Last year, Pickering Interfaces, the manufacturer of modular signal switching and simulation products for use in electronic test and verification, launched a new generation of 1 amp PXI matrices that delivers twice the density of competing modules. One of the key components in any module of this type is the switching element – the reed relay – and it was developments made by sister company Pickering Electronics to its reed relay product range that has enabled Pickering Interfaces to get a performance advantage over its rivals.
Unlike the age-old chicken/egg conundrum, (NB: it was the egg), there is no disputing which Pickering company came first. Pickering Electronics began producing reed relays in 1968; Pickering Interfaces was formed in the mid 1980s to broaden the company’s product portfolio when it appeared that solid state relays might eventually supersede reed and electromechanical products. It has transpired though that over 30 years later, the applications for reed relays has not declined. Pickering Interfaces has grown to become a leader in its field, claiming to offer the largest range of switching and simulation products in the industry for PXI, LXI, and PCI applications. Pickering’s products are specified in test systems installed throughout the world and have a reputation for providing excellent reliability and value.
The company’s latest product is the BRIC™ ultra-high-density large PXI matrix range (model 40-559), robust 1 amp/20 watt switching modules, with up to 4,096 crosspoints. The matrices are available in 2, 4, or 8-slot PXI sizes and are designed for high-performance matrix requirements. They are used in many industries including automotive ECU and semiconductor package testing. Keith Moore, CEO at Pickering Interfaces, claims that the new BRIC PXI matrices “deliver twice the density of any competing module” – similar-sized offerings from close competitors “can only claim around 0.3 amp/3 watts”, he adds.
The key to this significant increase in capability is Pickering Electronics’ continual innovation of the reed relay. Despite advances by electromechanical relays – which can be cheaper but are slower and shorter-lived than reeds – and solid state switches – which are faster than reeds but which suffer from having a lower insulation resistance, a higher capacitance and path resistance – “reed relays are still the critical building block for PXI matrices”, claims Moore.
Over the years, Pickering Electronics has pioneered several developments in reed relay design. One was the incorporation of a Mu-metal magnetic screen which enables relays to be packed very tightly together without risking operational failure due to magnetic interaction with adjacent relays. To this day still, some competitor’s products are unscreened.
Another development introduced by Pickering Electronics is the use of a former-less coil construction. Having a self-supporting coil dispenses with the supporting bobbin commonly found in competing reed relays. This increases the space available for coil winding by about 50% greatly improving magnetic efficiency. It also enables further product miniaturisation.
When designing its new BRIC PXI matrix range, Pickering Interfaces wanted to create a product that would enable a complete Functional ATE switching system to be housed in a single 3U PXI chassis and allow the use of much lower cost 8 or 14-slot PXI chassis. The model 40-559 matrices are designed with built-in, high-performance screened analogue busing to minimize the cost and complexity of cable assemblies to the device under test as well as to instrumentation. The range also includes Pickering’s Built-in Relay Self-Test (BIRST) and is also supported by their eBIRST Switching System Test Tools. These tools provide a quick and simple way of finding relays that have been damaged or reached their end of life within the modules.
Pickering Interfaces’ previous generation PXI modules use series Pickering Electronics’ series 117 reed relays, which, when launched, were the smallest products available on the market, with a footprint measuring 6.86 x 3.81 mm (0.27 x 0.15 inches). In order to achieve the doubling in switching density required to achieve its goals for the new matrices, Pickering Interfaces approached its sister company to see if any further space savings could be achieved.
“It was definitely a case of the application leading the product development”, says Moore, “and the new 120 series product they have produced occupies the smallest board area – a mere 3.9 x 3.9mm – while enabling the highest packing density currently available”.
The new reed relays are available in two switch types: a general-purpose sputtered ruthenium switch rated for up to 20 watts and 1 amp and a low-level sputtered ruthenium switch rated at 10 watts and 0.5 amps. The reed switches are oriented vertically within the package, significantly improving the packing density. However, the small package size cannot accommodate an internal diode: back EMF suppression diodes are included in many relay drivers, however, if they are required, depending on the drive method, diodes can be provided externally. Internal mu-metal magnetic screening is included – otherwise dense packing of the relays would not be possible due to magnetic interaction. While Pickering do not normally recommend connectors as they do compromise resistance integrity, it is understood that these are sometimes desirable to aid serviceability. The relay pins on 2mm pitch are compatible with some connectors in the market place and allow them to be stacked in either a row or in a matrix on a 4mm pitch.
