This short video explains the presence of thermal electromotive force (EMF) voltages within reed relays.
In applications, where very low signal levels are present or very low offset voltages, are required, thermal EMF voltages generated across the reed switches can introduce unacceptable errors where accurate measurements are important.
Understanding how thermal EMF voltages are generated, what affects the amplitude and the best way to use reed relays where this is critical, will help the user to select the best relay for their application.
Looking for additional information on reed relay technology?
Pickering has published an educational book, the Reed RelayMate, which provides an overview of how reed relays work, how they are constructed and how to interpret their specifications and make best use of them in their applications.
- Reed relay basics
- Comparing reed relays with other relay technologies
- Packaging reed relays
- Manufacturing test of reed relays
- Choosing a reed relay
- Placing and driving a reed relay coil
- Loading reed relays
- Understanding specifications
- Relay terminology
To view or download your free copy click here.
If you found this video helpful / interesting please take a look at our other useful videos which can be found here.
About Pickering Electronics
Pickering Electronics was formed over 50 years ago to design and manufacture high quality reed relays, intended principally for use in Instrumentation and Test equipment. Today, Pickering’s Single-in-Line (SIL/SIP) range is by far the most developed in the relay industry with devices 25% the size of many competitors. These small SIL/SIP reed relays are sold in high volumes to large ATE and Semiconductor companies throughout the world.
Pickering Electronics is part of the privately-owned Pickering Group; made up of three electronics manufacturers, including Pickering Interfaces who design and manufacture modular signal switching and simulation products, and Pickering Connect who design and manufacture cables and connectors.
We join in the UK Nation’s sadness at the news of the death of Her Majesty The Queen. We extend our deepest condolences to the Royal Family and everyone mourning her loss around the world.
Pickering will be closed on Monday 19th September for the Queen’s State Funeral. We aim to respond to any enquiries within 2 working days once we re-open on Tuesday 20th September.
Pickering Electronics launches Series 113RF screened relays for digital switching systems that demand low RF losses and low insertion loss
Pickering Electronics, the reed relay company which has pioneered miniaturization and high performance for over 50 years, has launched a new miniature coaxial reed relays for high frequency RF systems up to 3GHz. Series 113RF SIL/SIP reed relays feature a 2mm spacing footprint, enabling them to be stacked at very high densities.
When asked “When would reed relays be the best solution for RF and high speed, digital switching systems?” Kevin Mallett, Technical Specialist at Pickering Electronics answered: “These small, screened reed relays are faster and smaller than electromechanical relays (EMR), have a lower insertion loss and better DC capabilities than solid state relays (SSR), and better hot switching performance than micro-electromechanical machine (MEM) products.”
Series 113RF reed relays are suitable for switching up to 10W, 0.5A. 1 Form A configurations (SPST normally open) are available with 3V or 5V coils with coil resistances of up to 100/300 ohms respectively.
At low levels, the typical life expectancy of Series 113RF reed relays is greater than 250 million reliable operations. The relays employ the highest quality instrumentation grade reed switches with sputtered ruthenium contacts and are ideal for automatic test equipment.
All Series 113RF reed relays feature an internal mu-metal magnetic screen to enable high-density stacking of relays without the risk of adjacent devices interfering with each other resulting in faulty operation.
Pickering offer over a thousand standard catalogue reed relays, but for those that require something a little more bespoke, the company offers many standard build options, all the way to a fully customized part, depending on requirements. For example, if you are interested in the 113RF but require a different footprint, specific coil voltages or resistance figures the company would be happy to work with the designer to meet the exact specification request.
Reed Relay Basics
Reed relays contain a reed switch, a coil for creating a magnetic field, an optional diode for handling back EMF from the coil, a package and a method of connecting the reed switch and the coil to the outside of the package. The reed switch is a simple device and relatively low cost to manufacture.
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 serves to both hold the metal blades in place and to provide a hermetic seal that prevents any contaminants from entering the critical contact areas 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 make contact and electrical contact is made.
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 reed switch is considered to have no moving parts, and that means there are no parts that mechanically wear. 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 design variable on the reed switch is its size. Longer switches do not have to deflect the blades as far (measured by the angle of deflection) as short switches to close a given gap size between the blades. Short reeds are often 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.
Generating the magnetic field:
To create 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.
Different reed switches require different levels of magnetic field to close the contact, and this is usually quoted in terms of the ampere turns (AT) – simply the product of the current flowing in the coil multiplied by the number of turns. Again this creates a great deal of variation in the reed relay characteristics. 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 require more power.
The use of different wire gauges for the coil and number of turns creates relays with different drive voltage requirements and different 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 relays, the termination leads from the coils are skeined with several strands of wire twisted together to increase their physical strength.
Larger coils can be used to reduce power consumption, but that increases the size of the relay.
Part Two will continue to explain the wonders that are reed relays, including information on changeover reed switches and two pole relays.
Looking to browse our Reed Relay range?
From high voltage reed relay options to surface mount relays, find out more about our reed relays here