8-Pin Relay Diagram: How to Wire an Octal DPDT Ice-Cube Relay
This is a free printable 8 pin relay diagram: download the diagram as SVG or open it and print to paper or PDF.
An 8-pin relay is a double-pole double-throw (DPDT) octal device with 2 coil pins and 6 contact pins, used to switch two independent circuits simultaneously from a single control signal.
The 8-pin octal relay — often called an ice-cube relay because of its transparent square housing — is one of the most widely used general-purpose switching devices in industrial control panels, HVAC systems, and automation circuits. Understanding its pinout is essential before wiring any control circuit.
The 8 pins divide into two functional groups. Pins 2 and 7 are the coil terminals: apply the rated control voltage (commonly 12 V DC, 24 V DC, 24 V AC, or 120 V AC depending on the relay variant) across these two pins to energise the electromagnet and actuate the contacts. Polarity matters only on DC coils — check the datasheet for the correct polarity mark, which is usually a diode symbol or a plus (+) on pin 2.
The remaining 6 pins form two independent SPDT (single-pole double-throw) switching circuits:
- Pole 1: Pin 1 = Common (COM), Pin 3 = Normally Closed (NC), Pin 4 = Normally Open (NO) - Pole 2: Pin 5 = Common (COM), Pin 6 = Normally Closed (NC), Pin 8 = Normally Open (NO)
When the coil is de-energised, current flows from COM to NC on each pole. When the coil is energised, the armature pulls down and current is routed from COM to NO instead — the NC contact opens and the NO contact closes simultaneously.
The octal socket (8-pin round base) allows the relay to be inserted and removed without disturbing wiring. The socket itself is wired once and permanently mounted; the relay body snaps in and is retained by a locking clip. This design makes field replacement straightforward, which is why these relays dominate machine control and building automation applications.
Always match the coil voltage rating exactly to the control supply. Using a 24 V relay on a 12 V supply will result in unreliable or no actuation; over-voltage will overheat the coil. On DC circuits, always fit a freewheeling diode (flyback diode) across the coil terminals to suppress the inductive voltage spike when the coil is de-energised, protecting transistors or PLCs driving the relay.
How to wire 8 pin relay diagram
- Identify coil voltage and contact rating Read the relay body label to confirm coil voltage (e.g. 24 V DC), coil current, contact current rating (e.g. 10 A), and contact voltage rating (e.g. 250 V AC). Select a relay whose ratings exceed your application requirements.
- Mount and wire the octal socket Secure the 8-pin octal socket to the DIN rail or panel. Wire the load circuit to the contact pins on the socket. Pole 1: socket positions for COM (pin 1), NC (pin 3), NO (pin 4). Pole 2: COM (pin 5), NC (pin 6), NO (pin 8). Terminate control wiring to coil positions (pins 2 and 7).
- Install the flyback diode on DC circuits For DC coil relays, solder or clip a 1N4007 diode in reverse-parallel across the coil terminals on the socket (cathode to the positive coil terminal, pin 2). This suppresses inductive kickback when the control signal switches off.
- Insert the relay body into the socket Align the relay body pins with the socket holes — the mechanical key prevents incorrect insertion. Press firmly until the relay seats fully, then snap the retaining clip over the relay body to prevent vibration-induced loosening.
- Apply control voltage and verify operation Apply rated coil voltage across pins 2 and 7. The relay should operate audibly (a soft click). Use a multimeter in continuity or resistance mode to confirm that COM (pin 1) has switched from NC (pin 3) to NO (pin 4), and similarly for pole 2.
- Verify load circuit continuity With the relay energised, test continuity or voltage at the NO contact load terminals. With the relay de-energised, verify that the NC contact is closed and NO is open. Confirm both poles switch correctly before applying full load current.
- Label and document the installation Apply a tag to the relay and socket identifying the coil voltage, load circuit controlled, and socket wiring reference. Update the panel schematic to reflect the installed relay type, tag number, and pinout.
Specifications
| Relay type | DPDT (Double-Pole Double-Throw), octal 8-pin |
|---|---|
| Coil terminals | Pins 2 and 7 |
| Pole 1 contacts | COM pin 1, NC pin 3, NO pin 4 |
| Pole 2 contacts | COM pin 5, NC pin 6, NO pin 8 |
| Typical coil voltages | 12 V DC, 24 V DC, 24 V AC, 120 V AC, 240 V AC (model-dependent) |
| Typical contact rating | 10 A at 250 V AC or 30 V DC (resistive load) |
| Typical coil resistance (24 V DC) | Approximately 720 Ω (verify against specific relay datasheet) |
| Operating temperature range | -40 °C to +70 °C (typical; check manufacturer datasheet) |
Safety warnings
- Always isolate and verify dead (lock-out/tag-out) the load supply before wiring or replacing a relay. Confirm absence of voltage with a calibrated tester before touching any terminals. Follow IEC 60364, NEC/NFPA 70, BS 7671, or the applicable local electrical code.
