The standard 8-pin octal control relay (such as the Omron LY2, Schneider RXM2, or Finder 40-series) uses pins 2 and 7 for the electromagnetic coil, pins 1, 3, and 4 for the first DPDT contact set, and pins 8, 6, and 5 for the second. Understanding the control relay wiring diagram requires translating the schematic symbols into physical socket terminals, respecting coil polarity on DC models, and isolating the control circuit from the load circuit.
Decoding the Symbols and Physical Terminals
On a schematic, the relay coil is drawn as a rectangle or a circle with the letter 'K' or 'CR' (Control Relay) inside. The contacts are drawn as standard switch symbols, but they are mechanically linked to the coil by a dashed line. A Normally Open (NO) contact is drawn with the switch blade open and pointing away from the contact point. A Normally Closed (NC) contact is drawn with the blade overlapping the contact point.
When you look at the physical 8-pin socket (typically wired to an 11-pin or 8-pin DIN-rail base like the Omron P2R-08), the pins are numbered in a specific U-shape or dual-row pattern. Here is the exact terminal mapping for a standard DPDT (Double Pole, Double Throw) 8-pin octal relay:
| Pin Number | Function | Schematic Symbol Equivalent | Physical Location (Standard Socket) |
|---|---|---|---|
| 2 | Coil Terminal 1 (+ for DC) | Coil Rectangle (Left/Top) | Top Row, 2nd from left |
| 7 | Coil Terminal 2 (- for DC) | Coil Rectangle (Right/Bottom) | Bottom Row, 2nd from right |
| 1 | Contact Set 1: Common (COM) | Switch Pivot Point | Top Row, Far Left |
| 3 | Contact Set 1: Normally Closed (NC) | Switch Closed Contact | Top Row, 3rd from left |
| 4 | Contact Set 1: Normally Open (NO) | Switch Open Contact | Top Row, Far Right |
| 8 | Contact Set 2: Common (COM) | Switch Pivot Point | Bottom Row, Far Left |
| 6 | Contact Set 2: Normally Closed (NC) | Switch Closed Contact | Bottom Row, 3rd from right |
| 5 | Contact Set 2: Normally Open (NO) | Switch Open Contact | Bottom Row, Far Right |
Node-by-Node Trace: 24V DC Coil with 120V AC Load
To understand how the control relay wiring diagram functions in reality, we will trace a specific, common industrial scenario: using a 24V DC PLC output to energize the relay coil, which in turn switches a 120V AC load (like a motor contactor or indicator light) using the dry contacts. We are using a 24V DC coil relay (e.g., Omron LY2N-D2) which contains an internal flyback diode.
The Control Circuit (Coil Side)
- Source to Switch: The 24V DC positive supply leaves the power supply (+V) and routes to the PLC output terminal or a manual momentary pushbutton switch.
- Switch to Coil Pin 2: The switched 24V DC positive travels to Pin 2 on the relay socket. Polarity Note: Because this DC relay has an internal flyback diode, Pin 2 must be positive. Reversing it will short the power supply through the diode and blow the control fuse.
- Coil Pin 7 to Return: The current flows through the coil's copper windings and exits Pin 7 (Negative). This wire routes back to the 24V DC power supply common (0V/GND).
The Load Circuit (Contact Side)
- AC Source to COM: A 120V AC hot wire from a separate branch circuit breaker lands on Pin 1 (Common for Contact Set 1). The relay provides galvanic isolation here; the 120V AC never touches the 24V DC coil.
- NO to Load: A wire runs from Pin 4 (Normally Open) to the A1 terminal of a 120V AC motor contactor coil.
- Load to Neutral: The contactor's A2 terminal wires to the 120V AC neutral bus.
- Ground Path: The Equipment Grounding Conductor (EGC) bonds the metal DIN rail, the relay socket's ground screw (if equipped), and the motor contactor chassis directly to the panel ground bar. The ground path never routes through the relay's switching pins.
