Comparing reed relays with other relay technologies
Electro mechanical relays (EMRs)
Electro Mechanical Relays 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 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 are often enclosed in plastic packages which 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 arts in reed relays compared to EMRs.
Reed relays require less power to operate the contacts than EMR.
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 may also increase wear on the contact plating.
EMRs can have much higher ratings than reed relays because they use larger contacts, reed relays are usually limited to carry currents of up to 2 or 3 Amps. Because of their larger contacts EMRs can also often better sustain current surges.
EMRs typically have a lower contact resistance than reed relays because they use larger contacts and can normally 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 relays
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 MOSFET 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 also non-linear. The on resistance can also be non-linear, 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 nF 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 (Micro Electro-Mechanical) switches
MEMs switches 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 micro machined (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 availability in commercially viable volumes at the time of writing is very limited. The technology challenges involved 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 leads 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.
About the author
Kevin Mallet is Product Manager at Pickering Electronics UK – www.pickeringrelay.com
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