How to choose among magnetic switch technologies

Because no magnetic switch technology best fits every application, Hermetic Switch Inc. and technology consulting firm TekMark Growth Partners recently analyzed four magnetic switch technologies, so designers can more readily match requirements to the strengths and limitations of available switches.

05/04/2006


System Integration

Think small. Hermetic Switch provides the following magnetic switches (top to bottom): Hall Effect, MEMS, GMR, Reed (PRX+2452), and Reed (HSR-0025).

Because no magnetic switch technology best fits every application, Hermetic Switch Inc . and technology consulting firm TekMark Growth Partners recently analyzed four magnetic switch technologies, so designers can more readily match requirements to the strengths and limitations of available switches. The analysis identifies and compares many performance criteria for electromechanical (reed and MEMS) and solid-state (Hall and GMR) magnetic switch products, covering reliability, proven design, operating environment, power consumption, magnetic sensitivity, and design flexibility, the companies say. Various manufacturers and numerous product data sheets were accessed for this analysis. A summary of each switch's strengths and limitations follows.

Reed (electromechanical) switches have several strengths. They do not consume power in the "off" mode, making them suited for applications where conserving battery power is critical. These switches have hermetically sealed contacts, which makes them applicable to dirty, hostile environments. The reed switch family is highly resistant to electrostatic discharge (ESD). These devices provide switching and sensing functions in one package, saving on cost and labor. Reed switches use an established technology (dating back to 1936 when Dr. W.B. Elwood invented it at Bell Telephone Laboratories) and have a long-proven history of reliability. Limits: Capital cost of reed switches may be more per unit than other magnetic switches, but quality, performance, and reliability reduce overall cost. Reed switches have been larger in size than other magnetic switches. Some have come down in size; one overmolded package length is 0.210 in. (5.33 mm).

MEMS (micro electromechanical) switches' strengths include a small footprint, as small as 0.110 in. long (2.8 mm); high magnetic sensitivity, as low as 1.7 mT. They can withstand physical shock to 15,000 G and are relatively inexpensive, making them a good choice for high-volume, low-cost commercial applications. Limits: They are ESD sensitive. Switch contacts are not hermetically sealed so electrical contacts can become contaminated, causing sticking or other malfunctions. MEMS switches generally have higher contact resistance than other magnetic switch technologies.

Hall-effect (solid-state) switches are inexpensive and suited for high volume, low-cost commercial applications. They have very long life expectancy. When operated within electrical specifications, Hall switches can operate for billions of cycles. These switches have no contact bounce. They are also durable and resistant to shock and vibration. Limits: They require constant power, even when in the "off" mode, reducing battery life. Hall effect switches cannot switch loads, so if switching is needed, so are additional components, cost, and labor. They have a low signal output, usually requiring amplification circuitry. Hall-effect switches also are ESD sensitive.

GMR (giant magnetoresistance) switches are true solid-state devices so they have no moving parts. They have a small footprint, some as small as 0.04 in. (1.0 mm) square. They are very magnetically sensitive. Some have magnetic sensitivity as low as 1.0 mT and can maintain a very tight operating point. This switch family operates in a wide temperature range, -40 to 150 °C. Limits: GMR switches require constant power, even in the "off" mode. They are ESD sensitive. These switches are based on a relatively new, evolving technology.

A more in-depth report, "Guide to Magnetic Switch Technologies," is available upon request from Hermetic Switch.

—Edited by Mark T. Hoske, Control Engineering editor in chief,
MHoske@cfemedia.com





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