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Relay Seal-In and Holding Circuits

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A relay can remember that START was pressed

The classic relay seal-in, holding, or self-holding circuit is one of the clearest physical examples of memory.

RELAY SEAL-IN / HOLDING CIRCUIT L+0V STOP NC START NO K1 auxiliary NO K1 coil K1(next) = STOP_OK • (START + K1) After K1 energizes, its own auxiliary contact provides a second path around START. Opening STOP breaks both paths and releases the relay.

Follow it one event at a time

1. Resting: STOP is closed, START is open, K1 is released.
2. Press START: current reaches the K1 coil and K1 energizes.
3. K1's auxiliary NO contact closes: this creates a path around START.
4. Release START: current still flows through K1's own auxiliary contact. The circuit has remembered START.
5. Press STOP: the NC STOP contact opens, K1 releases, and its auxiliary contact opens again.

Try the memory action

K1 RELEASED / STOPPED

The relay circuit as Boolean state logic

K1(next) = STOP_OK • (START + K1)

The final K1 on the right side represents the relay's present state feeding back into the decision about its next state.

This is feedback

The output of the relay affects an input path through its own auxiliary contact. That feedback is what lets the circuit preserve a state after the momentary START signal disappears.

Try these yourself

What is the purpose of the K1 auxiliary contact placed in parallel with the START pushbutton? RELAY SEAL-IN / HOLDING CIRCUIT L+0V STOP NC START NO K1 auxiliary NO K1 coil K1(next) = STOP_OK • (START + K1) After K1 energizes, its own auxiliary contact provides a second path around START. Opening STOP breaks both paths and releases the relay.
Once K1 energizes, its own NO auxiliary contact closes and provides a second path around the momentary START button.
Why is STOP normally closed in the usual seal-in circuit?
The normally closed STOP contact carries current during normal operation; pressing STOP opens the circuit and releases K1.
Which Boolean relationship best describes the holding circuit?
The coil can remain on when STOP_OK is true and either START is pressed or K1 is already holding itself on.
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