- Do not exceed the relay contact voltage or current ratings. Overloading contacts causes arcing, contact welding, and fire risk. Always fuse the load circuit at or below the relay contact rating.
- On DC coil relays, always install a flyback diode across the coil. Omitting this diode can destroy the switching transistor, PLC output, or microcontroller pin driving the relay.
- Ensure the relay coil voltage exactly matches the control supply voltage. Over-voltage causes coil overheating and insulation failure; under-voltage causes unreliable or no actuation.
- In high-inductive load applications (motors, solenoids), fit a snubber circuit (RC network) or varistor across the load contacts to reduce contact erosion from switching arcs.
Tools needed
- Digital multimeter (continuity and voltage measurement)
- Flathead and Pozidrive screwdrivers (terminal screws)
- Wire ferrule crimping tool
- Wire strippers
- DIN rail cutter or hacksaw
- Insulation tape or heat-shrink tubing
- Voltage tester or non-contact tester
Common mistakes
- Confusing NC and NO contacts: failing to verify which contact state is needed for the application before wiring. Always test with a multimeter before applying load power.
- Omitting the flyback diode on DC coil circuits, causing repeated failure of the driving transistor or PLC output module.
- Inserting the relay body before completing socket wiring, resulting in energised terminals being touched during wiring.
- Using the wrong coil voltage variant — e.g. wiring a 120 V AC relay to a 24 V DC control supply — causing no operation or relay damage.
- Failing to retain the relay in its socket. Without the retaining clip, vibration in industrial environments can cause the relay to partially unseat and intermittently fail.
- Overloading the contacts by switching a load exceeding the relay's rated current, leading to contact welding and circuit faults.
Troubleshooting
- Relay coil energises but contacts do not switch
- Cause: Mechanical failure of the armature or contacts welded from previous overload Fix: Replace the relay body. Inspect the load circuit for the overcurrent condition that caused contact welding. Add or reduce the load fuse rating to match the relay contact specification.
- Relay will not energise despite control voltage applied
- Cause: Incorrect coil voltage, blown control fuse, broken coil winding, or poor socket connection Fix: Measure voltage directly at the socket coil terminals (pins 2 and 7). If voltage is correct, measure coil resistance — an open coil reads infinite resistance. Replace the relay if the coil is open-circuit. Check and replace the control fuse if blown.
- Relay chatters or hums continuously when energised
- Cause: AC coil relay operated at low voltage, or DC coil relay receiving ripple-heavy supply; on AC types, a damaged shading ring Fix: Measure coil supply voltage under load. Ensure AC supply voltage is within relay tolerance (typically ±15% of rated). For DC relays, add filtering capacitance to the control supply. Replace AC relay if shading ring is damaged.
- Intermittent switching under vibration
- Cause: Relay body not fully seated in socket, or retaining clip missing Fix: Power down the circuit, reseat the relay body firmly, and secure the retaining clip. Inspect socket terminal screws for tightness.
- Control transistor or PLC output fails repeatedly when relay is switched off
- Cause: Flyback diode absent or incorrectly oriented — coil inductive spike reaches driving device Fix: Fit a 1N4007 diode in reverse-parallel across the coil socket terminals (cathode to positive). Verify orientation with a multimeter diode test before powering up.
Frequently asked questions
What is the difference between an 8-pin and 11-pin relay?
An 8-pin relay is DPDT (2 poles, 3 positions each = 6 contact pins + 2 coil pins). An 11-pin relay is 3PDT — three independent SPDT poles (9 contact pins + 2 coil pins) — providing an extra switching circuit. Both use octal-style round sockets but the socket bases are not interchangeable.
Which pins on an 8-pin relay are the coil pins?
Pins 2 and 7 are the coil terminals on a standard 8-pin octal relay. Apply the rated control voltage across these two pins to energise the relay. On DC coil types, observe polarity: pin 2 is typically positive.
Can I use an AC coil relay on a DC supply?
No. AC and DC coil relays are wound and rated differently. An AC coil has a shading ring to prevent chatter at the zero crossings of the AC waveform. Using an AC coil on DC will cause the relay to hum excessively or fail to pick up. Always use the coil voltage type specified on the relay body.
What is a flyback diode and why is it needed with a relay?
A flyback (or freewheeling) diode is placed in reverse-parallel across the relay coil. When the coil is de-energised, the collapsing magnetic field generates a large voltage spike that can damage transistors or PLC output cards. The diode provides a safe path for this inductive current to dissipate.
What does 'ice-cube relay' mean?
The term 'ice-cube relay' refers to the transparent square or rectangular plastic housing of plug-in control relays, which visually resembles an ice cube. The housing allows the internal contacts and armature movement to be inspected without dismantling the device.
Related diagrams
- 8 pin timer relay wiring diagram
- 11 pin relay wiring diagram
- 12v 5 pin relay wiring diagram
- 12v relay wiring diagram 4 pin
- 14 pin relay diagram
- 2 pin flasher relay wiring diagram