A Comparison of Electromechanical and MOSFET Relays

09/14/2022 Know-How

Analog Technology in the Digital Age

The world is changing at an ever-faster pace—and so, too, are electronics. Components are expected to provide ever more performance while offering smaller, more compact constructions, and need to satisfy ever more sophisticated technical specifications to boot. So that means with relays, we will be going with MOSFETs in the future instead of electromechanical designs—right? Much like other so-called “dinosaurs,” electromechanical relays have long been declared obsolete. However, they are still hanging around—they still have their advantages, just like the MOSFET relays that were meant to replace them. Therefore, the choice is dependent on the individual circumstances—we need to compare the most important parameters. A relay’s key parameters include service life, trigger current, switching speed, electrical isolation, and, of course, price.

Service Life

MOSFET relays have a longer service life than electromechanical designs, as they are not switched mechanically—they are activated by a light signal (LED) that is converted into an electrical voltage. The LED is the only factor here that limits the life of the relay.

In an electromechanical relay, on the other hand, applications involving high inrush currents for switching purposes can result in the contacts melting, which can cause the contact tabs to be fused. To prevent this, some manufacturers have developed special high-inrush relays. The G5RL from Omron, for example, can handle trigger currents of up to 100 A. This is achieved in part by the use of silver tin oxide (AgSnO₂) as a contact material. At the same time, it must be noted that inrush surges are not prevented by the use of high-inrush relays. The relay is only capable of handling these peak currents.

Trigger Current

A current as little as 0.2 mA, for example the G3VM from Omron (high-sensitivity types), can trigger MOSFET relays. With battery-powered applications, this ensures a long service life, and when using multiple MOSFET relays, it prevents mains overload. Electromechanical relays require a trigger current of at least 5 mA.

Switching Speed

In terms of switching speed, MOSFET relays clearly have the advantage, needing barely 0.2 ms while electromechanical relays require all of 5 ms.

Noise Levels

MOSFETs switch entirely silently as they are triggered by a light signal. In electromechanical relays, switching is performed—as the name suggests—mechanically. This results in a clicking sound. For applications where noise levels are a factor, however, there are low-noise relays available. The G5RL from Omron, for example, does not exceed 30 dB.

Electrical Isolation

Both relay types are electrically isolated, albeit with a difference—a MOSFET is only electrically isolated on the load terminal, while an electromechanical relay is isolated on both the load and trigger terminals. This might be a critical argument in safety-related applications in favor of an electromechanical relay.

Price

Electromechanical relays are currently cheaper than MOSFET models. However, if we incorporate the service lives into cost-of-ownership analyses, they will be more expensive in some applications due to the addition of maintenance costs.

Conclusion

Electromechanical relays continue to have a purpose, especially in safety-critical and cost-sensitive applications such as solar power generation products, energy storage facilities, and electromobility. Where other requirements play a key role, MOSFET relays have the advantage. This is why both technologies tend to supplement one another than compete against each other. The most important thing is to be aware of their respective advantages and disadvantages in order to make optimum use of the components.

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