Introduction to Industrial Automation

2.1.7   Latching  Relays

A latching relay, after its activation, maintains its contact position although the coil power supply 

has been removed, and therefore has two relaxed states as an electronic, bistable flip-flop. As we have 

seen from the description above, the classical relay should have a coil voltage applied to it at all times 

that it is required to stay energized. Such a situation is not necessary in latching relays, where their 

contacts are mechanically or magnetically locked in the ON state until the relay is reset manually 

or electrically. Mechanical latching relays use a locking mechanism to hold their contacts in their 

last set position until commanded to change state, usually by means of energizing a second coil. 

Figure 2.11 shows a simplified schematic of a latching relay operated by two coils, each one with a 

corresponding control button. The relay is energized or “set” by pressing the ON button and deacti-

vated or “tripped” by pressing the OFF button. Due to the mechanical latching, the locking strength 

will not be reduced over time and the self-consumed power of the relay is null. Some conventional 

power relays (contactors) can be converted into latched contactors by adding a block containing the 

mechanical latching device with electromagnetic impulse unlatching or manual unlatching.

Magnetic latching relays are typically designed to be voltage polarity sensitive and hence can be 

driven directly from a reversible power supply. They require one short pulse of coil power to close 

their NO contacts, and another redirected short pulse to open them again. Repeated pulses from the 

same input have no effect. Magnetic latching relays can have either single or dual coils. On a single 

coil device, a permanent magnet is designed to hold the contacts in the energized position, while the 

C NC NO