If used correctly, a reed relay is a superbly reliable device. The switch contacts are hermetically sealed, so they don’t suffer from oxidization or contamination in the same way as an open electromechanical relay. In reality, relays are often considered slightly mundane and little thought is given to them, which sometimes leaves them vulnerable. This guide will help you to maximize the reliability of your design.
High-current or high-power inrushes are the most damaging and most frequent cause of contact damage. Reed relays have specified maximum current, voltage, and power ratings. The power figure is simply the product of the voltage across the open contacts before closure and the instantaneous current they first make.
We at Pickering have lost count of the number of times that we’ve heard something like, “I was only switching 5 V at 50 mA onto this CMOS logic board,” when the user has completely disregarded the current inrush into the liberal sprinkling of decoupling capacitors and several microfarads of reservoir capacitance on that board.
Don’t rely solely on electronic current limiting of power supplies to protect relay contacts. Electronic current limiting takes a finite time to react, and decoupling capacitors are often on the output of a power supply. There’s nothing better than resistive current limiting.
As well as inrushes due to charging capacitive loads, discharging capacitors can be an even greater issue, since the current is often only limited by the resistance of the reed switch and PC tracks. Even capacitors charged to quite low voltages can cause current inrushes of tens of amps. And, although they may be for microseconds only, such capacitors can cause damage to small reed switches.
As voltages increase for some applications, inrushes can become an even greater issue; for example, when discharging cables after high-voltage proof testing. The energy stored in a capacitance is equal to ½ CV2 joules, so it will increase with the square of voltage. Increasing from 10 to 1000 V will boost the stored energy by 10,000 times.
If you’ve ever had a relay contact stick closed, only to free up with a slight tap, or had a longer than expected release time, then more than likely it’s caused by a micro-weld due to a current inrush.
“Hot” vs. “Cold” Switching
Reed relays generally have a higher carry-current rating than their “hot” switching-current rating. Contact damage usually occurs during “hot” switching due to the resulting arc across the contacts as they open or close. A severe current overload will quickly melt the contact area, causing the two surfaces to fuse together, creating a hard weld as soon as the contact closes.
Less severe current inrushes will cause a milder weld or gradually build up a “pip” on one contact and erode a “crater” on the other, according to the direction of current flow. These can eventually lock together. Arcs can occur when contacts open, particularly when the load is inductive. Back EMFs from inductive loads should always be limited, usually with a simple diode in the case of dc loads, or by a snubber or varistor for ac loads.
One way to reduce or remove these issues is to “cold” switch. This is a common technique in test instrumentation, where the current or voltage stimulus isn’t applied to the switch until after the relay has been operated and contact bounce finished. In the same way, the stimulus is removed before the contact is opened. Therefore, no arcing or switched current inrushes will occur, and the relay will achieve maximum life, often into billions of operations.
When calculating the delay time between switching on the relay coil and applying the current to the switch, it’s important to consider the effects of high ambient temperature if it appears likely to occur. The maximum operate time and bounce figures given on datasheets are at a 25°C ambient level. At higher temperatures, the resistance of the coil winding will increase at a rate of 0.4%/°C, this being the coefficient of resistance of the copper coil wire. Correspondingly, both the coil current and level of the magnetic field generated to operate the reed switch will fall. This lower drive level will increase the operate time slightly.
The timing figures on datasheets are normally quite conservative, so this is unlikely to be an issue up to the normal ambient specification of 85°C. However, if there’s any additional self-heating within the relay due to a high carry current and switch resistance (I2R W), it will be necessary to consider this and allow a little more time before turning on the current through the switch.
You can view the original article here.
Pickering Electronics, a leading provider of Reed Relays, will be showcasing its latest high density range at electronica on 8 – 11, November 2016 in booth B1.550.
The Pickering Series 115, Series 116 and Series 117 are three ranges of small Single Pole (1 Form A) reed relays ideal for the construction of high density matrices or multiplexers. These three ranges have identical pin configurations allowing a common PCB for all types but allowing the designer a range of switch ratings according to which part is fitted. The reed switches are vertical within the package which permits a common footprint with a board area of only 3.8mm x 6.6mm. Only the profile height changes with the increasing power or current ratings.