Pickering Interfaces provides a standard three-year warranty and guaranteed long-term product support on all its products. This means that the reed switches that it uses must also be of a very high quality. Pickering Electronics only makes high quality relays and does not sell parallel lower quality ‘budget’ ranges. As Keith Moore says: “the risks of using an inferior or unsuitable product for a high performance application such as automotive or semiconductor testing far over-shadow any perceived cost savings – to put it another way, product recalls and lost reputations mean that one must consider the total cost of each component chosen very carefully indeed”.
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The reed relay was invented in 1936 by Bell Telephone Laboratories. Since that time, it has gradually evolved from very large, relatively crude parts to the small, ultra-reliable parts we have today. Production methods and quality systems have improved a great deal over that time, and costs have been radically reduced.
Pickering Electronics, an established reed relay manufacturer, was founded in 1968, and even then some were saying that these electromechanical devices would have a limited lifetime. Instead, the market for high-quality reed relays has increased into areas that were inconceivable in those days.
Part 1 of this two-part series answered the question, “What is a reed relay?” This article delves into the differences between reed relays and other switching technologies.
Electromechanical relays (EMRs) are widely used in industry for switching functions and often can be the lowest cost relay solution available to users. Manufacturers have made huge investments in manufacturing technology to make the relays in high volumes.
There are some notable differences between reed relays and EMRs which users should be aware of:
- Reed relays generally exhibit much faster operation (typically between a factor of 5 and 10) than EMRs. The speed differences arise because the moving parts are simpler and lighter compared to EMRs.
- Reed relays have hermetically sealed contacts, which lead to more consistent switching characteristics at low signal levels and higher insulation values in the open condition. EMRs often are enclosed in plastic packages that give a certain amount of protection, but the contacts over time are exposed to external pollutants, emissions from the plastic body, and oxygen and sulphur ingress.
- Reed relays have longer mechanical life (under light load conditions) than EMRs, typically of the order of between a factor of 10 and 100. The difference arises because of the lack of moving parts in reed relays compared to EMRs.
- Reed relays require less power to operate the contacts than EMRs.
- EMRs are designed to have a wiping action when the contacts close, which helps to break small welds and self-clean their contacts. This does help lead to higher contact ratings but also may increase wear on the contact plating.
- EMRs can have much higher ratings than reed relays because they use larger contacts; reed relays usually are limited to carry currents of up to 2 A or 3 A. Because of their larger contacts, EMRs also often can better sustain current surges.
- EMRs typically have a lower contact resistance than reed relays because they use larger contacts and normally can use materials of a lower resistivity than the nickel iron used in a reed switch capsule.
Reed relays and EMRs both behave as excellent switches. The use of high-volume manufacturing methods often makes EMRs lower cost than reed relays, but within the achievable ratings of reed relays, the reed relay has much better performance and longer life.
Solid State Reed Relays
The term “solid-state relay” refers to a class of switches based on semiconductor devices. There is a large variety of these switches available. Some, such as PIN diodes, are designed for RF applications, but the most commonly found devices that compete 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 compared to a reed relay:
- All solid-state relays have a leakage current associated with their semiconductor heritage; consequently, they do not have as high an insulation resistance. The leakage current is nonlinear. The on-resistance also can be nonlinear, varying with load current.
- There is a compromise between capacitance and path resistance. Relays with low-path resistance have a large capacitive load (sometimes measured in nanofarads for high-capacity switches), which restricts bandwidth and introduces capacitive loading. 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 higher 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 have much higher power ratings.
- In general, reed relays behave much more like perfect switches than solid-state relays since they use mechanical contacts.
MEMS switches still are largely in the development stage for general usage as relays. MEMS switches are fabricated on silicon substrates where a three-dimensional structure is micro-machined (using semiconductor processing techniques) to create a relay switch contact. The contact then can be deflected either using a magnetic field or an electrostatic field.
