standards

With these general guidelines to the basic function, performance, and recognized standards for RTD’s, anyone can specify the right device for the application.

Setting the specifications for any sensor or instrument can be a difficult process, and RTD’s (resistance temperature detectors) are no exception. No one can be expected to be an expert in all fields, and frankly, no one needs to be. With these general RTD guidelines, along with a little common sense and background information on the application, you will successfully detail the specifications of an RTD that will satisfy your requirements.

THEORY OF OPERATION

A basic physical property of a metal is that its electrical resistivity changes with temperature. All RTD’s are based on this principle. The heart of the RTD is the resistance element. Several varieties of semi-supported wire-wound fully supported bifilar wound glass, and thin film type elements are shown here.

Some metals have a very predictable change of resistance for a given change of temperature; these are the metals that are most commonly chosen for fabricating an RTD. A precision resistor is made from one of these metals to a nominal ohmic value at a specified temperature. By measuring its resistance at some unknown temperature and comparing this value to the resistor’s nominal value, the change in resistance is determined. Because the temperature vs. resistance characteristics are also known, the change in temperature from the point initially specified can be calculated. We now have a practical temperature sensor, which in its bare form (the resistor) is commonly referred to as a resistance element.

Through years of experience, the characteristics of various metals and their alloys have been learned, and their temperature vs. resistance relationships are available in look-up tables. For some types of RTD’s, there are also equations that give you the temperature from a given resistance. This information has made it possible for instrument manufacturers to provide standard readout and control devices that are compatible with some of the more widely accepted types of RTD’s.

RTD SPECIFICATIONS

Eight salient parameters must be addressed for every RTD application to ensure the desired performance. Many will be specified by the manufacturer of the instrument to which the RTD will be connected. If it is a custom circuit or special OEM application, the designers must make all the decisions. The four specifications dictated by the instrumentation or circuitry are: sensor material, temperature coefficient, nominal resistance, and, to some extent, wiring configuration. Sensor Material Several metals are quite common for use in RTD’s, and the purity of the metal as well as the element construction affects its characteristics. Platinum is by far the most popular due to its near linearity with temperature, wide temperature operating range, and superior long-term stability. Other materials are nickel, copper, balco (an iron-nickel alloy), tungsten, and iridium. Most of these are being replaced with platinum sensors, which are becoming more competitive in price through the wide use of thin film-type resistance elements that require only a very small amount of platinum as compared to a wire-wound element.

Temperature Coefficient

The temperature coefficient (TC), or alpha of an RTD is a physical and electrical property of the metal alloy and the method by which the element was fabricated. The alpha describes the average resistance change per unit temperature from the ice point to the boiling point of water. Various organizations have adopted a number of different TC’s as their standards (see “Temperature Coefficient Standards”).

Nominal Resistance

Nominal resistance is the pre-specified resistance value at a given temperature. Most standards, including IEC-751, use as their reference point because it is easy to reproduce. The International Electrotechnical Commission (IEC) specifies the standard based on 100.00 Ohms at 0°C, but other nominal resistance’s are quite common. Among the advantages that thin film technology has brought to the industry are small, economical elements with nominal resistance’s of 500, 1000, and even 2000 ohms.

Wiring Configuration

The wiring configuration is the last of those parameters typically specified by the instrument manufacturer, although the system designer does have some control based on the application. An RTD is inherently a 2-wire device, but lead wire resistance can drastically reduce the accuracy of the measurement by adding additional, uncompensated resistance into your system. Most applications therefore add a third wire to help the circuit compensate for lead wire resistance, and thus provide a truer indication of the measured temperature.

