PLC programming – Latching relays

The Latching relays are designed to keep the relay closed after the coil power has been disconnected. The usual relay logic opens the contact instantaneously after the coil is released.

Latching relays are used where it is necessary for contacts to stay open and/or closed even though the coil is energized only momentarily. The program elements that are assigned as Latching Relays will remain ON once they are energized. Only Relays and Outputs may be assigned as Latching Relays.

Latching instruction:

The latching relay uses two coils to accomplish the function as shown here:


  • When the ON button is momentarily actuated, the latch coil is energized to set the relay to its latched position.
  • The contacts close, completing the circuit to the pilot light, and so the light is switched on.
  • The relay coil does not have to be continuously energized to hold the contacts closed and keep the light on.
  • The only way to switch the lamp off is to actuate the OFF button, which will energize the unlatch coil and return the contacts to their open, unlatched state.
  • In cases of power loss, the relay will remain in its original latched or unlatched state when power is restored.

Example program:

Here, the output motor coil is latched. When the start button is pressed the motor coil turns ON as well the motor latch. Even the start is de-energised the motor coil stay ON until the stop button energise.

Latch relay:

An electromagnetic latching relay function can be programmed on a PLC to work like its real-world counterparts.

Latch-relay can be used in place of self-latching (Seal) circuits, a latch-relay in an interlock section will not be cleared when the interlock occurs.

Both the latch and unlatch outputs must have the same address. The OTL (latch) instruction can only turn a bit on and the OTU (unlatch) instruction can only turn a bit off. OTL sets the bit to “1” when the rung becomes true and
retains its state when the rung loses continuity or a power cycle occurs.

OTU resets the bit to “0” when the rung becomes true and retains it.


Instrumentation Engineer

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