8-pin relay — read the coil, contacts and socket orientation
Try the contact-logic exercise on this page, no sign-in ↓
The goal is to separate the terminals that power the coil from those that switch the load. Start with NO and NC contacts. A relay moves contacts with an electromagnet rather than your finger.
Numbering used in this lesson
| Role | First set | Second set |
|---|---|---|
| Coil | 2/7 | Same operating coil |
| COM | 1 | 8 |
| NO | 3 | 6 |
| NC | 4 | 5 |
These numbers describe the site's model and the referenced exam diagram. A circular pin drawing and a socket's screw layout are different views. Check the viewing direction, locating key and printed numbers before mapping one to the other. Do not wire by memorizing left and right positions alone.
The coil and contacts are separate paths
- The correctly rated supply across 2/7 energizes the coil.
- COM 1 switches from NC 4 to NO 3.
- COM 8 switches from NC 5 to NO 6.
- Removing coil power returns this model to its resting state. Energizing a coil does not automatically supply power to the contact outputs: their circuit needs its own complete supply path.
| Coil state | Closed paths | Open paths |
|---|---|---|
| OFF | 1–4, 8–5 | 1–3, 8–6 |
| ON | 1–3, 8–6 | 1–4, 8–5 |
| OFF again | 1–4, 8–5 | 1–3, 8–6 |
Eight pins do not guarantee compatibility
The exam instructions specify using the supplied 8P sockets for several listed devices. That exam convention is not a guarantee that all real eight-pin devices can be interchanged. Timers, level controllers and flicker relays can assign different functions to their pins.
For hardware, check the exact model, AC/DC coil type and voltage, contact load rating, internal connections and compatible socket. For example, OMRON's MKS2P DC24 has a 24 V DC coil, and different NO/NC resistive-load ratings: 10 A and 5 A respectively at 250 V AC. This is a product-specific example, not permission to install it in the site's 220 V exam circuit. The same product family also includes different internal connection variants.
Diagnose a connection mistake
| Symptom | Possible cause | Check |
|---|---|---|
| Coil on, lamp off | No supply to COM | Trace contact-side supply and return separately |
| Output appears inverted | NC used instead of NO | Compare terminal numbers with the state table |
| Socket layout appears reversed | Wrong viewing direction or model variant | Read printed numbers and that model's internal diagram |
Exercise and next step
Question: A lamp's complete circuit passes through COM 8 and NC 5. What happens when the coil turns on? Answer: The lamp turns off: 8–5 opens while 8–6 closes.
Open the homepage relay trial →. Press and release green PB1, then stop with red PB2. The trial uses an NO holding path. Follow the complete explanation in the self-holding lesson.
References
- Q-Net public problems, 2025-08-04 edition: problem 14, page 11-9 for component diagrams and 11-3 for socket instructions. Check subsequent official amendments.
- OMRON MK-S datasheet: page 3 for coil/contact ratings and page 7 for bottom-view internal connections. This manufacturer example is separate from the exam's specified equipment.
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: Coil and contact paths
Predict which path will close before energizing the coil. A load still needs a separate complete supply path through the contacts.
A contact-logic learning model, not a hardware wiring or ratings check. Active buttons are highlighted.
- COM 8 → NO 6
- Open
- COM 8 → NC 5
- Closed
Read the holding-path explanation · Open the homepage wiring simulator