An 8-pin DPDT (Double Pole, Double Throw) relay, such as the industry-standard Omron MY2N-D2 DC12, utilizes two completely isolated switch circuits controlled by a single electromagnetic coil. When reading a wiring diagram of relay circuits, the primary objective is to mentally separate the low-power control side (the coil) from the high-power load side (the contacts). The control circuit dictates when the relay switches, while the load circuit dictates what is being powered. Below is a complete terminal mapping, symbol translation, and node-by-node trace to help you wire, troubleshoot, and verify an 8-pin DPDT relay on the bench or in a control panel.
Terminal Pin Mapping and Diagram Symbols
Before tracing the wires, you must map the physical pins to their schematic symbols. In IEC 60617 standard schematics, the relay coil is drawn as a rectangle (often with a diagonal line or the letter 'K'), while the contacts are drawn as switch blades. The 8 pins are divided into two groups: two for the coil, and six for the two independent switch poles.
| Pin Number | Function | IEC Diagram Symbol | Typical Wire Color (12V DC) | Max Rating (Omron MY2N) |
|---|---|---|---|---|
| 13 | Coil Positive (+) | Rectangle top terminal | Red (18 AWG) | N/A (Control) |
| 14 | Coil Negative (-) | Rectangle bottom terminal | Black (18 AWG) | N/A (Control) |
| 9 | Common 1 (COM1) | Switch blade pivot (Pole 1) | Red (14 AWG) | 10A @ 250VAC / 5A @ 24VDC |
| 5 | Normally Open 1 (NO1) | Open contact point (Pole 1) | Blue (14 AWG) | 10A @ 250VAC / 5A @ 24VDC |
| 1 | Normally Closed 1 (NC1) | Closed contact point (Pole 1) | Yellow (14 AWG) | 10A @ 250VAC / 5A @ 24VDC |
| 10 | Common 2 (COM2) | Switch blade pivot (Pole 2) | Red (14 AWG) | 10A @ 250VAC / 5A @ 24VDC |
| 6 | Normally Open 2 (NO2) | Open contact point (Pole 2) | Blue (14 AWG) | 10A @ 250VAC / 5A @ 24VDC |
| 2 | Normally Closed 2 (NC2) | Closed contact point (Pole 2) | Yellow (14 AWG) | 10A @ 250VAC / 5A @ 24VDC |
For a deeper understanding of how these electromechanical components operate internally, refer to the Electronics Tutorials relay guide, which breaks down the magnetic flux paths and armature mechanics.
Node-by-Node Circuit Trace: Source to Load
A wiring diagram is useless if you cannot trace the current path. Let us trace a standard 12V DC control circuit energizing a 12V DC load through the Normally Open (NO) contacts. We will explicitly track polarity and the ground return path.
1. The Control Circuit Trace (Energizing the Coil)
- Source Origin: Current leaves the positive terminal of the 12V DC power supply.
- Switching Device: The current passes through a manual toggle switch or a PLC transistor output.
- Coil Entry: The wire lands on Pin 13 (Coil +). Polarity matters here if your relay has a built-in flyback diode or an LED indicator; reverse polarity will prevent the LED from lighting and could short the coil if a diode is present.
- Electromagnetic Action: Current flows through the internal copper winding, generating a magnetic field that pulls the armature down.
- Coil Exit: Current exits via Pin 14 (Coil -).
- Ground Return: The wire from Pin 14 routes directly to the DC ground bus bar (0V), completing the control circuit.
2. The Load Circuit Trace (Powering the Device)
- Load Source Origin: Current leaves the positive terminal of the load power supply (this can be the same 12V supply or a completely isolated 120V AC source, as the contacts are galvanically isolated from the coil).
- Common Entry: The wire lands on Pin 9 (COM1). This is the pivot point of the internal switch blade.
- Contact Closure: Because the coil is energized, the internal blade is pulled away from Pin 1 (NC) and makes physical contact with Pin 5 (NO1).
- Load Entry: Current exits Pin 5 and travels to the positive terminal of the load device (e.g., a DC motor or solenoid valve).
