Honey I shrunk the relay

Honey I shrunk the relayIan Purcell
Since its birth in 1837, the electro-mechanical relay, invented by Samuel Morse, has been re-invented several times to reach its modern day guise. The most recent major overhaul took place during the 1970’s when Hans Sauer, founder of SDS Relais, pioneered the second generation. He changing from iron to a permanent magnet as the core of the relay’s coil, enabling both the power consumption and overall size of the relay to be drastically reduced.
Relays race… These relays are about as up-to-date as you can get. The shiny one can switch 20W of RF power at 2.5GHz and is only 20 x 12 x 9mm high. The top one on the right is claimed to be the first surface mount relay for automotive use and is rated at 20A continuous operation and 40A for two minutes. Bottom centre is a half-size power switching relay for automatic test equipment. The remaining three relays are two-third-size (40 per cent less volume) telecoms relays with different mounting options. These enable 64 rather than 32 relays to be fitted on a telecoms line-card.  
 
The rapidly expanding telecommunications industry is the current driving force behind relay development. Its demands for increasing function and reducing footprints have forced manufacturers to change the physical layout of the relay.
To reduce PCB occupancy the latest generation of relays has moved away from the traditional 0.1in pin grid spacing. Telecommunications relays now standardise on a generic 2.2mm grid spacing. This new arrangement has not compromised industry standard isolation and insulation requirements, continuing to allow the required 1.6mm electrical isolation gap to be achieved after the relay has been fitted to a PCB.
Surface mount technology (SMT)is now becoming common place in all component areas including electromechanical relays.
The move to SMT for relays has not been easy as new plastics and assembly techniques had to be developed to ensure that they can withstand higher assembly and soldering temperatures. SMT relays still have limitations compared with other SMT components. PCB manufacturing pick-and-place machines are generally unable to handle components with a height over 10mm, and relays frequently exceed this.
The demand for greater circuit function, and falling relay size, has increased the number of relays used on PCBs. With increasing relay density has come higher demands on the system power supplies. As a result, pressure is being exerted on relay manufacturers to provide less power-hungry relays. Until recently, the average telecommunication relay had a power consumption between 150 and 200mW. It is now possible to produce an otherwise equivalent relay that consumes just 50mW. This has been achieved through developments in the wire used for winding the internal coil.
The increasing demands of mobile communications companies on the radio frequency capabilities of relays are leading to the development of a new generation of high frequency and high power relays.
Frequencies up to 18GHz will soon be switchable using a new generation of co-axial relays developed using years of knowledge gained from electro-mechanical relays. For example, relays due to be released soon will be able to control up to 20W at 2.5GHz.
This may not initially appear to be that great until you realise that previously the maximum switchable power was only 10W in a relay of equivalent size.
The automotive industry is another sector gaining from relay developments. It has traditionally used bulky plug-in relays to control high current loads. Greater demands for car-based commodities has forced careful thinking about both relay design and specifications. More and more electrical circuits are being loaded in the vehicle and this has decentralised switching components away from the under bonnet compartment and fuse box to any available space.
PCB-mounted relay technology has been adapted to meet the challenge of reduced space in vehicles and even SMD relays, of up to 40A switching capability, are beginning to appear.
The majority of automotive loads, typically DC loads such as motors and solenoids, are only active for a short period. This feature can be exploited to make smaller relays, rating the device for the full power, but only for intermittent operation.
Further recent automotive developments include the concept of a modular relay family footprint. This system enables circuit designers to create a common PCB that could be used on a number of different vehicle platforms, populated to suit the individual specification of the car.
In-car use has also prompted the development of quiet relays, typically between 20 – 25dB less noisy than standard automotive relays. At Matsushita this has been achieved by using double insulation and sound absorbing materials to eliminate sound transmission and by adding damping structures to eliminate vibration noise.
The photo-MOS semiconductor relay is beginning to eat away at the electro-mechanical relays market, for certain applications where ultimate reliability or physical size is important. But this highly-successful interloper may soon be under attack from its own stable. The micromachined relay, a single chip device where electrostatic charge on deposited electrodes is used to induce motion into micro-contacts fabricated on-chip could form the next generation of electro-mechanical relays.
Ian Purcell is the UK and Ireland marketing manager for Matsushita Automation Controls. Matsushita researches, manufactures and sells electro-mechanical and semi-conductor relays. Photo-MOS…
The latest generation of photo-MOS semiconductor relays is now accepted as a viable design solution. However, it has taken over five years for this technology to gain industry confidence.
The photo-MOS relay technology is a marriage between the long-established optoisolator and MOSFET technology. Inside, an LED illuminates a photovoltaic generator. This, in turn, supplies voltage to the gate of a switching MOSFET, or a pair of back to back MOSFETs in AC applications. The MOSFET has the advantage of offering a cl ean cut ON or OFF state and the well-understood LED driving characteristics are simple to accommodate in circuit design. Voltage isolation up to several thousand volts is provided by the optical link and the technology is able to control loads up to 1500V at currents up to 6A.
Reliability in normal operation is high because there are no moving parts. This makes photo-MOS relays ideal for use in applications where serviceability would prove expensive or difficult or where ultimate reliability needed.
Relays of this type do suffer from non-trivial on-resistances, up to a few ohms. This resistance is reducing as photo-MOS relays reap the benefits of the fast-moving MOSFET technology race occurring elsewhere in the industry. In the last 12 months some devices have seen a 66 per cent board area reduction, to only 18.9mm2.
Cost is also dropping as volumes increase and production techniques improve.


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