Self-holding circuit — trace start, hold and stop
Try the contact-logic exercise on this page, no sign-in ↓
The goal is to explain a circuit that keeps running after START is released but always drops out on STOP. Read NO/NC contacts and the relay pin guide first. This is a virtual exercise, not an instruction to connect live equipment.
The example and its labels
The homepage trial uses green PB1 for START, red PB2 for STOP, relay R and white lamp WL. Other exam circuits may call STOP PB0. The example uses coil terminals 2/7 and holding contact 8/6. L1 and L3 are different supply phases; L3 is not a neutral conductor N.
Read the connections in order
- Start path: L1 passes through PB1 NO, PB2 NC, and the R coil to L3. Before PB1 is pressed, this path is open.
- Hold path: R contact 8/6 spans the same two nodes as PB1. Once R energizes, this parallel path closes and keeps the coil supplied after PB1 is released.
- Stop path: PB2 sits between the junction of the two branches and the coil. Both possible supply paths must pass through it.
- Indicator path: A second NO contact controls WL. The lamp shows the result; it is not the contact that provides the holding path.
STOP could instead sit before the parallel branches. What matters is whether it interrupts every supply path to the coil, not its position on the page.
Predict each state
| Action | R coil | Holding contact | WL | Explanation |
|---|---|---|---|---|
| Power only | OFF | Open | OFF | Neither start nor hold path is closed |
| Press PB1 | ON | Closed | ON | Start path energizes R |
| Release PB1 | ON | Closed | ON | R bypasses the start button |
| Press PB2 | OFF | Open | OFF | Common path is interrupted |
| Release PB2 | OFF | Open | OFF | Another start command is needed |
Diagnose three mistakes
| Symptom | Possible cause | Check in the virtual circuit |
|---|---|---|
| Drops out when START is released | Missing or wrong holding contact | Does R 8/6 connect across the same two nodes as PB1? |
| Will not start at all | NO holding contact placed only in series | Is there a start path around the initially open contact? |
| STOP cannot turn it off | Holding branch bypasses STOP too | Does PB2 interrupt both paths? |
Switch simulation power off before editing. See terminal wiring for routing connections through the same electrical node.
Exercise and answer
Question: Replace PB2 NC with an NO contact in the same position. Can PB1 alone start the circuit?
Answer: No. The new stop-position contact is open at rest, so pressing only PB1 leaves the series path incomplete. Check the series path before adding more holding contacts.
Try it without signing in
Open the homepage trial circuit →
- Check that the trial is in simulation mode with power enabled.
- Press and release green PB1. WL should stay on.
- Press and release red PB2. WL should stay off.
- Press PB1 again to restart. This link opens the homepage trial, not a complete exam project.
References and scope
This lesson describes the site's original homepage example; its state transitions have been checked through the simulation engine. Actual coil ratings and terminal arrangements depend on the product. See the relay lesson for reading manufacturer specifications. For exam wiring requirements consult the relevant edition of Q-Net's public problems.
Test the contact logic here
Open the interactive exercise below →. No sign-in is needed; the instructions and results explain exactly which part of the circuit is being modeled.
Try without signing in: Start, hold, stop and wiring faults
Try START → release → STOP with normal wiring. Select a wiring fault and repeat. Changing the fault resets the exercise.
A contact-logic learning model, not a hardware wiring or ratings check. Active buttons are highlighted.
- R coil / WL
- OFF
- Holding path
- Open
Read the holding-path explanation · Open the homepage wiring simulator