DPDT Switch Wiring Diagram: Double Pole Double Throw Explained
A DPDT switch is really two SPDT switches mechanically ganged on a single actuator. One toggle moves two independent sets of contacts simultaneously. That coupling is what makes it useful -- you can switch two separate circuits at once, or use both poles on a single circuit to achieve polarity reversal.
This guide covers the terminal layout, common wiring configurations (polarity reversal, on-off-on center, latching), and how a DPDT compares to related switch types.
Terminal Layout
A DPDT switch has six terminals arranged in two rows of three:
Pole 1: [COM1] [NO1] [NC1]
Pole 2: [COM2] [NO2] [NC2]
In physical toggle switches, the terminals are usually on the back or underside. The labeling varies by manufacturer -- you may see:
- C1/NO1/NC1 and C2/NO2/NC2
- 1/2/3 and 4/5/6 (with a datasheet telling you which is which)
- A/B/C and D/E/F
When the switch is in position 1 (default/off):
- COM1 connects to NC1, COM2 connects to NC2.
When the switch is in position 2 (actuated/on):
- COM1 connects to NO1, COM2 connects to NO2.
DPDT vs Other Switch Types
| Type | Poles | Throws | Terminals | Common Use |
|---|---|---|---|---|
| SPST | 1 | 1 | 2 | Simple on/off |
| SPDT | 1 | 2 | 3 | Select between two paths |
| DPST | 2 | 1 | 4 | Switch two circuits simultaneously (like a 2-pole breaker) |
| DPDT | 2 | 2 | 6 | Polarity reversal, dual-circuit switching |
The DPDT is the most versatile mechanical switch. A DPDT can do everything a DPST can do, plus polarity switching.
Configuration 1: Polarity Reversal (H-Bridge with DPDT)
This is the most common reason to reach for a DPDT. You want to reverse a DC motor direction with a single switch throw. The wiring creates an H-bridge.
Wiring
Assume a 12V supply and a motor with terminals M+ and M-.
- Connect +12V to both NC1 and NO2.
- Connect GND (0V) to both NO1 and NC2.
- Connect COM1 to M+ (motor terminal A).
- Connect COM2 to M- (motor terminal B).
Position 1 (switch open/default):
- COM1 → NC1 → +12V (motor A gets positive)
- COM2 → NC2 → GND (motor B gets negative)
- Motor runs forward.
Position 2 (switch thrown):
- COM1 → NO1 → GND (motor A gets negative)
- COM2 → NO2 → +12V (motor B gets positive)
- Motor runs reverse.
One switch, two throwing positions, full motor reversal. No microcontroller required.
Important: Never throw the switch while the motor is running at speed on an inductive load without adding flyback protection. A 1N4001 diode across the motor terminals (cathode to positive rail, anode to negative rail) absorbs the back-EMF spike when polarity reverses.
Configuration 2: On-Off-On Center (3-Position DPDT)
Some DPDT switches have a center-off position, making them ON-OFF-ON. In the center position, all contacts are open -- no connection between COM and either throw.
This is useful for motor control with three states: forward, stop, reverse. Wiring is identical to the polarity-reversal circuit above, but the center position lets the motor coast to a stop (neither terminal powered).
For applications where you want braking (actively stopping the motor), a center-on (ON-ON-ON) DPDT is more appropriate -- in the center, both motor terminals connect to the same rail, shorting the motor and creating regenerative braking.
Check the switch datasheet carefully. "ON-OFF-ON" and "ON-ON-ON" are both available but differ in center-position behavior.
Configuration 3: Dual-Circuit Switching
A DPDT used as a DPST -- just ignore one set of throw terminals and use only the COM and NO (or NC) terminals on each pole.
Example: Switch 120V AC line and neutral simultaneously.
- COM1 connects to line (L). NO1 connects to load line.
- COM2 connects to neutral (N). NO2 connects to load neutral.
