Lessons · Industrial control · how a seal-in holds
How a seal-in holds
The coil's own contact, in parallel with the start button, keeps the rung true after the button is let go.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
It is how a momentary button runs a motor, and it is the oldest pattern in ladder.
How to think about it
One scan of Start writes the coil. On the next scan the coil's own contact is made, so the rung is true without Start. The stop, in series with the whole thing, is what ends it.
Worked example
{ [Start] | [Motor] } [/Stop] (Motor)Start pressed on scan 2, Stop held on scan 3 and still held with Start pressed again on scan 4. Motor: 0 1 0 0Your turn
What holds the rung after Start is released?
the own contact
Run a rung and watch the scan
The trap
Releasing Stop does not restart it. There is nothing left holding the rung true, which is exactly what makes the pattern safe.
Practise how a seal-in holds on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.