Self-holding contactor circuit

The self-holding (latching) circuit lets a momentary Start button energise a contactor that then keeps itself energised through its own auxiliary contact, until Stop is pressed. It is the basis of almost every motor control circuit.

What you will build

A 24 V DC control circuit: fuse F1, stop button S0 (normally closed), start button S1 (normally open) in parallel with the holding contact K1 13-14, the coil of contactor K1, and a second branch with contact K1 23-24 and indicator lamp H1.

Steps

1. New sheet. Open the app, click New → Blank drawing.
2. Supplies. From Supply place L+ at the top and M at the bottom (choose the type in the properties). Points with the same name are connected, so you can place M twice, one under each branch.
3. Fuse. From Protection place a fuse under L+. It is named F1 automatically; leave 6 A.
4. Stop button. From Operators place a push button and set its contact to NC in the properties. It becomes S0 — rename it if needed.
5. Start button. Place another push button, NO, below S0: S1.
6. Holding contact. From Contacts place an auxiliary contact NO next to S1 and give it the name K1; its terminals become 13-14.
7. Coil. From Relays & timers place a relay coil below: K1 (24 V DC).
8. Wires. Drag from terminal to terminal: L+ → F1 → S0 → S1 → K1 coil → M. Connect the holding contact K1 13-14 in parallel with S1 (same two nodes).
9. Lamp. Place a second auxiliary contact named K1 (it gets 23-24) from the node after S0, then a lamp H1 (24 V DC) from Signalling, and wire it to M.
10. Check. Click the design check button: it should report no unconnected terminals.

Simulate

1. Press F5. Nothing is energised yet; S0 is closed, S1 open.
2. Click S1: coil K1 pulls in, contacts 13-14 and 23-24 close, lamp H1 lights. Release — the contactor stays on through 13-14. That is the self-holding.
3. Click S0: the circuit is interrupted, K1 drops out, the lamp goes off.
4. Try a fault: wire L+ directly to M on an empty part of the sheet and start the simulation — F1 blows and reports the short circuit. Click the fuse to replace it.

Variations to try

• Replace the relay coil with a contactor and add its main contacts in a power circuit with L1 L2 L3, a motor protection breaker and a three-phase motor.
• Add a thermal overload relay with its 95-96 contact in series with S0.
• Do the same with a PLC: S0 and S1 on inputs, K1 on an output, and the self-holding in Ladder — see PLC and Ladder.