The Series 117 has a height of 9.5mm and is rated at 0.5 Amps switching at 5 Watts. The Series 116 has a height of 12.5mm and is rated at 0.5 Amps switching at 10 Watts. The Series 115 has a height of 15.5mm and is rated up to an impressive 1.0 Amp switching at 20 Watts. Double pole (2 Form A) versions are also available in the Series 116 and 117.
One benefit of the very small size of these relays is that it often makes it possible to increase the functionality of existing designs without increasing the size of the printed circuit boards.
All feature instrumentation grade reed switches with sputtered ruthenium contacts, making them an ideal choice for low level or ‘cold’ switching applications.
They have the option of an internal diode across the coil connections for Back EMF suppression and feature Pickering unique SoftCenter® construction as well as 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 size without magnetic screening would be totally unsuitable for applications where dense packing is required.
To learn more visit Pickering on booth B1.550 at Electronica this November, or contact Pickering today to request your free sample board of Pickering ultra-high density Reed Relays
You can view the original article here.
Philip Stoten interviews Graham Dale, Technical Director of Pickering Electronics on their booth at SemiconWest 2016. The interview focuses on Pickering’s new high voltage Reed Relay ranges including the Series 119 relays for up to 3kV and Series 67/68 for up 10kV. These new relays help to maximize packing density of test equipment, with the Series 119 being the industry’s smallest HV Reed Relay yet.
You can view the interview on Pickering’s YouTube channel.
Pickering Electronics’ newly released Series 67 and 68 reed-relay range are available for up to 10-kV stand-off, 7.5-kV switching, with an option of either PCB or flying-lead switch connections. Similar in specification to the long established Pickering Series 60/65, these new relays are manufactured in a single-in-line (SIL) format using former-less coils, which dispense with the more usual coil supporting bobbin, allowing a smaller package than similar rated devices.
The unusual package design does present some interesting packing possibilities for high-density applications such as multiplexers and matrices in instrumentation and test systems. See the nearby image of Pickering Series 67 relays mounted on a 3U PXI 12-way high-voltage multiplexer module that illustrates these possibilities.
Available as standard are 5-, 12-, and 24-V coils, and other voltages can be supplied to special order, as can variations in the lead length of the Series 68 range. The relays are suitable for high-voltage transformer and cable test and some electro-medical applications such as defibrillators.
Pickering Electronics is a worldwide manufacturer and distributor of high-quality reed relays for ATE, low thermal EMF, RF switching, and other specialist applications, available in surface mount, single-in-line (SIL), dual-in-line (DIL), and many other popular package styles.
Learn more about Pickering Electronics and its reed-relay ranges at SEMICON West in July 12-14 in San Francisco.
Thr original article can be seen here.
Pickering Electronics have expanded their high voltage Single-in-line (SIL) Reed Relay range to include three new series, that all offer higher packing density. The Series 67 and 68 dry Reed Relay range for up to 10kV, and the Series 119 Micro-SIL range for up to 3kV, will all be showcased this January at Semicon Korea on stand 2509.
The recently released Series 67 and 68 Reed Relay range are available for up to 10kV stand-off, 7.5kV switching, with an option of either PCB or flying lead switch connections. These new relays are manufactured in a SIL format using former-less coils which dispense with the more usual coil supporting bobbin allowing a smaller package than similar rated devices.
The unusual package design presents some interesting packing possibilities for high density applications such as multiplexers and matrices in instrumentation and test systems. 5, 12 and 24 volt coils are available as standard, and other voltages can be supplied to special order, as can variations in the lead length of the Series 68 range. The relays are suitable for high voltage transformer and cable test and some electro-medical applications such as defibrillators.
The new Series 119 is the industry’s smallest high voltage SIL Reed Relay for up to 3kV stand-off. These can be stacked side-by-side for maximum packing density (all Pickering SIL Relays have full magnetic screening allowing side by side operation).
This new relay is intended for voltages considerably higher than standard small SIL relays, ideal for Cable and Backplane Testers and Mixed signal ATE. Pickering also offer other Single-in-Line High Voltage dry Reed Relays with the Series 104 for use up to 3kV stand-off, 25 Watts switching.
To find out more about the new Reed Relays visit Pickering on stand 2509 at Semicon Korea 2016, or alternatively go to the website.
The original article ican be seen here.
