December, 2017
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Page 63
Reliability Fundamental to the Reed Relay’s Long Success Continued from previous page
Changing the wire gauge for the coil and the
number of turns creates relays with different drive- voltage requirements and coil powers. The resistance of the wire coil controls the amount of steady-state cur- rent flowing through the coil and the power the coil consumes when the contacts are closed. Whenever fine wires are used in Pickering Electronics relays, the ter- mination leads from the coils are skeined with several strands of wire twisted together to increase their phys- ical strength. A relay may have up to tens of thousands of turns of wire.
Packaging Reed Relays Most Pickering reed relays are constructed using
former-less coils, which dispense with the usual sup- porting bobbin. This leaves more room for the coil winding, permitting smaller relays or higher coil resistance figures. Reed relays are available in many package styles, such as dual-in-line, sin- gle-in-line and surface-mount. Often, reed relays are molded
using very hard materials that can cause stress on the delicate glass/metal seal of the reed switch capsule. Pickering uses a soft inner encapsula- tion, which provides a buffer to protect the switch. Without this, stress can dis- tort the reed switch slightly, changing the contact area and degrading perform- ance and contact resistance stability. An internal mu-metal magnetic screen enables Pickering relays to be stacked closely together, without the magnetic field from one relay affecting adjacent parts. This allows a high level of packing density.
Solid-State Relays The term “solid-state relay” refers
to a class of switches that are based on semiconductor devices. There is a large variety of these switches available. Some, such as PIN diodes, are designed for RF applications. Devices that com- pete with reed relays are based on FET switches. A solid-state FET switch uses two MOSFETs in series and an isolated gate driver to turn the relay on or off. There are some key differences
when compared with a reed relay. All solid-state relays have a leakage cur- rent like other semiconductor devices. This leakage current is nonlinear. Consequently, they do not have as high an insulation resistance. The on-resist- ance can also be nonlinear. There is a compromise between
capacitance and path resistance. Relays with low path resistance have a large capacitive load, which restricts bandwidth and introduces capacitive loading. This load is sometimes meas- ured in nanofarads for high-capacity switches. As the capacitive load is decreased, the FET size has to decrease, and the path resistance increases. The capacitance of a solid- state FET switch is considerably high- er than a reed relay. Reed relays are naturally isolated
by the coil from the signal path. Solid- state relays are not, so an isolated drive has to be incorporated into the relay. Solid-state relays can operate faster and more frequently than reed relays. Solid-state relays can also have much higher power ratings. In general, reed relays behave much more like per- fect switches than solid-state relays since they use mechanical contacts.
Electromechanical Relays Electromechanical relays (EMRs)
are also widely used for switching and can be the lowest-cost solution avail- able. Manufacturers have made huge investments in technology to make these relays in high volumes. There are notable differences
FlexLink is part of Coesia, a group of innovation-based industrial solutions companies operating globally headquartered in Bologna, Italy.
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Illustration showing coils. Actual
reed relays may have tens of thousands of coils.
between EMRs and reed relays. Reed relays general- ly exhibit much faster operation (typically between a factor of 5 and 10) than EMRs. The speed differences are due to the reed relay’s
simpler and lighter moving parts. Reed relays have hermetically sealed contacts, which offer more consis- tent switching characteristics at low signal levels and higher insulation values in the open condition. EMRs are often enclosed in plastic packages that offer a cer- tain amount of protection. Over time, however, the contacts are exposed to external pollutants and emis- sions from the plastic body, as well as oxygen and sul- phur ingress. Reed relays typically have 10 to 100 times the
mechanical lifetime as EMRs, under light load condi- tions. The difference arises because of the lack of moving parts in reed relays. Reed relay contacts require less power to operate than EMR contacts. EMRs are designed to have a wiping action when the
Continued on next page
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