When thrown, both line and neutral are switched together. This is how double-pole switches work in main electrical panels for 240V circuits and some European 230V appliances.
Do not break neutral on a single-pole switch feeding a live circuit -- the appliance remains energized through the line. A DPDT (or DPST) switches both conductors together, fully isolating the load.
Configuration 4: DPDT as a Transfer Switch
A DPDT can select between two input sources for a single load:
- Source A lines connect to NC1 and NC2.
- Source B lines connect to NO1 and NO2.
- Load terminals connect to COM1 and COM2.
Flip the switch to select Source A or Source B. Used in small-scale transfer switching (backup battery vs mains), audio source selection, and bench test setups.
DPDT Relay vs DPDT Switch
The six-terminal layout is identical whether the device is a manual DPDT switch or a DPDT relay. The difference is the actuator -- a toggle lever vs an electromagnetic coil.
A DPDT relay uses exactly the same polarity-reversal wiring for motor control as a manual DPDT switch, but the control signal triggers the coil (usually 5V, 12V, or 24V) rather than a human hand. The physical contact arrangement is the same six terminals.
You can draw both configurations in CircuitDiagramMaker -- the relay and switch symbols are separate, but the wiring topology is identical, which makes it easy to prototype a manual version and then swap to relay control without rethinking the circuit.
Ratings and Selecting a DPDT Switch
Key ratings to check on the datasheet:
- Current rating: Varies widely. PCB-mount DPDT switches rated for 0.5A are common for signal switching. Panel-mount power types can handle 10A--30A.
- Voltage rating: AC and DC ratings are different. A switch rated 10A/125VAC may be rated 6A/28VDC.
- Contact material: Silver alloy contacts work for most applications. Gold-plated contacts are specified for low-level signal switching (< 100mA) to prevent oxide buildup.
- Mechanical life: Panel-mount DPDT toggles typically 10,000--50,000 cycles. Specify a sealed type for wet or dusty environments.
Common Wiring Mistakes
- Confusing NC and NO: In the center of a 3-position switch, both throws open. In a 2-position DPDT, NC is the rest position -- it is connected without any power applied to the coil/actuator.
- Not protecting inductive loads: Motors, solenoids, and relay coils generate back-EMF when switched. Always add a flyback diode or snubber network.
- Exceeding DC current rating: Some switches are rated for AC only or derate significantly at DC because AC naturally crosses zero (aiding arc extinction). Check the DC rating explicitly.
- Floating unused terminals: In a polarity-reversal circuit, NC1 and NO2 carry +V, and NO1 and NC2 carry GND. Leaving any of these unconnected breaks the circuit in one direction.
Create Your Own DPDT Wiring Diagram
DPDT circuits are easy to get confused on paper but straightforward once drawn out clearly. With CircuitDiagramMaker you can:
- Place DPDT switch and DPDT relay symbols side by side
- Draw the H-bridge polarity-reversal circuit with colored wires for positive and negative rails
- Annotate each terminal (COM1, NO1, NC1, COM2, NO2, NC2)
- Simulate DC operation to confirm forward and reverse motor states
- Export the diagram as a reference for the bench
Create your own DPDT switch wiring diagram -- free
Key Takeaways
- A DPDT switch has six terminals: COM1/NO1/NC1 and COM2/NO2/NC2 -- two independent SPDT poles on one actuator.
- Polarity-reversal H-bridge wiring: +V to NC1 and NO2; GND to NO1 and NC2; motor terminals to COM1 and COM2.
- ON-OFF-ON (center-off) stops the motor by opening all contacts; ON-ON-ON (center-on) applies braking by shorting motor terminals.
- A DPDT relay and a DPDT switch use the same six-terminal topology -- the actuator is the only difference.
- Check DC current rating separately from AC rating -- DC arcs do not self-extinguish at zero-crossing.
- Always protect motor and solenoid loads with flyback diodes when switching inductors.
- Use gold-plated contacts for low-level signal switching below 100mA to prevent oxide buildup.