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?”1 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.
Reed relays contain a reed switch, a coil for creating a magnetic field, an optional diode for handling back EMF from the coil, and an encapsulating package with connection terminals. In many ways, a reed relay, if used correctly, is a near perfect device with a low-resistance metallic switch path and inherent isolation between the control voltage operating the coil and the signal being switched.
The Reed Switch Explained
The reed switch has two shaped metal blades made of a ferromagnetic material (roughly 50:50 nickel iron) and a glass envelope that holds the metal blades in place and provides a hermetic seal that prevents any contaminants from entering the critical contact area inside the glass envelope. Most (but not all) reed switches have open contacts in their normal state.
If a magnetic field is applied along the axis of the reed blades, the field is intensified in the reed blades because of their ferromagnetic nature, the open contacts of the reed blades are attracted to each other, and the blades deflect to close the gap. With enough applied field, the blades touch, and electrical contact is made (Figure 1).
The only movable part in the reed switch is the deflection of the blades; there are no pivot points or materials trying to slide past each other. The contact area is enclosed in a hermetically sealed envelope with inert gasses, or in the case of high-voltage switches a vacuum, so the switch area is sealed against external contamination. This gives the reed switch an exceptionally long mechanical life.
Another reed switch design variable is size. Longer switches do not have to deflect the blades as far (measured by angle of deflection) as short switches to close a given gap size between the blades. Short reeds often are made of thinner materials so they deflect more easily, but this clearly has an impact on their rating and contact area. Smaller reed switches allow smaller relays to be constructed—an important consideration where space is critical. The larger switches may be more mechanically robust and have greater contact area, improving their signal-carrying capability.
Various plating materials and methods are used for the switch contact areas: most commonly, rhodium, iridium, or ruthenium—all rare platinum group metals. These all provide hard, wear-resistant surfaces with good resistance stability for a long life, often into billions of operations. For very high voltages, 5 kV to 15 kV, tungsten tends to be the preferred material due to its very high melting point and resistance to welding through arcing across the contacts. Reed switch contacts can be coated by either electroplating or vacuum deposition (sputtering). Pickering Electronics’ relays intended for low-level instrumentation use sputtered ruthenium contacts.To operate a relay, a magnetic field needs to be created that is capable of closing the reed switch contacts. Reed switches can be used with permanent magnets (for example, to detect doors closing), but for the reed relays, the field is generated by a coil, which can have a current passed through in response to a control signal. The coil surrounds the reed switch and generates the axial magnetic field needed to close the reed contacts.
Generating the Magnetic Field
Different reed switches require different levels of magnetic field to close the contacts, and this usually is quoted in terms of the ampere turns (AT)—simply the product of the current flowing in the coil multiplied by the number of turns. Stiffer reed switches for higher power levels or high-voltage switches with larger contact gaps usually require higher AT numbers to operate, so the coils need more power.
Changing the wire gage 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 current flowing through the coil and therefore the power the coil consumes when the contacts are closed. Whenever fine wires are used in Pickering Electronics relays, the termination leads from the coils are skeined with several strands of wire twisted together to increase their physical strength.
While Figure 2 shows only a few turns of wire, in reality there can be hundreds, thousands, or even tens of thousands of turns.
Packaging Reed Relays
Most Pickering reed relays are constructed using former-less coils, which dispense with the usual supporting 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, single-in-line, and surface-mount.
Often, reed relays are molded using very hard materials that can cause stresses on the delicate glass/metal seal of the reed switch capsule with the risk of damage. Pickering, instead, uses a soft inner encapsulation, which provides a buffer to protect the switch. Without this, stresses can distort the reed switch slightly, thereby changing the contact area and degrading performance 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 the highest level of packing density.
The second article in this two-part series will look at the future of reed relays and compare them with other switching technologies.
The original article can be seen here.
About the author:
Graham Dale began his career in telecommunications with the U.K. Ministry of Defense. In the 1970s while working on a research project at The University of Essex sponsored by the British General Post Office (later to become British Telecom), he met John Moore, the founder of Pickering Electronics. This was in the early days of computer-controlled switching systems, and the project became a user of Pickering reed relays. Dale started working for Pickering at the beginning of 1975 and remains with the company today as a technical director and applications engineer.