The 5 important things to consider when selecting a relay

Although newer technologies have replaced electromagnetic relays in some applications, electromechanical relays still have a lot to offer. These robust and versatile relays suit high voltage applications or those that require galvanic isolation between terminals. They come in almost every conceivable variety for signal and power applications. Their large, robust contacts mean they can withstand voltage surges, but it makes them comparatively slow to switch (5 to 15ms) and larger in size overall. Latching types are also available which save energy by only putting a short pulse through the coil to switch; they don’t require energy to maintain their switched state, only to switch.

Reed relays, which have thinner contacts, can offer faster switching (in the order of ten times faster), but they are susceptible to arcing in certain conditions, which damages the contacts. They have longer mechanical lifetimes than other types of electromechanical relay, but are generally not recommended for low voltage applications as they can produce a thermal EMF at high temperatures that could interfere with a low voltage signal.

For low voltages, semiconductor relays switch faster

Solid state relays (SSRs) use a MOSFET as the switching element, doing away with any mechanical parts to improve reliability and eliminate audible noise. Typically they use optical isolation – the control circuit turns an LED on or off and the photo-sensitive MOSFET switches accordingly. This is much faster (1ms or less, limited by the LED switching time), but although there is isolation between the control side and the switching side, there is no isolation between the contacts. Contact resistance is significantly increased, compared to electromechanical types, as it’s based on the MOSFET’s on-resistance. These components are often used in high reliability, long life applications.

Other types of semiconductor relays include FET switches, which basically use a series of CMOS transistors, without any isolation. These are the fastest to switch of all the relay types and also the smallest overall, offering long lifetime but at the expense of relatively high on-resistance, as with SSRs.

MEMS relays combine some of the properties of both types

MEMS has been proposed as a solution that potentially combine the features of electromechanical and semiconductor relays. In premise, it’s basically a very tiny electromechanical switch on the scale of CMOS devices, though it uses electrostatic forces to bend the tiny contacts together, not electromagnetism. They can be made very small and dense, with good linearity – perfect for the switching of RF signals. They also offer excellent power consumption as it takes very little energy to switch or maintain the on state. However, isolation, reliability and cost remain the main issues, with moisture ingress and mechanical weakness close behind.

Reliability figures are prone to specmanship

Like most figures of merit for electronic components, reliability figures for relays are prone to specmanship (manipulation of data to improve the specifications of a product or process). It pays to look closely at how the manufacturer is presenting its reliability figures and make sure you are comparing apples with apples.

Manufacturers test their relays by selecting a sample representative of each batch they produce and testing them until they fail. This testing should be carried out at the rated load, at the device’s maximum operating temperature. Typically they test until they reach the minimum rated life of the relay – then any failures in the sample size are used to calculate the overall reliability. Make sure to check that the sample size is big enough (no ‘projecting’ from small sample sizes), and check the operating conditions for the tests. Relays should be tested at maximum operating temperature and at full load, but this may not be consistent across manufacturers. What condition is defined as a failure? For multiple pole/throw relays, check how one cycle or one operation is defined too.

Reliability figures don’t reflect real life

Of course, the figures you find in the data sheets don’t really reflect the reliability you’ll experience in real life applications. For a start, the quoted figure is not applicable for all load conditions. The operating conditions are also likely to be very different from the manufacturer’s test environment; real life systems may be subject to heat, dust, dirt, humidity and/or corrosive gases, which can cause problems. Electrical factors that reduce lifetime include the ratio of on time to off time (for SSRs), exposure to voltage transients from the line, and hot switching.

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