Seven questions about Reed Relays
The following interview explains what Reed Relays are, their unique features and what needs to be considered when using them within switching solutions. Graham Dale, Technical Director of Reed Relay manufacturer, Pickering Electronics, explains.
Why should engineers use Reed Relays in switching applications?
Reed Relays remain an attractive solution for switching applications. Having a metallic path, they do not suffer from a relatively high contact resistance and high off-state leakage current usually associated with solid state relays. Having hermetically sealed contacts, reed relays offer a better low level performance than conventional electro-mechanical relays as they do not suffer from oxidization or films building up on the contacts. They also have the advantage of faster operate and release times, critical in today’s instrumentation and test systems.
How do Reed Relays work?
A reed relay consists of two ferro-magnetic switch blades that exit the opposite ends of a hermetically sealed glass tube. This tube is filled with an inert gas, usually nitrogen. A vacuum is normally used instead in the case of high voltage reed switches. The overlapping blades are attracted to each other when they become magnetized by the field generated by a coil wound around the switch and the contacts will close. These contacts are plated using various methods and materials appropriate to the application. Pickering favour Sputtered Ruthenium plating for low level switching.
In general, the most common construction for a reed relay offers no magnetic screening, involves the use of a hard moulded package and an operating coil wound on a plastic bobbin surrounding the reed switch capsule. Pickering offer a technically superior solution including SoftCenter construction, mu-metal magnetic screens and former-less coil winding techniques.
What is SoftCenter technology?
Instead of the more usual hard moulded package, Pickering use our SoftCenter® technology, whereby a soft inner material is used to protect the delicate glass to metal seal of the reed switch capsule against expansion mismatches. This de-stresses the switch ensuring accurate alignment of the switch blades which in turn gives a longer and more reliable life.
What is former-less coil winding?
Relay operating coils are commonly wound on bobbins. The great majority of Pickering relays are manufactured with self-supporting coils, thus avoiding the space required for these bobbins. In the case of the smaller relays types, this gives around 50 percent more room for the coil winding, allowing the use of less sensitive reed switches with their inherent advantages of higher operating and restoring forces. In some ranges, this technique allows Pickering to achieve extremely high coil resistance figures.
What is the purpose of the Mu-metal magnetic screen?
Reed switches are operated by the magnetic field from a coil which is wound around the reed switch capsule. Without a magnetic screen, this field will spread some way outside the confines of the relay package and will de-sensitize other reed relays mounted alongside. As an example, an unscreened relay mounted on 0.2 inches (5.08mm) pitch, will require a coil voltage about 25 percent higher to operate it, when other relays alongside are also in an operated state.
This falls to between 1 and 5 percent, according to type, if magnetically screened. Smaller relays on 0.15 inches (3.81mm) pitch, would have an interaction figure as high as 40 percent if unscreened.
