If you think optically isolated relays (a.k.a. optical MOSFET-based relays) are just for test-and-measurement applications, think again. These solid-state devices can also switch and protect small motors, power supplies, and control devices with load currents up to 6A.
In fact, these industrial uses represent the next wave of applications for optical MOSFET technology, which has been widely accepted as a way to switch high-precision data acquisition and measurement systems. These systems take advantage of the relays’ high switching speeds, low on-resistance, low capacitance, and tiny packages.
The same technical advantages apply to industrial devices, too. But motors, power supplies, and controls can reap other benefits by moving from traditional electromechanical relays to optical MOSFET solid-state relays.
A typical optically isolated relay requires 10 to 20 times less power than an equivalent electromechanical relay. For example, a 5mA optical MOSFET can often do the same job as an electromechanical relay that requires 50mA to 100mA, depending on the electromagnetic force needed to close the coil. A few milliamps here or there may not sound like a big deal, but in plants with many small devices, the savings add up quickly.
Some devices contain a built-in protective circuit allowing them to safeguard motors, power supplies, and other industrial devices from possible disturbances on the output side. These disturbances -- such as voltage peaks or overcurrent conditions -- can arise due to short circuits or improper use. The protective circuit is located on the output side of the component and recognizes high currents. This arrangement protects both the D-MOSFET on the output side and the load circuit against overcurrent conditions. As soon as a dangerous load current arises, the load circuit switches off completely. It can be switched on again only after the input signal has been reset.
The optically isolated protective circuit can play an important role when the relay must perform at elevated temperatures. Because the voltage drop across the shunt increases as rising temperatures drive up resistance in the component, the protective circuit responds to lower current levels as temperatures rise. In essence, it exhibits a negative temperature coefficient, which allows it to offset the increased power dissipation associated with elevated temperatures.
Solid-state relays really shine when it comes to reliability. With no moving parts, they typically have an excellent mean time to failure (MTTF). In general, solid-state relays tolerate shock and vibration loads that threaten electromechanical relays. Solid-state relays also eliminate the buzzing that can affect electromechanical relays driven by PWM and other methods intended to conserve input power.
Solid-state relays may cost more than electromechanical relays, but the total cost over the relay’s lifecycle tips the scales back in favor of solid-state technology. Most of the operating cost advantage comes from reductions in power consumption and a longer lifecycle for fewer relay replacements. Factor in the cost benefit of motor protection, and the value proposition becomes even more compelling. And keep in mind that the savings can be greater in applications that require the relay to remain in its closed state for long periods of time. Solid-state relays can be operated closed without the elevated temperatures and extra current draw of their electromechanical counterparts.
Finally, integrating the protective mechanism in the relay, rather than relying on a separate component, saves space. And it speeds development time because there’s one less component to work into your design.
— Aiman Kiwan is an engineering manager at Panasonic Electric Works of America. He has more than 20 years of experience and extensive knowledge in semiconductors with a particular focus on MOSFET and TRIAC products.