Latching vs. Failsafe Electromechanical Switches

The primary difference between latching and failsafe coaxial switches lies in how they react to a loss of electrical power. Failsafe switches require continuous current to remain in their energized state. If power is interrupted, an internal spring returns the contacts to their default, de-energized position. This behavior is critical in applications where a known default signal path must be established during system failures or power outages.

In contrast, latching switches use a mechanical mechanism or permanent magnet to hold their position without continuous power. When power is lost, a latching switch remains in its last commanded state. While this preserves the signal path during transient power disruptions, it means there is no automatic return to a default state during a total system shutdown.

Because failsafe switches require constant current to hold the energized position, they continuously dissipate power as heat. This heat generation raises the internal temperature of the switch, which can cause subtle physical expansions and affect electrical characteristics such as isolation and insertion loss.

Latching switches avoid this continuous heat generation because they only draw power during the brief period it takes to change states. For example, the N1810TL has a maximum switching time of 15 ms, and the 87104A transitions in less than 15 ms. To prevent continuous coil heating, latching designs often include internal mechanisms to cut off current. The 87104A features a self-interruption mechanism that automatically disconnects power after switching, while the 8762B includes a coil cutoff feature. This zero-power idle state ensures the switch remains cool, helping to maintain strict RF performance. For instance, both the N1810TL and 87104A guarantee a maximum insertion loss repeatability of 0.03 dB over a lifetime of 5,000,000 cycles.

Driving a failsafe switch is straightforward: applying voltage switches the state, and removing it reverts the state. The control driver only needs to sustain a constant DC voltage.

Latching switches require more complex control logic. They need a pulse of electrical power to set the switch to one position, and a separate pulse—often of opposite polarity or directed to a different coil—to reset it. Some latching configurations simplify this interface by offering optional TTL/CMOS control, as seen on the N1810TL. This allows standard digital logic levels to control the switching state, hiding the underlying complexity of driving the physical coils.