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Reed switches and relays offer long life and versatility.

Reed Switches are frequently overlooked when the topic of switches comes up. Basically a reed switch is a glass tube with a lead coming out of each end. Inside the glass tube are two leads that make up what would he considered the contacts. These leads or contacts are spread apart with an air gap of 0.001". Current or voltage is switched with the use of a magnet.

Most reed switches in use today have a glass tube length of 10 mm to 20 mm and a diameter of approximately 2 mm. Hasco presently sells a reed switch that starts out as nickel iron and then plated with rhodium over gold. To ensure a life cycle of tens of millions of operations, or possibly years of inactivity, the leads go through a special process to remove impurities. This makes the leads in the glass immune to contact resistance. The contacts are also sealed with nitrogen in the glass tube. Other reed switch companies use ruthenium as a plating method. Both production methods are good as long as there are adequate quality control methods in place.

Long Life for a Variety of Applications

There are two ways in which reed switches offer long life. First, reed switches can be operated in low signals to operate a couple of hundred million cycles. The second advantage is that they can sit around for years in non-use without the problems of contact resistance closing up the contact points.

Reeds are found in security applications that monitor doors and windows. They can also be found in printers or other devices where an enclosure is opened and the power gets turned off. One of the largest applications for reed switches is in the automotive industry. Applications include the monitoring of oil, windshield washer and brake fluid. There are also designs to use reeds as a proximity device in transmission and braking areas as well as safety related areas. Reeds can be found in different environments. There are reeds in space as well as underwater in diving computers and underwater communications equipment to turn on and off those devices. Versions of reeds come in Form A (SPST) and Form C (SPDT) types.

Although reed switches come with straight leads out of the box, customization should not be an issue. Reeds can be cut and bent to fit in PC board holes or made to go surface mount. Any good supplier should be able to process millions of reeds to client specifications every year. Each manufacturer charges usually no more than a few cents for the customization. Reeds can also be packaged in epoxy or plastic to protect it in abusive environments. There are also many packages for reeds and magnets encapsulated in plastic for mounting in holes or on surface areas with wires attached.

Relays--Reed and Electromechanical

Relays are one of the oldest electronic devices. Crude versions have been around since Samuel Morse was relaying signals across the country. Reed relays are a separate component from reed switches. While reed switches are classified under switches, reed relays are categorized under relays. This device inexpensively switches current or voltage by use of a coil being activated. The coil simply fits around the reed switch and is usually molded in a single in line (SP) or dual in line (DIP) package. Reed relays offer all the advantages over electromechanical types when it comes to operations. They are very fast, having operating times of 0.3 milliseconds, yet at their rated load they can operate at 100 million cycles. Loads range from switching 0.3A to 1A carrying 0.5A to 2.5A. Reed relays are also available for PC boards (through hole or surface mount). Excellent areas for their uses include ATE (because of their long life and low failure rates), modems, fire and security applications, and anything that requires quick, reliable, low level signaling.

Grabbing the lion's share of the market are the general purpose, or electromechanical relays. The difference between a reed relay and an electromechanical relay is basically the crux of it. As previously mentioned, a reed relay is a switching device, but it uses a reed switch and coil to activate it. The standard electromechanical relay uses metal armatures that close when a coil gets energized. The advantage to electromechanical relays is that they can handle and switch loads greater than 1A and usually up to 70A.

Electromechanical relays are also less expensive when they have more than one pole to them. Reed relays are much more expensive in SPDT, DPDT, 3PDT and 4PDT configurations.

Electromechanical relays are designed into some of the best devices in the world due to their price. An average of prices on the market for an engineer designing in a 10A relay in a volume application is approximately $.30; a 30A relay is in the mid- to upper $.60 range and a 70A or 80A relay can be acquired for less than $1.00. Price also varies on ultimate volume as well as whether the coil is switching AC or DC current. For the lower-current telecom-type, a miniature DPDT relay that can fit on a dime can cost less than $.75. Overall, relays offer a significant value for any type of application.

Tab Hauser is President of Hasco Components International Corp. Hasco has been producing and marketing relays, reed relays and reed switches since 1976. He can be reached at (516) 328-9292 or tabh@hascorelays.com; www.hascorelays.com.

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Title Annotation:Product tech highlight: switches/relays
Author:Hauser, Tab
Publication:ECN-Electronic Component News
Geographic Code:1USA
Date:Aug 1, 2003
Words:920
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