Reed relay
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As a relay is a switch controlled by an electromagnet, so a reed relay is one or more reed switches controlled by an electromagnet. The contacts are of magnetic material; thus the electromagnet acts directly on them rather than requiring an armature to move them. Sealed in a long, narrow glass tube, the contacts are protected from corrosion, thus are ordinarily plated with silver rather than precious metals. The most common reed relays of the late 1930s through the 80s had two reed switches inserted into holes in the bobbin. Since the moving parts are small, reed relays are capable of faster switching than most others [1].
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[edit] Memory device
A few million reed relays were used in the 1930s through the 60s for memory functions in Bell System electromechanical telephone exchanges. Usually one of the reeds latched the relay, and the other performed a logic function or more often a memory function. Most reed relays in the Crossbar switching systems of the 1940s through the 1970s were packaged in groups of five. Such a "reed pack" was able to store one decimal digit, encoded in a two-out-of-five code (74210 variant) for easy validity checking by wire spring relay logic.
[edit] Crosspoint switch
In the Bell System Stored Program Control exchange systems of the 1970s, reed relays were no longer needed for data storage, but tens of millions of them were packaged in arrays for voice path switching. In the 1ESS switch, the cores were made of a magnetically remanent alloy, so the relay could latch magnetically instead of latching electrically. This "Ferreed" method reduced power consumption and allowed both contacts to be used for voice path. The coils were wired for coincident current selection similar to a magnetic core memory, so operating the contacts for one crosspoint would release the other crosspoints in its row and column.
Each input of the array had, besides the two talk wires, a P lead for controlling the crosspoints on that level. Two coils on each crosspoint were wired in series with all the others on that level, to the P lead. Each output of the array also had a P lead with two coils on each crosspoint of that output level. The two windings controlled by the same level were unequal, and were wound around opposite ends of the reed, in opposing polarity. When a pulse passed through the crosspoints of a level, the two ends of each reed were magnetized north to north or south to south, thus repelled each other and opened the crosspoint in all except the selected crosspoint.
The selected crosspoint had current passing through both its input P lead and its output P lead, thus through all four windings. On each end of the ferreed, the windings provided by the two different P leads were opposed to each other, and the greater one predominated when both were energized. This being the input P lead at one end of the ferreed, and the output P lead at the other end, the two ends of that particular ferreed were magnetized north to south, hence attracted each other and closed the contact. Current was applied by the pulser only to set up the connection. The P leads remained dry and the crosspoint remained closed until such time as another connection was made involving one of the levels.
Because the individual crosspoints were more expensive than those of crossbar switches, while the control circuitry was cheaper, reed arrays usually had fewer crosspoints and were more numerous. This required them to be arranged in more stages. Thus, while a telephone call in a typical crossbar exchange like 5XB passed through four switches, a call in a reed system such as 1ESS typically passed through eight.
In the later 1AESS, the reeds themselves were of remnant magnetic material. This "Remreed" design allowed further reduction in size and power consumption. A "grid" of 1024 2-wire crosspoints, arranged as two stages of eight 8x8 switches, was permanently packaged in a box. Despite the sealed contacts, plating with silver rather than with precious metals resulted in reed arrays being less reliable than crossbar switches. When one crosspoint failed, the grid box was quickly replaced whole, and either repaired at a local workbench or shipped to a repair shop.
Stromberg-Carlson made the similar ESC system, whose reeds were called crossreeed.
Reed arrays passed out of use in the early 1990s, being unnecessary in true electronic (digital) telephone systems such as DMS-100 and 5ESS switch. Reed relays continued in their old uses outside the telephone industry.