Four-wire RTD’s provide slightly better compensation, but are generally found only in laboratory equipment and other areas where high accuracy is required. When used in conjunction with a 3-wire instrument, a 4-wire RTD will not provide any better accuracy. If the fourth wire is not connected, the device is only as good as the 3-wire RTD; if the fourth wire is connected, new errors will be introduced. Connecting a 3-wire RTD to a 4-wire instrument can cause serious errors or simply not work at all, depending on the instrument circuitry. A 2-wire RTD can be used with either a 3 or a 4 -wire instrument by jumping the appropriate terminals, although this defeats the purpose and reintroduces the un compensated resistance of the leads. To get the optimum performance, it is generally best to specify the RTD according to the instrument manufacturer’s recommendations.

Two other parameters are more application dependent;

the temperature range of the application; and,
the accuracy.

Temperature Range

According to the ASTM, platinum RTD’s can measure temperatures from -200°C to 650°C. (IEC says -200°C to 850°C).

You must consider the temperature limitations of all the materials involved, where they are applied, and the temperatures to which each will be exposed.

A few quick examples to illustrate this point:

TFE Teflon should not be used for wire insulation if it will be exposed to temperatures above 200°C (250°C for some).

Moisture proof seals are commonly made with various types of epoxy that generally have limits below that of the Teflon insulation.

Many wire insulating materials become brittle at subzero temperatures and therefore should not be used for cryogenic work.

So state the intended temperature range right up front and let the applications engineer assist you, especially since it may affect the materials chosen for internal construction of the probe.

Accuracy

You are probably wondering why accuracy was not the first topic covered, because RTD’s are generally known for their high degree of accuracy and it is typically one of the first specifications laid out. Well, the subject is not quite that simple, and it requires a bit of discussion. First, we must establish the difference between accuracy, precision, and repeatability. In the case of temperature, accuracy is commonly defined as how closely the sensor indicates the true temperature being measured, or in a more practical sense, how closely the resistance of the RTD matches the tabulated or calculated resistance of that type RTD at that given temperature.

Precision, on the other hand, is not concerned with how well the RTD’s resistance matches the resistance from a look-up table, but rather with how well it matches the resistance of other RTD’s subjected to that temperature. Precision generally refers to a group of sensors, and if the group has good precision at several temperatures, we can also say that they are well matched. This is important when interchangeability is a concern, as well as in the measurement of temperature gradients. Repeatability can best be described as the sensor’s ability to reproduce its previous readings at a given temperature.

Here’s an example. An ice point reading is done with an RTD that is then used to take readings at 100°C, 150°C, 37°C, and again at 0°C.

A comparison of the first and last ice point readings will give you an indication of the sensor’s repeatability under those conditions. A note of caution, however: an RTD’s repeatability is very application-dependent. So when you get right down to it, accuracy without repeatability is worthless. If you start with a sensor that is ±0.03°C at 0°C but is found to have repeatability only around ± 0. 5°C, what you have is a sensor whose readings are far less reliable than a standard-accuracy probe with good repeatability. A high-accuracy RTD installed in a field application also does not ensure that you will be getting a highly accurate signal back at the control room.

Most 4-20 mA transmitters and many display units and controllers have adjustable zero and span controls that if improperly adjusted will destroy the high accuracy of the RTD signal.

The best solution for applications of this type is to have both the RTD and the transmitter, or display, or whatever, calibrated as a unit by a certified calibration laboratory.

Fortunately, the requirements for this degree of accuracy best solution for applications of this type are few and far between. For more on this subject see, Accuracy Standards.

Our final two parameters are application dependent and vary from the specification of a bare resistance element to a large industrial assembly with thermowells, connection heads, and possibly field -mounted transmitters. We will discuss only the most basic areas: physical dimensions and size restrictions, and material compatibility.

Dimensions and Size

The physical dimensions and size requirements can be more complicated than you might think. On the low end, a resistance element to be used in the construction of a sheathed RTD generally requires only that the element is small enough to fit into the desired sheath ID. For cylindrical elements, such as wire-wound units, this is obvious-just don’t forget to allow for the wall thickness of the sheath. For thin film-type elements, we must apply the Pythagorean theorem; we need to know the width of the element, w, and the thickness of the element at its largest point, t. Then the minimum ID of the sheath will be given by; ID > (w2 + t2).

