A DPDT (Double Pole, Double Throw) relay symbol represents an electromechanical component with two isolated switch circuits (poles), each capable of connecting a common terminal to either a Normally Open (NO) or Normally Closed (NC) contact. In standard 8-pin packages like the ubiquitous Omron G2R-2 or Finder 40.52, this translates to two coil pins and six contact pins. Below is the definitive reference for interpreting these symbols and mapping them to physical hardware.
The DPDT Relay Symbol & Pinout Reference Table
The table below maps the standard schematic symbol elements to the physical pins of an 8-pin flat-blade DPDT relay (using the industry-standard Omron G2R-2 pinout as the baseline). This is your master reference for bench wiring and schematic decoding.
| Symbol Element | Pin Number (8-Pin Flat) | Function in Practice | Typical Wire Color (US/NEC) |
|---|---|---|---|
| Coil (Rectangle/Circle) | 2 and 7 | Energizes the electromagnet. Polarity matters only if an internal flyback diode is present. | Red (+) / Black (-) for DC; Black/White for AC |
| Pole 1 Common (COM1) | 1 | The moving contact for the first switch circuit. Connects to the load source. | Black (Line) or Red (Switched Leg) |
| Pole 1 NC (Normally Closed) | 4 | Connected to COM1 when the coil is de-energized. Breaks when the coil pulls in. | Blue or Yellow (Switched Return) |
| Pole 1 NO (Normally Open) | 3 | Connects to COM1 only when the coil is energized. Used for the primary 'ON' load. | Red or Orange (Switched Hot) |
| Pole 2 Common (COM2) | 8 | The moving contact for the second isolated switch circuit. | Black (Line) or Red (Switched Leg) |
| Pole 2 NC (Normally Closed) | 5 | Connected to COM2 when de-energized. | Blue or Yellow (Switched Return) |
| Pole 2 NO (Normally Open) | 6 | Connects to COM2 only when the coil is energized. | Red or Orange (Switched Hot) |
Regional Standard Variants (IEC vs. ANSI vs. Old UK)
Reading a DPDT relay symbol correctly requires knowing which drafting standard the engineer used. A symbol that looks like a simple circle in North America might be drawn as a rectangle in Europe, but the electrical behavior is identical.
- IEC 60617 (Global/European Standard): The coil is drawn as a simple rectangle. The contacts are drawn as separate switch symbols linked by a dashed mechanical line to the coil. This is the most common standard in modern PLC schematics and international datasheets. See the Electrical Engineering Portal's guide on IEC vs ANSI symbols for visual comparisons.
- ANSI/IEEE 315 (North American Standard): The coil is traditionally drawn as a circle (or a circle with a letter inside, like 'CR' for Control Relay). The contacts use a hinged line notation. You will see this heavily in older US industrial control panels and NEMA-style motor control schematics.
- Old UK (BS 3939): Largely superseded by IEC 60617, but you will still encounter it in legacy British infrastructure. It used a distinct 'box with a diagonal line' for the coil and specific semi-circle notations for NO/NC contacts. If you are retrofitting a panel in an older UK facility, expect these anomalies.
Rows People Get Wrong & Faded Marking Troubleshooting
Even experienced makers and electricians make specific errors when translating the DPDT relay symbol to physical wiring. Here are the most common pitfalls and how to recover when the physical relay's silkscreen is worn off.
The 'Rows People Get Wrong' Notes
- Assuming Pins 1 and 8 are the Coil: On many 8-pin octal plug-in relays (like the Potter & Brumfield KRPA-11), the coil is indeed on pins 2 and 7. However, beginners often assume the outermost pins (1 and 8) are the coil because they sit on the edges. On the Omron G2R-2 PCB layout, pins 1 and 8 are the Commons. Wiring 120VAC into the coil pins by mistake will instantly vaporize the coil wire and trip your breaker.
- NO vs. NC Orientation Inversion: The DPDT relay symbol shows NO and NC clearly, but when you flip a physical relay upside down to solder it or wire a socket from the back, the left/right orientation of pins 3/4 and 5/6 reverses. Always verify continuity with a multimeter rather than relying on visual memory of the pinout.
- Ignoring the Flyback Diode Symbol: If the schematic shows a diode symbol across the coil rectangle, the physical relay has an internal protection diode. If you wire the coil backward (positive to the cathode side), the diode will short the power supply, likely destroying the driving transistor or blowing the control fuse.
Safe Interpretation When Markings are Faded or Missing
Industrial relays in harsh environments often lose their silkscreen pinout markings. If you pull a dusty Finder 40.52 or Omron G2R-2 from a panel and can't read the pins, do not guess. Use this DMM (Digital Multimeter) procedure:
- De-energize and Isolate: Remove the relay from its socket or circuit. Never test continuity on a live circuit.
- Find the Coil: Set your DMM to resistance (Ohms). Probe the pins. The coil pins will read a specific resistance (e.g., ~650Ω for a 24VDC coil, or ~15kΩ for a 120VAC coil). All other pin combinations will read 'OL' (Open Loop) or show a dead short if the contacts are welded.
- Identify Commons, NO, and NC: Set the DMM to continuity mode (the beep setting). With the relay unpowered, probe the remaining 6 pins. The two pins that beep with the 'middle' pins are your NC contacts. The two pins that beep with the 'outer' pins when you manually press the relay's armature down with a non-conductive tool (like a plastic spudger) are your NO contacts. The pivot points are your Commons.
For a deeper dive into component testing, refer to Electronics Tutorials' comprehensive guide on relay symbols and testing.
Frequently Asked Questions
What does the DPDT relay symbol look like on a PLC wiring diagram?
On a modern PLC wiring diagram (typically drawn to IEC 60617 standards), the DPDT relay is rarely drawn as a single unified block. Instead, the coil is drawn in the control logic section (e.g., rung 4 of a ladder diagram) as a rectangle labeled 'CR1' or 'K1'. The two sets of contacts (Pole 1 and Pole 2) are drawn separately in the power/load sections of the diagram, linked only by the same alphanumeric label (e.g., 'K1-1' and 'K1-2'). This decoupled drawing style is standard practice to keep schematics readable across multiple pages.
How do I wire a DPDT relay for motor reversing using standard symbols?
A DPDT relay is the simplest way to reverse a DC motor. In the schematic, the symbol will show the motor connected across the two Common terminals (Pins 1 and 8). The power supply positive is wired to Pin 3 (NO1) and Pin 5 (NC2). The power supply negative is wired to Pin 4 (NC1) and Pin 6 (NO2). When the coil is de-energized, current flows one way through the motor. When the coil energizes, the Commons switch to the NO contacts, reversing the polarity across the motor terminals. Always add physical or electrical interlocks if using two separate relays to prevent a dead short if both pull in simultaneously.
Why does my DPDT relay symbol show a diode across the coil?
That symbol indicates a flyback (or freewheeling) diode. When a relay coil de-energizes, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback) that can exceed 100V, easily destroying the sensitive MOSFET or BJT driving it. The diode symbol across the coil tells the builder that this protection is either internal to the relay module or must be added externally. If it is an external diode (like a 1N4007), the cathode stripe must face the positive supply voltage. If your schematic shows a Zener diode symbol instead, it is designed for faster relay drop-out times, which is critical in high-speed pneumatic valve switching.
