The automotive relay race

Compared to just a decade ago, the electrical systems within a modern automobile are staggering in number and complexity. This is set to grow even further as the leading OEMs look to incorporate more and more systems, including those that interface with the Internet. Indeed it was predicted that by 2014 the automotive industry would be manufacturing the third fastest growing connected consumer device after phones and tablets.

Cars used to use simple CANBus solutions (they still do, but they are much more extensive), which provided minimal feedback and very simple data transmission. Alongside this simple network was a completely separate in-car entertainment system, which often simply comprised a radio and a pair of speakers. These days OEMs have integrated everything together and are even deploying Ethernet and its derivatives to handle the sheer volume and speed of data required by modern driver-centric solutions, such as periphery cameras and sensors, lane guidance systems, entertainment and video systems.

For all these advances in technology, some of the basic core functionality of cars has remained unchanged and still relies on decades-old technology, such as the simple on-off switch delivered by relays, traditionally electromechanical models. However, with silicon playing an ever-increasing role in the modern car, it makes sense that solid-state relays could usurp their electromechanical peers; but sometimes it is not that simple.

Both variants have their advantages and disadvantages, and in most instances these pros and cons will have a direct correlation on their suitability for specific automotive applications. To gauge application capability, it is necessary to define their method of operation and their physical construction and then decipher how these features define their suitability.

In their simplest guise, relays are used to control higher-power circuits through the use of a lower-power signal. In a car this might be a signal from the engine management unit to start the cooling fan. The signal would come from a low voltage/low current controller but the action would take place in a much higher voltage/current circuit.

Electromechanical relays, such as those supplied by RS from Omron, Schneider Electric and Panasonic, use solenoids, in which an electro magnet (triggered and energised by the lower-powered circuit) moves an iron armature to close (or open, depending on the role/construction) a pair of contacts. This movement then makes (or breaks) a connection in the higher-powered circuit. Electromechanical relays used in automotive applications will sometimes also include a diode to dissipate any potential voltage spikes generated by the collapsing magnetic field, which could be detrimental to semiconductor components.



Figure 1: The Omron MY relays series is ideal for sequence control and power switching applications


Solid-state relays, including those available from RS supplied by TE Connectivity, Finder and Crydom, perform an identical role but do so without the use of any moveable components. Instead of a solenoid, solid-state relays typically use a light emitting diode (transmitter) coupled to a phototransistor (receiver) as the interface between the two circuits. In operation, the LED emits light, which is reflected onto photo diodes, which generate a voltage, which is then used to trigger the gates within two MOSFETs (metal oxide semiconductor field-effect transistors) bridging the higher-power circuit.



Figure 2: Example of a solid-state relay from Crydom


Both have operational restrictions and limitations in automotive applications. Electromechanical relays can face contact degradation caused by arcing and they can also ‘bounce’ when triggered causing initial voltage fluctuations. Like all moving mechanical components, wear and tear becomes an issue and their operational life is finite, depending on the frequency and ‘severity’ of actuation. When matched to their application though, in terms of voltage and operation, they are incredibly capable devices as illustrated by their widespread use throughout industry.

A major issue with solid-state relays is that they are never completely on or off. While ‘on’, heat is generated by the current flow and resistance, so they may need to rely on heat sinks that can be many times the size and weight of the device. This means that remedial design decisions have to be made in applications where significant external heat is present, such as in an engine bay, or where less-than-ideal heat sinks have to be deployed.

Another immediate difference in the two types of devices, and arguably the first feature used to define their deployment, is package size. The size of an electromechanical relay is defined by the size and arrangement of its internal mechanical components. In comparison, a solid-state relay is only limited by the size of the semiconductor components and can therefore be significantly smaller.

Solid state relays are also much faster in operations than electromechanical relays, with some capable of switching in milliseconds; and their lack of moving parts means less wear and none of the contact-degradation issues faced by electromechanical relays. This immunity to wear also elevates solid-state relays on the reliability scale. They are also less sensitive to environmental factors such as vibration, humidity and external shocks; and although they are less affected by external magnetic fields they may be falsely triggered by transients.

From a purely electrical perspective solid-state relays also offer higher input to output isolation voltages than electromechanical relays, while electromechanical relays typically have an output capacitance of less than 1 pF, making them more suitable for high-frequency applications.

Although they both have significant advantages and disadvantages, the higher reliability and longer service life of solid-state relays is leading to much wider deployment in many industries, including automotive. Variants are expanding and operational characteristics are always changing as technology evolves meaning that the devices will only get better, further increasing their application capabilities.