When we begin to discuss RTD probes and assemblies, the subject becomes more demanding. We need to examine the mounting arrangement: will it be used for direct immersion or with a thermowell? Or will it be something special, like an exposed airflow probe or surface mount sensor? Probe designs are endless in their configurations, and it seems that most applications have some unique requirements that make this a rather creative field in itself.

In many applications, the probe is immersed in a small vessel or piping system. Dimensions here are generally limited to sensor diameter (which affects response time); immersion depth into the fluid; and the mounting arrangement, i.e., will the sensor be screwed into a threaded port, typically with ANSI tapered threads, or will it be used in con-junction with a fluid seal already in place? Or will some other special considerations need to be made? There may be other variables, such as pressure limitations or high flow, depending on the complexity of the application. It is always best to look at the whole picture. and then discuss it with your applications engineer.

Thermowells are generally used for larger vessels and systems so that the system will not have to be drained in the event the sensor requires calibration or changing. Assuming the thermowell has already been specified, we need only to specify the probe diameter (typically ¼ in. OD for a 0.260 in. bore well), the depth of the thermowell bore, and how the RTD will be secured into the well (typically spring-loaded through a ½ in. NPT nipple or hex-nipple).

Material Compatibility

Most people specifying RTD probes have to pay attention only to the chemical compatibility that will prevent corrosion. This is generally straightforward and guidelines can be taken from other materials used in the system in which the RTD will be installed. If the piping system is constructed of 316 S.S., then the probe probably should be also. But always check a corrosion guide for corrosion rates and material recommendations if you have the slightest doubt.

For applications involving thermowells, the thermowell will carry the burden of corrosion protection. However, be sure to protect the connecting wires and any terminals or plugs from possible corrosion caused by splash or corrosives in the atmosphere.

SUMMARY

There are quite a few things to be considered when specifying an RTD probe or even resistance elements. But it’s just a matter of applying a bit of common sense and using information from the application environment to set down a clear set of requirements. And if there is something you are uncertain about, get your background information together and call that applications engineer. We can’t all be experts at everything

When an automated, oil filled, load-transfer switch, installed more than 20 years ago, failed and interrupted service to a platform-mounted transformer bank, we suspected a lightning strike as the culprit. The transformer fed a storm-drain pump, whose sole purpose was to drain flood waters off of an interstate highway system in a heavily traveled urban underpass. The circuit consisted of a primary selective system to provide two separate electrical circuits to ensure continued pumping in the case of a system failure. The switch, an externally mounted stored-energy mechanism with porcelain air bushings, was platform-mounted about 15 ft (4.6 m) above ground. After the failure, temporary repairs were made and one circuit was back on line in about 2 hr. Replacing the Switch Since the state in which we are located requires an automatic changeover switch, we advertised for a replacement. We expected that we would install the same kind of switch in the same circuit configuration. We received two bids, one from the company that provided the original oil switch and the other from Joslyn Power Products Corp. of Alsip, Illinois, U.S. for a SF6 switch. The SF6 switch was less expensive than the oil-filled switch. It also was smaller, easier to install and appeared to require less maintenance. Because we could eliminate the oil, which could be an environmental hazard, and because explosions were not a concern, we specified the gas-insulated switch. An Innovative Solution The installation resulted in some unexpected bonuses. The original transfer switch had three sets of bushings: one for the preferred source, one for the alternate source and one to serve the load. With this arrangement it was possible to switch between only two circuits. In addition, the oil switch had four potential transformers, three on the preferred feed and one on the alternate feed. The transformer primaries were unfused and connected line to ground. Donut-type current transformers ringed the load bushing and fed overcurrent relays, which latched to prevent operation in case of an overload. The PTs and CTs were inside the same tank as the switch and the operating mechanism was external and linked to the switch with a pipe linkage. A bonus in the new installation is that the SF6 scheme uses two separate switches installed on adjacent poles, which not only switches between two sources but will automatically sectionalize a down-stream fault. Voltage and current of all three phases of both the preferred and alternate feeders are monitored continuously without PTs or CTs using sensors developed for interfacing with remote terminal units of automated distribution systems. A self-contained local logic opens and closes the SF6 switches in the correct sequence to ensure reliable service to the pumping station. Pole mounting the SF6 switch scheme required creativity. The purchased switches were designed to be mounted on a crossarm. To simplify the installation, Aluma Form, Inc. of Memphis, Tennessee, U.S., constructed custom brackets that permitted installation of the switches directly to the pole. The switches were mounted to the brackets on the ground (Fig. 1) and then the assembly was hoisted using a block and tackle (Fig. 2). The installation was quick and simple. Making the connections was the most mechanical part of the job, since the switch and the motor operator mechanism were assembled at the factory (Fig. 3). The control cables were attached to the pole with standard staples (Fig. 4). The Future The experience has encouraged us to consider the SF6 as a standard since it is environmentally safe and it enhances design flexibility. TDW Clarence Wooddell, a graduate of Memphis State University, is supervisor of Electrical Distribution Engineering and has been with Memphis Light, Gas Water for 43 years. He is a senior member of IEEE and is a member of the Insulated Conductor Committee. Reggie Bowlin earned the BSEE degree from University of Tennessee - Knoxville and is design engineer in the Electrical Distribution Engineering Department. He is registered as a professional engineer in Tennessee

