SWITCHES & RELAYS FEATURE The relay from past to present
Robert Dixon, Product Manager at Panasonic Electric Works UK investigates the evolution of the reed relay and how this technology is being advanced with the latest PhotoMOS V’s reed relays
T
he Reed relay is a well-established mainstay of the test and
instrumentation industry since its mainstream introduction in the 1930’s. The reed relay had a number of advantages over other switching solutions of the time but surely the time has now come to consider technology of the 21st century? The reed relay certainly offered a
number of advantages over the more “contactor” styled electromechanical relays of its time. Manufactured as a pair of contacts mounted within a glass tube with sealed ends the standard reed relay has an intrinsically sealed construction. This gave a great advantage when compared to a non-sealed electromechanical equivalent since it could resist the effects of moisture and chemical exposure that would otherwise cause problems with both reed reliability and contact resistance. Switching speed was also much faster with many reed relays being able to operate at below 1mS switching time. Since the reed contacts were very
lightweight and relatively easily moved, the relay required very little in the way of power in order to set up an electromagnetic force strong enough to allow operation. This sensitivity allowed a tenfold decrease of energisation current when compared with older style electromechanical relays.
/ ELECTRONICS Figure 1:
Photomos relays from Panasonic Electric Work
They also had a disadvantage that the
contact ratings was typically very low and a limited, albeit extended, life operation factor. This life operation factor may never become an issue in a low switch speed application but when being used at a high operational frequency the number of operations can rise dramatically and very quickly exceed the operational life of the reed relay. To show an example of this:
operational life @ 100 cycles a minute = 100 x 60 x 24 = 144000 operations in 24hrs. If this was to run continuously the relay
will have reached 10 million operations in a little over two months of continuous use and that could be considered the end of the mechanical life of the relay being reached. At the end of life the reed relay would certainly start to lose reliability and this could lead to irregularities and failures. An obvious effect of this type of failures would be at the very least repeated service replacement costs. Panasonic Electric Works introduced
some years ago what many consider the natural replacement to the Reed relay - the Optically Isolated MOSFET relay. The company called this series of devices, the PMOS (PHOTO-MOS) relays. In external design this is often physically similar in appearance to an opto-isolator but that is where the similarities end. A PMOS relay is an optical MOSFET with true gate control logic giving a real
‘On/Off’ switch state unlike the more analogue response of an opto-Isolator that can depend on the factors of input control voltage. There are a number of parameters where a PhotoMOS relay is a clear match or better than a reed relay. Switching speed is one specification that can be shown as a good comparison. PMOS can be as fast as 0.2ms total switching time and this is far superior to all but the fastest of reed relay equivalents. And of course since the PMOS has a life factor that is not based on the actual number of operations it will not meet a natural limitation in this area. The PhotoMOS relay is also contact bounce free due to the lack of internal moving parts. This is another significant point when consideration of their use is made. Pulse counting applications cannot usually differentiate between certain switching actions and the associated bounce signals usually encountered in the equivalent electromechanical relay. One area where the MOSFET has caused concerns over the years is the channel capacitance, an unfortunate, in this instance, parameter of MOSFET construction. Panasonic Electric Works has resolved this with the introduction of the Low CxR variants, simply explained the Low CxR models are carefully specified MOSFETs where the company has tailored the channel to either have Low resistance or Low capacitance. This value is expressed as the picofaradOhm, with values as low as 5pFOhms. This allows engineers to take into account these parameters and design successful circuits suitable for signal processing. Surely the time is finally here to
consider the MOSFET relay as a more than worthy alternative to the “near century old” technology of the reed relay? Panasonic Electric Works used to make reed relays too, but as the company’s confidence, knowledge and ever evolving PhotoMOS product range grew, it ultimately discontinued them as we hit the 21st century.
Panasonic Electric Works UK
www.panasonic-electric-works.co.uk 01908231555
Enter 203 ELECTRONICS | DECEMBER/JANUARY 2015 15
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