Choose a Reliable Reed Relay Construction

Reed relays are considered to be the most reliable mechanical relay solution for low power switching. They have few moving parts and an inherently long mechanical life. However, not all reed relays are constructed to the same high standards. We only use reed relays assembled by our Relay Division (Pickering Electronics) because these offer the very best performance. Pickering reed relays are chosen by many ATE vendors because their life and reliability performance measures have the most consistent performance and long life. Because of these measures, these reed relays give our products a competitive advantage over other suppliers of reed relay switching systems. On this page, you will find an explanation of why these relays offer such improved reliability.

Reed Construction

A reed relay is a simple device; it is a reed assembled into a package that includes a coil and maybe a flyback diode. The most common form is the normally open (Form A) type, where the relay in the rest state is open circuit. Other types are available, but for a variety of reasons, they tend to be harder to control in manufacturing and less reliable. 

The reed itself is a sealed glass envelope that contains two (reed) blades made from magnetically sensitive material. The glass envelope is filled with dry inert gas to prevent oxidation of the contacts. The reed blades are selectively plated at the end, so when the two blades come together, they form a repeatable electrical contact. The blades are forced together by applying an axial magnetic field that causes the two blades to deflect and come into contact. In a reed relay, the magnetic field is produced by a coil that wraps around the glass envelope.  Take a look at this video that explains what a reed relay is and how it is constructed:

This construction is complicated by a choice of variables:

• What is the blade separation

• How much magnetic field does it take to close the contacts

• How well the glass envelope is sealed

• What plating materials are used on the contacts

• How much power, voltage, current can the contacts handle

• Overall length of the reed

• Stiffness of the blades

Pickering's relay division has had considerable experience in picking the right reed to use to construct a relay. Experience has shown, for example, that ruthenium contacts offer much better performance than alternative contact materials except in higher current applications. Reeds that are chosen in this way for the best performance are usually referred to as Instrument Grade reeds. We only use these ruthenium reeds for lower voltage switching. This gives us the significant advantage of improved reliability of the contact materials under low and medium switching power conditions.

Assembling a Reed Relay

Once the best reed switch component is chosen, those reed needs to be assembled into a reed relay. A reed relay encapsulates the reed switch into a package which contains the coil and a flyback diode to control back EMF's when the coil is released. A reed switch is a fragile device that can be mechanically damaged until it is packaged. It must be protected and supported at all times until the relay is complete, the packaging has a significant impact on reed reliability.

The image to the right shows a typical construction used by our relay division's competitors. It uses hard plastic molding material to encapsulate the reed, coil, and the diode in a package. The plastic materials support the reed relay components. Still, temperature cycling - caused either by environmental changes, coil operation or signal path losses heating the blades causes the differential expansion to stress the wires - the temperature coefficients of the parts are simply not matched. In any case, local heating in the assembly means that not all parts stay at the same relative temperature. The packaging method has a significant impact on the reed relay life.

The image below shows Pickering's reed relay construction. It uses a different construction method. The reed, coil and diode are mounted on an etched lead frame. The lead frame is mounted in a hard plastic shell and the inside of that shell is filled with a soft encapsulation material that allows differential expansion without unduly stressing the component integrity. 

There are other differences as well. Pickering uses a former-less coil for the reed. The coil is wound directly around the glass envelope, ensuring that the field from the coil is more closely coupled to the reed blade. They also include a magnetic shield that confines the external field to the relay - preventing the magnetic field of one reed relay from interacting with a closely spaced neighbor.