Industrial Electronic Thermostats – What is a Hybrid Switching Relay and Why is it Advantageous?

Electronic thermostats and controllers can use electromagnetic relays (EMR) or solid state relays (SSR) in their construction. Both have their own pros and cons. A hybrid switching relay leverages both EMR and SSR and ultimately takes the benefits of both while foregoing drawbacks of both.

What is a hybrid switching relay exactly?

A hybrid switching relay uses mechanical contacts and semiconductor components to achieve reliable and efficient switching, especially in applications that need long operational life and high power. EMRs use a coil of wire and a physical armature to create a magnetic field that closes or opens contacts, physically connecting or disconnecting a circuit. Whereas SSRs use semiconductor components like transistors to switch power without any moving parts, offering high speed switching and a long lifespan.

For a hybrid switching relay when switching on, the SSR is activated first to handle the transient switching stress, minimizing wear and electrical noise; followed by the EMR to handle the high-current load, minimizing the heat in the SSR and providing a low resistance path when closed. When switching off, the EMR contacts open first, followed by the SSR, to prevent arcing and damage.

Up until fairly recently, there were no standards or requirements for longer lifespan for thermostats. The International Electrotechnical Commission (IEC) recently changed that with the new IEC 60079 standard, and going forward all industrial thermostats need to be rated to a minimum of 250,000 power cycles. Mechanical thermostats today typically only offer a lifespan of 100,000 cycles as they traditionally only use an EMR. Conversely, solid state relays have an infinite life cycle so they meet this new IEC requirement, but come with their own drawbacks (namely the heat they generate and the requirement to have expensive heat sinks to dissipate this heat).

The EMR in a mechanical thermostat typically has two metal plates that either come together or apart, physically opening or closing the circuit. Once the mechanical thermostat closes these plates to energize the circuit, these two plates are very close together for a fraction of a second. While these plates are very close together, they may be subject to something known as micro arcing, where a small electrical arc bridges the gap of air between the plates which ultimately damages the plates. Since multiple of these micro arcs can occur every time the circuit is energized, over time the metal plates in the EMR will start to have little pits in them, and over time these pits will cause the relay to fail, which is why EMRs can only withstand 100,000 cycles. By using a hybrid relay – one is able to take the best features of both EMR and SSR. A hybrid relay can be set up where the SSR and EMR are wired in parallel and the programming of the thermostat dictates that whenever the circuit turns on, the SSR closes first for a fraction of a second, before the EMR also closes, and then when the circuit turns off, the same function happens in reverse. This allows the EMR to carry the majority of the heat over time and since EMRs don’t generate much internal heat a heat sink is not required. Additionally, since the SSR carries the current before the EMR closes, micro arcing is prevented, extending the lifespan of the relay.

Hybrid relays are a great solution as they provide many of the benefits of an EMR as well an SSR without the drawbacks.

The new Raychem Elexant 3500i electronic thermostat uses hybrid switching relay and meets the IEC 60079 standard which means it has 250,000 power cycles and thus a much longer design life than that of a mechanical thermostat. This can help reduce costly maintenance and operating costs for replacing failed mechanical thermostats. To learn more about the Elexant 3500i, please visit the product page here.