Reed relays switch to new designs, applications

Article By : Bill Schweber

Ancient electromechanical components still have an irreplaceable role...

Electromechanical relays are like dinosaurs who haven’t gotten the news that they are supposed to be extinct. Instead, they beget new generations, get designed into new applications, and even take over new territory.

Countless millions of these relays are used in new designs to solve often otherwise intractable problems from DC to RF, switching low-level signals, and power, while addressing conflicting objectives. Plus, of course, there is a large market for form/fit/functional replacements for ones that have failed or, more likely, for use in replacement boards and systems (relays are amazingly reliable under normal use.)

I was reminded again of the vitality of this ancient device when I saw a press release from UK-based Pickering Electronics Ltd, a vendor of reed relays (sometimes called “reed switches” but “relay” is more common). This release touted a new, smaller, and better reed relay they had engineered on a tight schedule for next- generation automatic test equipment (ATE) at ON Semiconductor (Figure 1 and References 1 and 2).

illustration of Pickering reed relays used on an ATE boardFigure 1 This ATE board, fully loaded with newly designed miniature reed relays, shows that this classic component still has a vital role in the latest generation of ATE systems. Source: Pickering Electronics Ltd

If you are familiar only with “conventional” coil/armature relays, reed relays employ a very different structure and are used extensively in instrumentation and ATE, as they can support high-voltage switching, as well as sensitive signals with low thermal EMF, and can be designed for DC through RF switching. The basic reed relay is a model of design elegance with the contacts in a sealed glass tube that can either have a vacuum or an inert gas, Figure 2 (see References 3-7 for more details).

illustration of the concept of a reed relayimage of a reed switch showing its basic constructionFigure 2 The simple concept of the reed relay (top) is seen in its basic construction (bottom). Sources: Electronics NotesWikipedia

The activating magnetic field is applied via an external electromagnet or even a permanent magnet (for this reason, reed relays are often used to detect door/window closures). Since the contacts are in a sealed tube, they are immune to dirt/dust build-up or contaminants in the air, which might otherwise degrade to contact resistance over time or encourage flashover and sparking.

Reed relays were developed in the 1930s at the venerable Bell Telephone Laboratories, which was looking for a small, lightweight, and extremely long-life relay for switching of telephone circuits. In line with the simplicity of the reed relay concept (if not the actual manufacturing), the 1941 patent (Reference 8) is amazingly short, with just one page of three basic drawings and a little over a page of text – that’s all there is.

If you are not familiar with reed relays or relays in general, it’s a good idea to get comfortable with them. Just because these electromechanical relays with their frowned-upon “moving parts” have been around forever doesn’t mean that they are obsolete – far from it. Yes, solid-state relays (SSRs, also called optocouplers) have taken over many of their traditional roles, but old-style relays still solve some nasty design problems with no headache, hassle, or surprises.

Are there any other “ancient” electromechanical components that have nominal electronic replacements, but you still regard as viable and useful? Are there any which have mostly disappeared but you wish were still commercially available?

Leave a comment