Start–Stop holding circuit: why does it run after Start is released?

1. Get familiar with the circuit before reading the diagram

Imagine controlling a pump with Start and Stop buttons. You want one press of Start to run the pump, rather than holding the button all day.

This example uses a 24 VDC control supply and a compatible KM1 contactor coil. We examine only the coil control circuit; motor power and protection are outside this diagram.

Start is normally open (NO): open when released. Stop is normally closed (NC): closed when released. The KM1 NO auxiliary contact closes when the KM1 coil is energized.

2. Which path carries current when Start is pressed and released?

Pressing Start completes the path: +24 V → Stop → Start → KM1 coil → 0 V. KM1 picks up and closes its auxiliary contact.

Releasing Start opens the button path. A second path is now available: +24 V → Stop → KM1 auxiliary contact → KM1 coil → 0 V. This keeps the coil energized and is called holding or seal-in.

Pressing Stop opens the common supply path to both branches. The coil drops out and its auxiliary contact opens. Releasing Stop does not restart the circuit; you must press Start again.

Look at both ends of the KM1 branch: it bypasses only Start. If it bypasses Stop as well, the circuit may hold but Stop will not interrupt it.

3. Understand the circuit one action at a time

Before any button is pressed, Start and the KM1 auxiliary contact are open, so the coil is unpowered. Start creates the initial supply path; the auxiliary contact then closes to maintain it.

Use the table to compare each action. Pay particular attention to releasing Start and releasing Stop: Start release retains holding, whereas Stop release after stopping leaves the circuit idle.

4. Build this circuit in EleRun

You do not need many devices to test the principle. Begin with a supply, two buttons, a coil, and an auxiliary contact.

If the coil picks up but the auxiliary contact does not close, inspect the symbol type and linking tag. Do not add wires to compensate for a contact that is not behaving correctly.

1. Place Start NO, Stop NC, the KM1 coil, and a NO auxiliary contact.
2. Assign the same KM1 tag to the coil and auxiliary contact in device properties.
3. Wire the supply through Stop and Start to the coil, then connect the supply return.
4. Wire the KM1 auxiliary contact in parallel with Start: branch beginning after Stop and ending before the coil.
5. Switch to Run. Press Start, release Start, press Stop, and release Stop. Compare with the table.

5. Try a few more situations to check your understanding

Holding after Start is a useful result, but test stopping and power restoration too. The table lists six scenarios to examine.

Loss of control power releases the coil and opens the holding contact. If power returns with Start released, the coil does not restart automatically. If Start is held or stuck closed, behavior may differ because the Start path remains available.

In this diagram, pressing Start and Stop together opens the common path at Stop, leaving the coil unpowered. If the tool cannot reproduce a particular action directly, trace the current path and explicitly record that it was not simulated.

6. Three symptoms that point to where you should look

Each fault gives a clue. Inspecting the relevant branch is faster than moving wires all over the drawing.

• Stops on Start release: inspect the holding branch, its endpoints, and the KM1 auxiliary contact.
• Still holds when Stop is pressed: find a coil supply path bypassing Stop.
• Picks up as soon as power is applied: check whether Start or the holding branch is already closed.