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issue: February 2007 APPLIANCE Magazine

Switches and Switching Devices
Adapting Electromechanical Switches to Meet an Application

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Despite the abundance of electromechanical switch varieties and configurations available to design engineers, an off-the-shelf product may not always be there to meet the needs of specific applications. As a result, the mechanical and/or electrical attributes of a switch may need refinement or wholesale modification.

The first step in specifying a switch involves a review of its critical electrical performance ratings and its physical packaging or configuration. Comparisons of these parameters to the intended application will then determine if changes need to be made.
From an electrical standpoint, the major selection considerations for a switch include the current rating, voltage rating and initial contact resistance. These considerations are often a reflection of the type of switch involved and are influenced, to a degree, by the switch’s physical packaging. For example, as the physical size of a switch decreases, in general, so does the electrical rating. On the other hand, big, bulky rocker switches can typically handle higher power.

A customized three-position power switch started with a standard two-position power rocker.

Current Ratings

Current ratings vary a great deal depending on the type of switch in question. For instance, the current ratings for a PCB-mounted DIP style switch could be as low as 0.4 V A. This type of switch rarely experiences customization due to its industry-standard footprint and other commodity-only attributes. Some switches, such as toggles or push buttons, may have current ratings starting at 0.4 V A, but by selecting optional contact platings, current ratings of a few amps or more (depending on the physical size of the contact) can be achieved. Further, other switches, by design, have more robust contact arrangements to handle higher current. Applications that require this type of switching in appliances or other commercial electronics include certain on/off power switching and selector switches carrying less load.
At the other end of the current spectrum is high-power switching to approximately 20 A. Switches carrying up to 20 A may include rockers. With such additional current requirements, the need often arises to customize a switch or to even develop an entirely new switch.

Tactile switches can be customized in terms of their sealing, button style, actuation force, and profile.

Adapting Electrical and Mechanical Attributes

Recently, as Tyco Electronics worked with a customer to design its next-generation product and as its designers evaluated the electrical power budget and other design needs, it was determined that most off-the-shelf rocker switches (for “on/off” control) did not meet requirements. In fact, since the application actually required an “on/off/on” feature, available switches were simply not viable. The main sticking point was the current rating. A switch was needed that would provide improved service life; thus, the overload performance of the switch was a major consideration. The switch had to meet UL-1054 overload, horsepower (hp) testing. Notably, the requirements of UL-1054 for 1 HP 125 V AC dictate no electrical or mechanical failures after 50 cycles at 96 A and 125 V AC. Likewise, for 2 hp 125 V AC, there can be no electrical or mechanical failures after 50 cycles at 144 A and 125 V AC.
As in many design scenarios, the designers reviewed off-the-shelf switches that were close to meeting the overload and current rating objectives. This approach was favored because designing from known values (with an existing, documented product) typically shortens the design process. After identifying a two-position on/off rocker switch, engineers at Tyco tested the switch’s performance to evaluate how to adapt it to meet the manufacturer’s requirements.
After examining the test results, the engineers concluded that the switch contacts needed to be “beefed up” in addition to modifying the switch to an on/off/on configuration. Although the packaging of the three-position (on/off/on) rocker would change the mechanical attributes of the switch, the housing of the switch had to stay the same in order to fit the customer’s design. Engineers increased the size on the contact buttons and improved the transition surfaces between the movable and stationary contacts. By changing the contacts in this fashion, the supplemental contact surface area allowed for additional overload current (2 hp, 144 A). Subsequent qualification testing certified the switch, as required, to UL-1054. Thus, through simple, but thoughtful, changes to an off-the-shelf product, a value-added solution emerged.

Ultraminiature microswitches come in a standard package, but items such as the actuating lever, mounting configuration or contact leads can be customized to meet application needs.

Physical & Mechanical Considerations

In addition to the functional changes that add value to off-the-shelf switches, designers may also require cosmetic or other packaging changes. For example, environmental conditions may dictate certain packaging changes. In particular, temperature resistance and sealing are two specific conditions that may influence switch design. Indeed, while there are standard switches available that have been designed to operate in certain temperature extremes, not all conditions can be covered with these off-the-shelf products. Thus, when dealing with applications subject to temperature extremes, changes to the switch’s materials should be considered, as the materials must maintain both mechanical integrity and electrical performance.
Sealing a switch from environmental factors, such as moisture or contamination, can be critical. Ingress of water, laundry detergents, cleaning agents, food, grease, and other household chemicals into electronic devices is clearly undesirable. Sealing a switch may involve simply adding a protective boot to the button or it may be integrated within the switch design. Sealing is also included in some switches that are soldered to printed circuit boards in order to protect against the solvents used in the solder wash process. For the most part, all three of these sealing methods conform to an ingress protection (IP) rating. A common IP rating for switches is IP 67, which prevents ingress of all dust and protects against the effect of immersion between 15 cm and 1 m. Some applications calling for a certain IP rating may require certain adaptations to the switch, including packaging it in an enclosure or other housing or developing internal sealing methods.
Other physical packaging considerations include the geometries in which a switch resides. Indeed, the size and configuration of a switch’s enclosure impact the way the switch connects to wires, printed circuit boards and other electrical factors. Also, the user interface (or actuator) of the switch may need to be modified for better ergonomics or handling. A simple modification, such as button height or actuation force, can easily be changed to enhance a product’s ergonomics. On the other hand, switches may be better suited with lower profiles and higher actuation forces to prevent unintentional actuation. Again, the application will dictate these items.
Even the most basic issue of how a switch mounts to a bezel, circuit board or enclosure should be considered. For example, adapting a switch to fit a certain hole or opening size with a minimum of additional components or special installation tools may be the design objective. Trends toward snap-in switching devices that do not require mounting hardware or tools may also require modifications. Additional investigations on wire termination may be warranted to determine more efficient wire connections.

In sum, there are numerous products to choose from when designs call for an electromechanical switch. However, many applications require a more customized product, beyond a standard off-the-shelf switch. Typically, the fastest and most efficient approach to resolving these unique design situations involves examining existing switches and evaluating possible modifications and adaptations to meet the new requirements. Naturally, testing and qualification of the newly adapted switch is a necessary and integral part of that approach.

This information provided by Mark Zitto, product manager, Tyco Electronics, Harrisburg, Pennsylvania, U.S.

Suppliers mentioned in this article:
Tyco Electronics

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