A switch is used to open or close a circuit. The most simple switch is the single pole single throw switch (SPST). It can either open or close one pathway. The pole is somewhat like the number of switches contained in the switch. If it is double pole single throw (DPST) it is the same as having two SPST switches, that when you flip on or off the DPST switch it is the same as throwing two SPST switches simultaneously. The throw is the number of positions the common point can move to. If it is a single pole double throw (SPDT) then the common connector on the switch can move from one point to another. So if you had the switch in one position, the common connector would be attached to one point. And if you then change its position it would move the common connector to the other point. You could use this kind of switch to turn off one object while turning on another when the switch changes position. And the double pole double throw (DPDT) is like having two SPDT switches.

You can also have more than two poles or two throws. You can have a triple pole triple throw (TPTT), or even more. The rotary switch normally has many throws, and poles. The one shown above is a single pole switch with five throws. A rotary switch is the kind you turn to move into different positions.

Some switches are shown above. Some switches, such as the 3rd from the left in the upper portion, are pulled. The one to the right of that one is twisted.

A switch is a mechanical device used to connect and disconnect a circuit at will. Switches cover a wide range of types, from subminiature up to industrial plant switching megawatts of power on high voltage distribution lines.

In applications where multiple switching options are required (e.g., a telephone service), mechanical switches have long been replaced by electronic switching devices which can be automated and intelligently controlled.

The prototypical model is perhaps a mechanical device (for example a railroad switch) which can be disconnected from one course and connected to another.

The switch is referred to as a “gate” when abstracted to mathematical form. In the philosophy of logic, operational arguments are represented as logic gates. The use of electronic gates to function as a system of logical gates is the fundamental basis for the computer—i.e. a computer is a system of electronic switches which function as logical gates.

With clock frequencies of a few hundred megahertz, today’s electronic systems are using pulse edges in the sub-nanosecond range. Networking interfaces deliver data rates approaching 1000 Mbits/s (Gigabit Ethernet and FDDI - fiber distributed data interface) and 155 and 622 Mbits/s (ATM - Asynchronous Transfer Mode). High quality video circuits also use pixel rates at sub-nanosecond rates.  These higher processing speeds present never-ending engineering challenges

One such challenge is RF interference, which originates from a fast change of electromagnetic energy. The faster the slew rate (rise/fall times) and the higher the voltage/current amplitude, the more problematic a circuit becomes.  As a result, electromagnetic compatibility (EMC) is harder to achieve today than ever before.

