Comparing reed relays with electromechanical relays
07 November 2017
When Pickering Electronics was first established in 1968 as a reed relay manufacturer, some said that these devices would have a limited lifetime. Instead, the market for high-quality reed relays has reached into areas that were inconceivable in those days...
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There are other relay technologies available to users, with different characteristics to reed relays; for example, electromechanical relays (EMRs), solid state relays and MEMS (microelectromechanical systems) switches. Some applications are best served by these alternatives, while others are best served by reed relays. This article is intended to give some objective comparisons between reed relays and other switching technologies.
EMRs are widely used in industry for switching functions, and can often be the lowest cost relay solution available to users. Manufacturers have made huge investments in manufacturing technology to make these relays cost-effectively in high volumes.
There are some notable differences between reed relays and EMRs which users should be aware of, including the following:
• EMRs typically have a lower contact resistance than reed relays because they use larger contacts and normally use materials of a lower resistivity than the nickel iron used in a reed switch capsule.
• 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 2A or 3A. Because of their larger contacts, EMRs can also better sustain current surges.
• EMRs are designed to have a wiping action when the contacts close, which helps to break small welds and self-clean their contacts. Whilst this helps lead to higher contact ratings, it can also increase wear on the contact plating.
• 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; however, 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 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.
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 usually has better performance and longer life.
Other relay technologies
A solid-state relay refers to a class of switches based on semiconductor devices. There is a large variety of these switches available, and some (such as PIN diodes) are designed for RF applications; however, the most commonly-found devices that compete with reed relays are based on FET switches. A solid-state FET switch uses two MOSFET in series and an isolated gate driver to turn the relay on or off.
One issue, however, with all solid-state relays is that they have a leakage current associated with their semiconductor heritage, and consequently, they do not have as high an insulation resistance.
The leakage current is also non-linear – and the same may apply to the on-resistance, depending on load current.
Another relay technology to consider is MEMS (microelectromechanical systems) switches – although these are still largely in the development stage for general usage as a relay. MEMS switches are fabricated on silicon substrates, where a three-dimensional structure is micromachined (using semiconductor processing techniques) to create a relay switch contact. The contact can then 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 at the time of writing, availability in commercially viable volumes is limited. The technology challenges involved have resulted in a number of MEMS vendors failing – and either ceasing to trade or closing down their MEMS programmes.
Like reed relays, MEMS can be fabricated so the switch part is hermetically sealed (either in a ceramic package or at a silicon level). This 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.
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