Much has been written about the promise of MEMS switches, particularly for RF switching, but availability in commercially viable volumes at the time of writing is very limited. The technology challenges have resulted in a number of vendors involved in MEMS failing and either ceasing to trade or closing down their programs.
Like reed relays, MEMS can be fabricated so the switch part is hermetically sealed (either in a ceramic package or at a silicon level), which generally leads to consistent switching characteristics at low signal levels. However, MEMS switches have small contact areas and low operating forces, which frequently lead to partial weld problems and very limited hot-switch capacity.
The biggest advantage for MEMS relays—if they can be made reliable—is their low operating power and fast response. The receive/transmit switch of a mobile phone, for example, has long been a target for MEMS developers.
However, at their present stage of development, it seems unlikely they will compete in the general market with reed relays as the developers concentrate on high value niche opportunities and military applications.
The Future for Reed Relays
In more recent years, there has been a constant quest for further miniaturization. Smaller parts have required more sophisticated methods, including lasers, to create the glass-to-metal hermetic seal of the reed switch capsule. Lasers also are sometimes used to adjust the sensitivity of reed switches by slightly bending the switch blades to change the size of the contact gap. Contact plating materials and methods also have changed, particularly in the areas of cleanliness, purity of materials, and the reduction of microscopic foreign particles or organic contamination, resulting in superb low-level performance.
Reed-relay operating coils also have become smaller and more efficient thanks to advanced coil-winding techniques with controlled layering of the coil-winding wire. In the case of Pickering Electronics’ relays, the coil-winding bobbin also has been dispensed with in favor of former-less coils, which has reduced package sizes. While reed relays are a relatively mature technology, such evolution will continue in the future.
A reed relay in many ways is a near perfect switching element with a simple metallic path. A well-designed and correctly used part will give a long and reliable life. Reed relays will certainly be around for many years to come.
Original article can be found here.
Pickering’s new Series 120 4mm2 TM reed relay range has attracted a lot of interest since being released in July at Semicon West in San Francisco, U.S.A. The relays require a board area of only 4mm x 4mm, making it the highest packing density currently available, taking up the smallest board area ever.
Two switch types are available, a general purpose sputtered ruthenium switch rated for up to 20 Watts, 1 Amp and a low level sputtered ruthenium switch rated at 10 Watts, 0.5 Amps.
These are the same reed switches as used in many other long-established Pickering Electronics ranges but are orientated vertically within the package, allowing this very high density. The small size of the package does not allow an internal diode. Back EMF suppression diodes are included in many relay drivers but if they are not, and depending on your drive methods, these may have to be provided externally.
The relays feature an internal mu-metal magnetic screen. Mu-metal has the advantage of a high permeability and low magnetic remanence and eliminates problems that would otherwise occur due to magnetic interaction. Relays of this small size without magnetic screening would be totally unsuitable for applications where dense packing is required.
To learn more about this industry changing Reed Relay range visit Pickering Electronics in booth E.5910 at Electronica China 2018 this March, 14 – 16, in the Shanghai New International Expo Centre, China.
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Pickering Electronics, a UK designer and manufacturer of Reed Relays, has announced they are now a member of the Electronics Representatives Association (ERA), the international trade organization for professional field sales companies in the global electronics industries, manufacturers who go to market through representative firms and global distributors.
Since 1968, Pickering Electronics have been manufacturing high quality Reed Relays for Instrumentation and Automatic Test Equipment (ATE), High voltage switching, Low thermal EMF, Direct drive from CMOS, RF switching and other specialist applications.
Pickering Electronics has developed a solid customer base in a wide range of industries/applications, including their sister company; Pickering Interfaces; designers and manufacturers of modular PXI/PCI/LXI Switching Systems. Pickering Interfaces are a large Reed Relay customer who work very closely with Pickering Electronics on leading edge Reed Relay designs, reliability testing, life testing, production engineering, amongst other things. This close relationship greatly benefits both companies and gives Pickering Electronics a strong insight into demanding functional test Reed Relay applications.
Consecutive years of double-digital growth in sales, and recent investments to expand the capacity of the organisation, have led Pickering Electronics to update their strategy in the USA, to begin establishing strategic partnerships with representatives and distributors that are focused on the electronic components/test and measurement market.