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When selecting a toggle switch, push button switch, or key switch, terms such as SPST, SPDT, DPST, and DPDT are commonly used.
These abbreviations describe the number of poles and throws inside the switch.
Understanding them helps equipment manufacturers select the correct circuit and avoid wiring errors.
What Do Pole and Throw Mean?
A pole refers to the number of separate circuits controlled by one actuator.
A throw refers to the number of output paths available to each pole.
Therefore:
• Single Pole means one circuit
• Double Pole means two circuits
• Single Throw means one switching path
• Double Throw means two selectable switching paths
SPST: Single Pole Single Throw
An SPST switch controls one circuit and has one switching path.
It is the simplest switch configuration.
Typical terminal count:
• Two terminals
Typical function:
• ON-OFF
When the switch is open, current does not flow.
When the switch is closed, the two terminals are connected.
Common applications include:
• Simple power control
• Lighting
• Equipment enable
• Auxiliary function control
• Single-circuit isolation
An SPST switch is often described as a simple on/off switch.
SPDT: Single Pole Double Throw
An SPDT switch controls one circuit and switches the common terminal between two output terminals.
Typical terminal count:
• Three terminals
Typical functions include:
• ON-ON
• ON-OFF-ON
• (ON)-OFF-(ON)
• ON-OFF-(ON)
The three terminals are commonly described as:
• Common
• Output 1
• Output 2
In one position, the common terminal connects to Output 1.
In the other position, it connects to Output 2.
If the switch includes a center OFF position, the common terminal is disconnected from both outputs in the center position.
Common applications include:
• Mode selection
• Direction signals
• Circuit changeover
• Two-output selection
• High and low settings
• Manual and automatic mode selection
DPST: Double Pole Single Throw
A DPST switch controls two separate circuits with one actuator.
Both circuits open or close at the same time.
Typical terminal count:
• Four terminals
Typical function:
• ON-OFF
A DPST switch can be considered as two SPST switches operated together.
Common applications include:
• Switching two power lines
• Controlling two related circuits
• Simultaneous isolation
• Dual-load activation
• Separating control and signal circuits
The two poles are electrically separate, even though they are operated by the same lever or button.
DPDT: Double Pole Double Throw
A DPDT switch controls two separate circuits, and each circuit can switch between two output paths.
Typical terminal count:
• Six terminals
Typical functions include:
• ON-ON
• ON-OFF-ON
• (ON)-OFF-(ON)
• ON-OFF-(ON)
A DPDT switch can be considered as two SPDT switches operated together.
Common applications include:
• Motor direction control
• Polarity reversal
• Dual-circuit changeover
• Forward and reverse control
• Mode switching
• Two-channel signal selection
Terminal Count Is Not Always Enough
Although terminal count can provide a useful indication, it should not be the only method used to identify the circuit.
Some switches include extra terminals for:
• LED illumination
• Ground connections
• Multiple lamp circuits
• Separate common terminals
• Customized internal connections
For example, an illuminated push button switch may have terminals for both the switch contacts and the LED.
The circuit diagram should always be checked.
Pole Configuration and Operating Function
The pole configuration describes the internal circuit arrangement.
The operating function describes the available switch positions.
These must be confirmed separately.
For example:
• SPST ON-OFF
• SPDT ON-ON
• SPDT ON-OFF-ON
• DPDT (ON)-OFF-(ON)
Two switches may both be DPDT but perform different functions.
Normally Open and Normally Closed
SPST and DPST switches may also be described as normally open or normally closed.
Normally open means the circuit is open in its normal position.
Normally closed means the circuit is closed in its normal position.
For maintained toggle switches, the terms ON and OFF are often more useful.
For momentary push buttons, NO and NC descriptions are common.
How to Read an SPDT Circuit
Consider an SPDT switch with terminals 1, 2, and 3.
Terminal 2 may be the common terminal.
In one position:
• Terminal 2 connects to Terminal 1
In the other position:
• Terminal 2 connects to Terminal 3
In an ON-OFF-ON version, Terminal 2 is disconnected from both outputs in the center position.
The actual terminal numbering varies by product, so the supplier’s circuit diagram should be followed.
How to Read a DPDT Circuit
A DPDT switch contains two independent SPDT circuits.
For example:
• Common A switches between Output A1 and Output A2
• Common B switches between Output B1 and Output B2
Both poles move at the same time.
This can be used to reverse the polarity applied to a DC motor, but the current rating and motor load must be suitable for the switch.
Which Configuration Should You Choose?
Choose SPST when one circuit only needs basic ON-OFF control.
Choose SPDT when one circuit must switch between two outputs or requires a center OFF position.
Choose DPST when two circuits must open or close together.
Choose DPDT when two circuits must switch between two output paths.
The final decision should also consider:
• Voltage and current
• AC or DC load
• Momentary or maintained action
• Number of positions
• Terminal type
• Sealing requirement
• Panel size
• Wiring method
Common Selection Mistakes
Assuming More Terminals Always Means More Poles
LED terminals and additional connections can increase the terminal count.
Confusing DPST with DPDT
DPST provides two ON-OFF circuits.
DPDT provides two changeover circuits.
Ignoring the Center Position
An ON-ON switch and an ON-OFF-ON switch may have the same pole configuration but different operating functions.
Assuming the Circuit from the Product Appearance
Two switches with the same lever and housing may use different internal contact arrangements.
Not Confirming Simultaneous Contact Timing
For some applications, the sequence in which contacts open and close may matter. Break-before-make and make-before-break should be confirmed when required.
Break-Before-Make and Make-Before-Break
In a double-throw switch, the common terminal may disconnect from the first output before connecting to the second. This is known as break-before-make.
In other designs, the second connection may be made before the first one is fully disconnected. This is called make-before-break.
Most equipment applications use break-before-make, but specialized circuits may require a specific contact sequence.
This information should not be assumed unless it is stated in the specification.
Conclusion
SPST, SPDT, DPST, and DPDT describe the number of circuits and switching paths controlled by a switch.
Selecting the correct configuration requires more than counting terminals.
Equipment manufacturers should confirm the circuit diagram, operating function, contact state, electrical load, and terminal arrangement before approving a switch.
CTA
Contact TOOWEI with your circuit diagram, required function, electrical rating, panel size, and terminal preference to identify a suitable switch configuration.