While fast changing pulses of current between two nodes of a circuit represent the so-called differential noise source, the fields surrounding this circuit can couple into other components and etch connections. The noise induced via inductive or capacitive coupling represents common-mode interference.  The RF interference currents are in phase with each other, and the system can be modeled as one which connects the source, “victim circuits” or “recipients” and the return path, which in many cases is represented by a chassis. Several factors are critical in defining the amount of the interference:

o Strength of the source

o Size of the area encircled by the culprit current

o Slew rate of the change

Thus, despite many possible causes of unwanted interference in a circuit, the noise is almost always the common-mode type.  Once there is some RF voltage present between a cable plugged into an I/O (input/output) connector and the enclosure or the ground plane, the resulting RF current of a few mA can be enough to exceed the allowable emission levels.

Typical Causes of RF Interference

Noise Coupling and Dissemination

Common-mode noise can be generated by less than an ideal layout. Some typical causes are an imbalance in the length of the individual conductors in differential pairs, or differences in distance to the power planes or the chassis.  Other source are imperfections of components - magnetic inductors and transformers, capacitors and active devices such as ASICs (Application Specific Integrated Circuit).

Magnetic components, especially the so-called “slug choke” type storage inductors used in power converters, always produce an electromagnetic field. An air gap in the magnetic circuit is equivalent to a large resistor in a series circuit, where most of the applied power is dissipated. Thus, the slug choke, which is built on a ferrite rod,  generates a strong field around the rod, with highest field density near the poles.

In switching power supplies using flyback topology, the transformer must have an air gap, which is associated with the high density magnetic field. Components that are best suited for “keeping the field to themselves” are toroids, which distribute the field through the length of the core.  This is one of the reasons the toroidal construction is preferred in high-frequency networking magnetics.

Circuits with inadequate decoupling often become the source of interference as well.  If a circuit requires high pulses of current and the local decoupling is not able to support the need due to low capacitance or relatively high internal impedance, the voltage generated by the supply loop drops. This is equivalent to a ripple, or fast change of the voltage between terminals. Through the stray capacitance of the package, this event can couple into other circuits, causing common-mode problems.

When a circuit intended for I/O interface is contaminated with common-mode noise, the problem has to be resolved before it passes through the connector.  Different applications suggest various ways of dealing with this problem. In video circuits, where I/O signals are single-ended and share the same common return, the solution is to filter out the noise with small LC filters. In lower frequency serial interface networking, some capacitive shunting to the chassis can be sufficient.

Differentially driven interfaces, such as Ethernet and FDDI, are normally transformer-coupled to the I/O area, with center taps provided on one or both sides of the transformer.  These center taps are connected via high voltage capacitors to the chassis, allowing shunting of the common-mode noise to the chassis without causing distortion of the signal.

Common-Mode Noise in I/O Area

There is no generic solution for all types of I/O interfaces.  Designers whose main goal is to get the circuit working, often overlook simple details. Some basic rules should be followed to minimize the amount of noise before it reaches the connector:

Basic Rules to Follow to Minimize Noise BEFORE it Reaches a Connector

* Locate decoupling capacitors close to the load
* Minimize the size of the loop of pulsed currents with fast edges
* Keep high-current devices (i.e., drivers and ASICs) away from I/O ports
* Evaluate signal integrity to assure minimum over-or-undershoot, especially in high current critical signals (i.e., clock, bus).
* Use local filtering such as RF ferrites where necessary to absorb RF interference
* Provide a low impedance bond or reference to the chassis in the I/O area

RF Noise and Connectors

Figure_1A_Jul23_98 1Even if the designer takes most of the precautions listed above to reduce the amount of RF noise in an I/O area, there is no guarantee that the efforts will be successful enough to meet emission requirements. Figure_1B_Jul23_98Some of the noise will be conducted, traveling from inside the circuit board as common-mode current. This source of the interference is between chassis and circuit etch.  Thus, this RF current needs to close the path through the lowest impedance available between the chassis and the carrier signal lines.  If the connector does not present low enough impedance (bond to the chassis), this RF current will travel via stray capacitance. While it is passing through the cable, the emissions are inevitably generated (Figure 1A).