- Load Exit & Ground: Current passes through the load, exits the negative terminal of the device, and returns to the DC ground bus bar, completing the high-power circuit.
If you were using the second pole simultaneously, the exact same sequence occurs from Pin 10 (COM2) to Pin 6 (NO2). For more on contact forms and how DPDT configurations isolate circuits, review Macromatic's breakdown of relay contact forms.
Verifying Connections with a Multimeter
Do not apply power to a newly wired relay panel without first verifying the paths with a digital multimeter (DMM). Here is the exact diagnostic sequence using a standard DMM.
Step 1: Coil Resistance Test (De-energized)
Set your multimeter to the Ohms (Ω) setting, specifically the 200Ω or 2kΩ range. Place the red probe on Pin 13 and the black probe on Pin 14.
Expected Reading: For a 12V DC Omron MY2N coil, you should read approximately 163Ω.
Failure Modes: A reading of "OL" (Open Loop) indicates a burnt-out internal coil winding. A reading of 0.0Ω indicates an internal short. In either failure case, the relay must be replaced.
Step 2: Contact Continuity Test (De-energized)
Set your multimeter to the Continuity mode (the diode/soundwave symbol).
Place probes on Pin 9 (COM1) and Pin 1 (NC1). The meter should beep, indicating a closed path (near 0.0Ω).
Move the probe from Pin 1 to Pin 5 (NO1). The meter should read "OL" and remain silent, indicating an open path.
Note: Manually press the relay's mechanical test button (usually a clear or red plastic tab on the top of the housing). The continuity should instantly swap, beeping on Pin 5 and going silent on Pin 1.
Step 3: Voltage Verification (Energized)
With the circuit powered and the control switch closed, set your DMM to DC Volts (20V range).
Place the black probe on a known ground bus bar. Place the red probe on Pin 13. You should read 12.0V to 12.6V.
Move the red probe to Pin 5 (NO1). You should read the exact same load supply voltage (e.g., 12.0V). If Pin 13 reads 12V but Pin 5 reads 0V, your coil is energized but the internal contacts have failed to close, or the load supply is disconnected from Pin 9.
Common Wiring Mistakes and Edge Cases
Even with a correct wiring diagram of relay connections, physical implementation introduces edge cases that schematic drawings often omit.
Flyback Diode Placement
When a DC coil de-energizes, the collapsing magnetic field induces a massive reverse voltage spike (inductive kickback) that can destroy the PLC transistor or microcontroller GPIO pin driving Pin 13. If your relay does not have a built-in suppression module, you must wire a flyback diode (like a 1N4007) directly across Pins 13 and 14. The diode cathode (striped end) must point toward Pin 13 (Positive), and the anode toward Pin 14 (Negative). Wiring this backward will create a dead short across your power supply the moment the switch closes.
AC vs. DC Contact Derating
A common mistake is assuming a relay rated for "10A at 250V AC" can also switch "10A at 24V DC". This is false. AC voltage crosses zero 120 times a second, which naturally extinguishes the electrical arc that forms when contacts open. DC voltage never crosses zero, meaning the arc sustains much longer, pitting and welding the contacts together.
As shown in the spec table above, a standard 10A AC relay is typically derated to 5A or less for DC loads. If you are switching a 12V DC motor that draws 8A, an 8-pin MY2N relay will eventually weld its contacts shut. For high-current DC loads, you must use a heavy-duty contactor or a solid-state relay (SSR) designed specifically for DC arc suppression.
Thermal Management in Enclosures
Relay coils generate heat. An 8-pin 12V DC coil draws roughly 73mA (12V / 163Ω = 0.073A), dissipating nearly 1 watt of heat continuously. If you pack ten of these relays tightly side-by-side in a sealed NEMA 4X control panel, the ambient temperature inside the enclosure will rise. Relay coil resistance increases with temperature, which reduces the coil current and can cause the relay to drop out (fail to hold the contacts closed) if the voltage sags below the 80% nominal threshold. Always factor in enclosure ventilation or derate the coil voltage expectations in high-density panels.
