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.
Pickering Interfaces only uses reed relays assembled by our sister company, Pickering Electronics, because these offer
the very best performance. Many ATE vendors choose to use Pickering Electronics reed relays because their performance
measures on life and reliability show them to have the most consistent performance and long life. That gives Pickering
Interfaces products a competitive advantage over other suppliers of reed relay relay switching systems.
On this page you will find an explanation why these relays offer such an improved reliability.
Reed relays are really simple devices, 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 manufacture and less reliable.
The reed itself is a sealed glass envelope that contains two (reed) blades made from a magnetically sensitive material. The glass envelope is filled with an 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 filed is produced by a coil which wraps around the glass envelope.
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 Electronics 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. Pickering Interfaces ONLY uses these ruthenium reeds for lower voltage switching. Reeds which are chosen in this way for the best performance are usually referred to as Instrument Grade reeds and they give Pickering Interfaces the major advantage of improved reliability of the contact materials under low and medium switching power conditions.
Having chosen the best reed switch component 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 which is easily mechanically damaged until it is packaged, it must be protected and supported at all times until the relay is complete, and the packaging has a major impact on reed reliability.
The image to the right shows a typical construction used by Pickering Electronics competitors. It uses hard plastic moulding material to encapsulate the reed, coil and the diode in a package. The plastic materials support the reed relay components, but temperature cycling - caused either by environmental changes, coil operation or signal path losses heating the blades causes differential expansion to stress the wires - the temperature coefficients of the parts are simply not matched and in any case local heating in the assembly means that not all parts stay at the same relative temperature. The packaging method has a major impact on the reed relay life.
The image below shows Pickering Electronics'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 Electronics use 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.
|SoftCenter construction used by Pickering Electronics|
The net result is the construction of a more reliable and better quality product that can be densely mounted on a PCB without fear of relay interaction. That enables Pickering Interfaces to make much denser and more reliable reed relay based switching products.
Further information on Pickering Reed Relays