Another mechanism for injecting common-mode currents in an I/O area is through coupling from nearby strong sources of interference. Even some of the “shielded” connectors with a metal cover over the top are not immune in such cases, since the culprit source can be located near the bottom side of the connector, as in PC environments.  If there is an opening between a connector and the reference chassis, the induced RFFigure_2A_Jul23_98 voltage between these two entities can substantially weaken the EMC performance (Figure 1B).

How to Minimize RF Interference with Connectors

Connectors with Metal Tabs and Gasketing

There are ways of packaging connectors with additional finger stock or gaskets. The connectors provide the bonding by filling the space between the face of the connector and the enclosure. This approach requires gFigure_2B_Jul23_98askets (Figure 2A).  Metal or metallic impregnated plastic gaskets work well if they are handled properly, that is, if the surface is free of residue from the installer’s hands, and if the pressure is enough to maintain good, low-impedance contact.

Other connectors are equipped with tabs or another means of making connections to the enclosure. The maximum area of contact in this arrangement is rather small, and it is restricted

Emissions can still “leak” between tabs and an enclosure panel
by the size of the tab and its flexibility.  In the case of using the cutout in the enclosure for a shielded connector, the sides of the cutout must be properly prepared by removing the paint (Figure 2B). Any slack in tolerance may result in this connector being recessed too deeply inside the enclosure and the bond becomes intermittent if the fingers are caught in any obstacle or otherwise damaged. Every EMC engineer knows the difference between the “golden” system qualified to meet emission requirements and the one from the production line in audit.  Loose gaskets or bent tabs mounted over paint over spray in critical areas (such as connector cutouts) will cause frustration.

For severe EMI conditions, gasketed connectors should be considered for the following reasons:

* Gaskets made of conductive fabric over foam are extremely flexible, and can be mounted around the whole connector.  In PCB mounted applications, a three-sided configuration is usually most appropriate.

Regal’s EMI/RFI gasket presses firmly against enclosure, helping prevent EMI/RFI emissions

* The mechanical engineer can position the connector within an acceptable through panel dimensional tolerance of the system package.

* The connector makes a low impedance bond to the chassis, eliminating concerns for the consistency of the contact.  A gasket that slides on the inner side of the enclosure wall can be much more forgiving with the masking requirements when the paint is applied.

* For designs with forced cooling, an optimum gasket configuration can provide an additional benefit: it helps to seal the gap between the connector and the wall, reducing air leaks.  In a dusty environment, a gasket helps to keep the inside of the system clean.

Meeting Emission Specifications and Other Cost Considerations

The total cost of implementing an EMC solution must be considered in the context of the situation, especially if the situation is dictated by a failure to meet an emission specification such as EN55024 and CISPR24. In these kinds of situations, an EMI/RFI problem is typically discovered in final testing at a testing lab.  When designers are faced with options that range from complete circuit redesign to swapping in EMI/RFI suppression connectors in key I/O areas, the swapping option is clearly the more favorable option—even though EMI/RFI suppressing connectors are more expensive. It is not unusual, once a connector based solution is identified, to implement a solution that is measured in days, as opposed to weeks and months of time-consuming circuit redesign and testing. The key is identifying the “right” EMI/RFI connector, or combination of connectors, that will effect the most cost-effective and timely solution.

Conclusion

Care must be taken to identify and understand the contribution levels and types of interference sources. The variety of connectors available on the market today enables designers to select the optimum design for the specific interface.