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issue: December 2009 APPLIANCE Magazine

APPLIANCE Engineer - Technology Report
Miniature Magnetic Sensing

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 A 3.7-mm-long magnetic reed switch—engineered to be the world’s smallest—has a narrow sensitivity range and is suited for applications with space constraints or low magnetic fields.

Using customized manufacturing processes, Cincinnati, OH, U.S.–based supplier Standex Electronics (www.standexelectronics.com) is able to produce what it says is the smallest magnetic reed switch ever made for mass production.

At 3.7 mm in length, the GR150 also maintains a very narrow sensitivity range of 2–4 ampere-turn (At). Paul Linsley, product manager, attributes this narrow range to the strict in-house production. “The manufacturing process Standex has developed allows for very accurate control of those variables that control both magnetic sensitivity and hysteresis,” Linsley explains. “The production processes have the ability to achieve over an 80% yield in a narrow 3-At band. Normal production processes for reed switches deliver a much broader range of magnetic sensitivity.”

The tight magnetic sensitivity of the GR150 helps engineers yield tighter performance characteristics in the end product, says Linsley. “The ability to specify a narrow sensitivity range substantially reduces how this variable (switch sensitivity) affects the application. In proximity applications, the narrow sensitivity range will substantially reduce the actuation point variability with a given magnetic source.”

With a typical operating time of 0.2 milliseconds and a release time of 0.1 milliseconds, this professional-grade ultra miniature reed switch is suited for demanding applications such as hearing aids, surgical devices, pill cams, cell phones, and other small consumer electronics products.

In the case of hearing aids, Linsley says, the purpose of the switch normally involves automatically switching the hearing aid to a mode that gives better performance when the end customer is using a telephone. “Since the normal magnetic source in a telephone is a lower-grade ceramic magnet located on the handset, the device used to detect this magnetic field has to be magnetically very sensitive,” says Linsley.

Simply increasing the sensitivity level of the switch, however, does not lead to better performance. “The other side of the coin is that the bias of the earth’s magnetic field—in the area of 1 At—could cause a switch to latch in the on state or not operate at all, depending only on how the switch is positioned relative to the earth’s magnetic field,” Linsley tells APPLIANCE. This is when the GR150’s small footprint becomes advantageous. “The smaller the sensor of the magnetic field, the more options the hearing aid manufacturer has to position the reed switch in his device to make better use of the field generated by the source magnet.”

Although the reed switch is minuscule in size, it retains certain important attributes of its larger counterparts that make it durable. For example, “the switches are hermetically sealed in nitrogen,” says Linsley, “and the contact surfaces use very hard precious metals, providing many millions of switching cycles.”

The GR150 reed switch can also function as a noncontact position sensor for applications where the available magnetic field is extremely low, or where the space limitations are a major design factor. In addition, Linsley says, when compared with some magnetic sensors, reed switches can represent more power savings. “In the off state, reed switches take no power, which can be very important in battery driven applications. Some other magnetic sensors do have a power drain while in the off state or standby mode.”

Available in many standard and custom configurations, the reed switch can be integrated into an existing product that uses a larger reed switch or other magnetically operated sensor. Linsley says the GR150 can easily match the magnetic sensitivity of other magnetically driven switches and can be provided in an axial form or with leads cut to customer needs. It can also be provided in SMD configurations, including being overmolded, to aid automatic handling, Linsley adds.



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