Heatsinks and Relays 387higher, plus, the breakdown voltage at these pF•ȍ levels
is much lower than that of a reed switch. The turn-off
time for SSRs is also longer than the 50ȝs needed by a
reed relay to reach its typical 10^12 ȍ off resistance.
Some feel that the reliability of reed relays compared to
solid-state devices is largely unjustified, due to contin-
uous technological improvements. Many reed relays
have demonstrated MCBF values of several hundred
million to several billion closure cycles at typical signal
switching levels.
PIN diodes are occasionally used for HF switching.
However, PIN diodes require relatively complex drive
circuitry compared to the simple logic circuitry that
drives reed relays. PIN diodes typically have a lower
frequency cut-on of about 1 MHz, while a reed relay
can switch from dc to its useful cut-off frequency. The
high junction capacitance of PIN diodes results in lower
RF isolation than a reed relay when the PIN diode is
biased open. When biased closed, the higher on-resis-
tance of the PIN diode can lead to Q-factor damping in
the circuit to which it is connected. PIN diodes can
exhibit significant nonlinearity, leading to gain
compression, harmonic distortion, and intermodulation
distortion, while reed relays are linear switching
devices.
Electromechanical relays (EMRs) have been devel-
oped with bandwidths to about 6 GHz, and isolation of
about 20 dB at that frequency. This isolation is better
than that of a reed relay, since the contacts can be
designed with bigger spacing, resulting in lower capaci-
tive leakage. This advantage must be weighed against
the increased size and cost of EMRs and lower reli-
ability. The EMR has a complex structure with more
moving parts than the simple blade flexure involved in
closing a reed switch, resulting in a lower mechanical
life. If higher isolation is required with a reed relay
solution, two relays can be cascaded together with a
combined reliability that is still higher than that of a
typical EMR.
MEMS switches (relays) are being developed based
on two technologies, electrostatic closure and pulsed
magnetic toggling between open and closed states. They
offer potential advantages in terms of small and low loss
high-frequency switching. However, adequate contact
reliability has not been demonstrated at the switching
loads required by automated test equipment (ATE)
applications. At present, though, MEMS relay tech-
nology is too immature for use in most applications
addressed by reed relays.
13.2.5.7 Dry Reed SwitchesA dry reed switch is an assembly containing ferromag-
netic contact blades that are hermetically sealed in a
glass envelope and are operated by an externally gener-
ated magnetic field. The field can be a coil or a perma-
nent magnet. The switches in Figs. 13-19A and 13-19B
can switch up to 175 Vdc at 350 mA or 140 Vac at
250 ma. The switch in Fig. 13-19C can switch 200 Vdc
at 1 A or 140 Vac at 1 A.Figure 13-19. Coto Technology dry reed switches.
Courtesy Coto Technology.Figure 13-20. Energizing a dry-reed switch with a coil.
Courtesy Coto Technology.A. RI-80 SMD SPST 5 W switch. B. RI-80 SPST 5W switch.C. RI-25 SPST 25 W switch.NSN SNSIcIcIcS NA. A dry-reed switch mounted within a coil.B. A dry-reed switch mounted outside a coil.C. A dry-reed switch biased by a permanent magnet
and operated by a coil.