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Design Considerations for Switches
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In human-machine interface devices, the individual electromechanical switching mechanisms are where the rubber really meets the road. After all, it is with the individual switches that the machine's human counterpart actually interacts with the device and controls its processes. As such, special attention should be paid when designing or selecting the individual components of each switch to be incorporated into an interface's design. This special attention to the considerations surrounding the switches' design ensures that the interface will accomplish its intended purposes and that the design of the overall device will have sufficient longevity to make the design and engineering process cost effective.

As with any project, the team you put together has deep implications for the success of your project. The primary and most important consideration in the selection of a switch should be the selection of a manufacturer that has a long track record within the industry — especially one with custom design and in-depth support should problems arise in your design. Many switch suppliers no longer offer these services, but taking the extra time to connect with one that does will pay big dividends in the end. Quality and consistency of the product should also be a considered when looking for a supplier. Because many of the more recent design tools require 3D schematics, engineers should determine if the switch manufacturer provides them or makes them available as a value-added service. When they are provided, 3D schematics can be slipped right into designs, making it simple to do the mechanical study needed and can often help the engineer reduce the amount of samples that are pre-purchased, saving them time and money.

Switch Load
Once a quality supplier has been chosen, the next switch design consideration that must be thought-out is load. Pure resistive loads are the simplest to switch and if there is very little arcing during the contact transfer, erosion is minimal. This means that contact life is maximized.

Following the manufacturers' switch specifications and taking into account any possible surge, inrush current, and voltage is crucial when determining the maximum load a switch can accommodate. Doing so will limit premature contact erosion and possible failure resulting from inductive loads of improperly-specified switches. Selecting switches with ratings exceeding anticipated surges will reduce the problem caused by arcing, but will increase cost.
Switch cutaway: Most switches contain the same basic parts.

Ratings are expressed in different values, depending on the load characteristics, and they account for voltage, contact material and electrical operational life cycle. For example, a switch rated 6A at 125VAC with a resistive load, silver contact materials and expected electrical life of 25,000 cycles can also be rated 4A at 125VAC with an inductive load (the electrical life would remain the same at 25,000 cycles).

Size, Location and Application
Once the load specifications are determined, thought should be given to the interface itself. Its size, location and application all bring up issues pertaining to the design of the switches that will be incorporated into it.

Switches come in an incredibly vast array of sizes. With today's, ultra-compact electronic devices, it is easy to succumb to the temptation to think that smaller is always better, but this is not necessarily the case with switch design. It is important to make sure that the size is small enough to be comfortably accommodated on the interface — switches that are too large can clutter the device and take away from its functionality — but in many instances a smaller switch might not provide the necessary capability or reliability for a particular design.

Environmental considerations are also crucial to switch design. Improperly installing a switch that is not designed for harsh environmental conditions on an interface that will be used in such surroundings can result in dramatically decreased switch life expectancy and even critical switch failure. Obviously, devices that will never see the light of day, a drop of water or even a fleck of dirt probably don't need extreme environmental barriers built into them, but designers should nonetheless consider all conceivable scenarios where the device may be exposed to adverse conditions that could result in premature switch failure and plan accordingly. In addition to determining if tactile feedback mechanisms, illumination, legends and easy field replacement or repair capabilities are necessary, engineers must determine the frequency with which the switches will be used and the reliability they provide under extreme use circumstances.

Consider for a moment switches used on medical devices. These switches are typically used with great frequency and in situations where a switch failure could be catastrophic. In this case, and in many others, switching cycles require close attention.

Terminal Type
Another interface-related consideration that needs to be accounted for is terminal type. Designers can choose from surface mount, PC mount, solder lug, quick connect or wire-wrap terminals. Each of these is best suited to a certain set of circumstances and it is important to determine which type is called for in the product design.

Actuator Type
Actuators come not only in a wide selection of categories themselves, such as pushbutton, toggle, rocker, paddle, rotary, keylock rotary and slide actuated, but within each category a multitude of sub options are available. The key to determining the best actuator type for a switch design is to determine what the switch will specifically be used for and by whom it will be used, and then logically consider each actuator option in those terms.
Types of switches: There are dozens of different types of switches.

Despite the vast array of choices, ranging from soft alloys to pure metals, silver and gold have emerged as the most widely used materials in the switch industry because of their unique properties as contact materials. Both of these metals have distinct advantages and disadvantages that suit them for certain applications. Silver is the most commonly used contact material as it greatly improves performance of the contacts. Its chemical, electrical, thermal and mechanical properties, as well as low cost and ready availability, make silver an ideal material for industrial applications. However, silver does tarnish easily and when this tarnishing occurs, a hard, insulative oxide layer forms on the surface. In rare instances, tarnish may affect performance of a switching mechanism by disrupting the contact's ability to convey current.

Gold as a contact material suits applications requiring low energy switching. Gold is the most non-reactive of all metals, and unlike silver, it never reacts with moisture, which means it does not tarnish. In addition to gold's resistance to corrosion, it is among the top three or four electrically conductive metals. In its pure form, gold's limitations as a contact material include high cost, higher contact resistance and polymer formation.

Engineers can obtain some of gold's advantages by using gold plating over silver. Gold plating over silver is useful in a double pole switch, where one pole switches the power circuit and the other pole switches the logic level. In a power circuit, the gold plating is vaporized leaving only the silver material to handle the power switching. Alternatively, the gold plating remains intact in the logic circuit, which requires low-energy switching, and provides tarnish-free contacts for long-term, reliable switching.

Illumination Choices
Many switch applications either require or can be greatly enhanced by using illuminated switches. This brings up another switch design consideration: LED or incandescent lamp illumination. Endurance, price and performance must all be taken into account here. At about 100,000 hours, LEDs have a virtually unlimited service life whereas incandescent lamps must operate in a self-destructive mode to produce light, which results in a typical lifetime of around 7,000 hours. However, incandescent lamps are inexpensive on the front end when compared to LEDs — though engineers should remember to factor in incandescent lamp replacement and labor costs as well. When it comes to performance, LEDs provide an unmatched level of brightness but are not easily filtered or diffused and so are limited in color. Incandescent lamps may not be as bright as LEDs, but they can be easily filtered to any color desired and they have omni-directional and broad-spectrum characteristics that make their emissions readily diffusible for illumination over even relatively large areas.

"Smart" Switches
A final consideration regarding switch design deals with technological advances. Traditional switches are what can be considered non-programmable — function keys which have one hardwired function. "Smart" switches, or programmable switches, on the other hand typically incorporate a small LCD display and can be programmed to carry out a multitude of different functions — combining the benefits of dedicated function keys with those of versatile touchscreen displays.
Power rated illum PB: Illuminated switches light up when actuated, offering a quickly identifiable status, which is very popular for high stress environments including broadcasting, food services and industrial.

Of course, the price of these switches is greater than that of traditional switches, as with most technology, because of their expanded capabilities. This allows engineers to consider space saving, multifunction alternatives and its long-term benefits rather than a single function traditional electromechanical switch. With so many considerations to take into account when designing or selecting a switch, and being such an important aspect the overall interface engineering process, it's not uncommon to feel overwhelmed. However, it is also perhaps the most exciting stage of the design as switches are the components that will actually meet with user contact and control the processes the machine is intended to accomplish. To help with this important part of the design, good switch manufacturers have a trained staff who can suggest switches and custom design solutions based on specific needs and applications.

Contact: NKK Switches, 7850 E. Gelding Dr., Scottsdale, AZ 85260 480-991-0942 fax: 480-998-1435 Web:

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