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December, 2017
Reliability —Fundamental to the Reed Relay’s Long Success
A
reed relay, if used correctly, is a near-perfect device with a low-resistance metallic switch path and inherent isolation between the con-
By Graham Dale, Technical Director, Pickering Electronics Another reed switch design variable is size.
trol voltage operating the coil and the signal being switched. Reed relays contain a reed switch, a coil for creating a magnetic field, an optional diode for handling back EMF from the coil, and an encapsulating package with connection ter- minals.
Two Metal Blades The reed switch has two shaped metal
blades made of a ferromagnetic material, roughly 50 percent nickel and 50 percent iron, and a glass envelope that holds the metal blades in place. The envelope also pro- vides a hermetic seal that prevents any con- taminants from entering the contact area. Most, but not all, reed switches have open contacts in their normal state. If a magnetic field is applied along the
axis of the reed blades, the field is intensi- fied in the blades because of their ferromag- netic nature, the open contacts of the reed blades are attracted to each other and the blades deflect to close the gap. With enough applied field, the blades touch, and electrical contact is made. The only movable parts of the reed
Longer switches do not have to deflect the blades as far (measured by angle of deflection) as short switches to close a given gap size between the blades. Short reeds are often made of thinner materials so they deflect more easily, but this has
used for the switch contact areas. Most commonly used are rhodium, iridium or ruthenium — all rare platinum-group metals. These provide hard, wear- resistant surfaces with good resistance stability for a long life, often into billions of operations. For very high voltages, from 5 to 15 kV, tungsten tends to be the preferred material, due to its high melting point and resistance to arc welding across the contacts. Reed switch contacts can be coated either by electroplating or vacuum deposition (sputtering). Pickering Electronics’ relays intended for low-level instrumentation use sputtered ruthenium contacts.
Generating the Magnetic Field To operate a relay, a magnetic field
switch are the blades. There are no pivot points or materials trying to slide past each other. The contact area is enclosed in a hermetically sealed envelope with inert gases, or in the case of high-voltage switches, a vacuum, so the switch area is sealed against external contamination. This gives the reed switch an exceptionally long mechanical life.
Reed relay without magnetic field applied (top) and with magnetic field (bottom).
an impact on their rating and contact area. Smaller reed switches allow smaller relays to be constructed — an important consideration where space is critical. The larger switches may be more mechanically robust and have greater contact area, improving their signal-carrying capability. Various plating materials and methods are
needs to be created that is capable of closing the reed switch contacts. Reed switches can be used with permanent magnets (for exam- ple, to detect doors closing), but for the reed relays, the field is generated by a coil, which can have a current passed through in response to a control signal. The coil sur- rounds the reed switch and generates the axial magnetic field needed to close the reed contacts. Different reed switches require differ-
ent levels of magnetism to close the con- tacts. This is usually quantified in ampere
turns (AT). Ampere turns are the product of the current flowing in the coil multiplied by the num- ber of turns. Stiffer reed switches for higher power levels or high-voltage switches with larger contact gaps usually require higher AT numbers to oper- ate, so the coils need more power.
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