Bench Verification: Proving the Circuit with a Multimeter
Before applying power, verify the physical wiring matches the control relay wiring diagram using a digital multimeter (DMM). Set your meter to the Ohms (Ω) setting for resistance and continuity checks.
1. Verify the Coil: Place your probes on Pin 2 and Pin 7. For a standard 24V DC octal relay, you should read a resistance between 600Ω and 750Ω (e.g., the Omron LY2N-D2 spec sheet lists the coil resistance at approximately 650Ω). If you read 0.0Ω, the coil is shorted. If you read OL (Open Loop), the internal winding is broken. Note: If your meter applies a high test voltage, it might forward-bias the internal flyback diode, showing a lower resistance in one probe direction than the other. This is normal.
2. Verify the Dry Contacts (De-energized): Place one probe on Pin 1 (COM) and the other on Pin 3 (NC). The meter should beep (continuity) or read less than 0.5Ω. Move the second probe to Pin 4 (NO). The meter should read OL (open circuit). Repeat this for Pin 8 (COM), Pin 6 (NC), and Pin 5 (NO).
3. Verify the Contacts (Energized): Apply 24V DC to Pins 2 and 7 (observing polarity). You should hear an audible click. While the coil is energized, check continuity between Pin 1 and Pin 4 (should now be closed) and Pin 1 and Pin 3 (should now be open). According to All About Circuits, this mechanical state change is the fundamental principle of electromechanical relay logic, allowing a low-power signal to dictate a high-power circuit state.
Control Relay Wiring Diagram FAQ
How do I read the control relay wiring diagram symbols for NO and NC contacts?
In IEC and NEMA standard schematics, the relay contacts are drawn in their "de-energized" or "shelf" state. A Normally Open (NO) symbol looks like a switch that is physically disconnected from the contact point. A Normally Closed (NC) symbol shows the switch blade resting on and overlapping the contact point. When the coil rectangle is energized, you mentally flip all linked switches: NO becomes closed, and NC becomes open. The dashed line linking the coil to the switches indicates they move together mechanically.
Why does my control relay wiring diagram show a diode or RC snubber across the coil?
When a DC relay coil is de-energized, the collapsing magnetic field induces a massive reverse voltage spike (inductive kickback) that can destroy sensitive switching transistors in a PLC or microcontroller. A flyback diode wired in reverse-parallel across the coil (cathode to positive, anode to negative) provides a safe recirculation path for this spike. For AC coils, a diode would short the AC circuit; instead, schematics will show an RC (resistor-capacitor) snubber network or a MOV (Metal Oxide Varistor) across the coil to suppress the AC arc and voltage transient. You can find more on inductive load suppression in the NFPA codes and standards regarding industrial control panel protection.
What wire gauge and ferrule size should I use for an 8-pin relay socket?
Most standard DIN-rail 8-pin relay sockets (like the Omron P2R-08 or Schneider RXZE2) feature cage-clamp or screw terminals rated for 10A to 15A. The physical terminal block accepts wire from 22 AWG up to 12 AWG. For control circuits carrying less than 2A, 18 AWG stranded wire is the industry standard. For load circuits pulling up to 10A, use 14 AWG. Always use a 14 AWG or 18 AWG insulated bootlace ferrule with an 8mm strip length to ensure a gas-tight connection and prevent the screw from fraying the copper strands.
Can I wire two separate voltage sources to the same 8-pin relay contacts?
Yes, but with strict isolation rules. Because the two contact sets (Pins 1/3/4 and Pins 8/6/5) are physically separated by the relay's internal plastic actuator, you can switch a 120V AC load on Set 1 and a 24V DC load on Set 2 simultaneously. However, you must never wire two different voltage sources to the Common and NO pins of the same contact set, as this will dead-short the two power supplies together when the relay pulls in. Furthermore, ensure the voltage difference between Contact Set 1 and Contact Set 2 does not exceed the relay's dielectric breakdown rating (typically 250V AC/DC for standard octal relays) to prevent internal arcing across the actuator.
