Kurze Antwort
SPST, SPDT, DPST, and DPDT describe how many circuits a switch or relay contact can control and how many output positions each circuit can connect to.
The first part describes the number of poli. A pole is one independent circuit path controlled by the device. The second part describes the number of throws. A throw is the number of output positions that each pole can connect to.
In simple industrial terms:
- SPST controls one circuit ON/OFF.
- SPDT switches one circuit between two outputs.
- Bipolare a una via (DPST) switches two circuits ON/OFF at the same time.
- DPDT switches two circuits between two output paths at the same time.
These terms are used for control switches, selector switches, cam switches, changeover switches, relays, timer relays, and many control panel devices.
What Do Pole and Throw Mean?
Before comparing SPST, SPDT, DPST, and DPDT, it helps to separate the two words.
Un polo is the number of independent circuits controlled by one actuator or relay coil. A single-pole device controls one circuit. A double-pole device controls two circuits at the same time.
Un throw is the number of possible output paths for each pole. A single-throw device has one ON path. A double-throw device can change over between two paths.
| Termine | Significato | Practical Interpretation |
|---|---|---|
| Unipolare | One controlled circuit | One line, one signal, or one contact path |
| Bipolare | Two controlled circuits | Two lines or two isolated contact paths |
| Single Throw | One output position | ON/OFF function |
| A doppia commutazione | Two output positions | Changeover or selection function |
This is why SPDT and DPST are very different, even though both have three or four terminal points in many physical designs. SPDT is one circuit switching between two outputs. DPST is two circuits switching ON/OFF together.
SPST, SPDT, DPST, and DPDT Quick Comparison
| Tipo | Modulo completo | Cosa fa | Common Industrial Applications |
|---|---|---|---|
| SPST | Unipolare a una via (SPST) | Controls one circuit ON/OFF | Basic switches, push buttons, simple control signals |
| SPDT | Unipolare a due vie (SPDT) | Switches one input between two outputs | Selector circuits, relay changeover contacts, interlocking |
| Bipolare a una via (DPST) | Bipolare a una via | Switches two circuits ON/OFF together | Disconnecting line and neutral, two-line control, paired circuits |
| DPDT | Bipolare a due vie | Switches two circuits between two output paths | Polarity reversal, motor direction logic, complex changeover control |
SPST Switch Explained
An SPST switch is the simplest type. It has one pole and one throw, so it opens or closes one circuit path.
When the switch is open, current cannot flow through the controlled path. When the switch is closed, current can flow. This makes SPST suitable for basic ON/OFF control.
Typical SPST applications include:
- simple control switches
- push button contacts
- auxiliary control circuits
- indicator lamp control
- simple enable/disable functions
In a relay, an SPST contact may be normally open (NO) or normally closed (NC). A normally open SPST contact closes when the relay coil energizes. A normally closed SPST contact opens when the relay coil energizes.
SPDT Switch Explained
An SPDT switch has one pole and two throws. It does not simply turn one circuit ON/OFF. Instead, it switches one common terminal between two output terminals.
This is why SPDT is often called a changeover contact. In relay terminology, it is commonly described as a contact with COM, NO, e NC terminals.
Typical SPDT applications include:
- selecting between two control signals
- relay changeover contacts
- alarm normal/fault indication
- manual/automatic selector circuits
- interlocking control logic
- timer relay output contacts
SPDT is very common in control panels because one contact can provide both a normally open and normally closed function. For deeper relay-specific wiring examples, see VIOX’s guide to SPDT vs DPDT time relay contact configurations.
DPST Switch Explained
A DPST switch has two poles and one throw. It switches two independent circuits ON/OFF at the same time using one actuator.
In many practical cases, DPST is used when two conductors must be opened or closed together. For example, a DPST switch may switch both line and neutral in a single-phase circuit where the design requires both conductors to be disconnected. It can also control two separate control circuits together while keeping the circuits electrically isolated.
Typical DPST applications include:
- switching line and neutral together where required by design
- two-line control circuits
- simultaneous switching of two loads
- isolated control paths operated by one handle
- safety-related auxiliary switching when designed appropriately
Do not assume DPST automatically provides safety isolation. The switch rating, utilization category, contact spacing, and applicable standards still matter.
DPDT Switch Explained
A DPDT switch has two poles and two throws. It is essentially two SPDT switches operated together by one actuator.
DPDT can switch two independent circuits between two output paths at the same time. This makes it useful for more complex control functions, especially where two conductors or two signals must change state together.
Typical DPDT applications include:
- polarity reversal in direct current control circuits
- motor forward/reverse control logic where suitable contactors or relays are used
- signal routing between two control paths
- changeover control circuits
- paired NO/NC relay output functions
- more complex interlocking arrangements
DPDT is powerful, but it is also easier to miswire. In motor control and power switching applications, the contact rating, load type, interlocking method, and protection device must be checked carefully.
How These Terms Apply to Relays
In relays, SPST, SPDT, DPST, and DPDT describe the contact arrangement, not the coil voltage.
A relay has two separate sides:
- Coil side: the control voltage that energizes the relay.
- Contact side: the switching contacts that control another circuit.
For example, a relay may have a 24V DC coil and SPDT contacts rated for a separate control circuit. The coil voltage tells you how to energize the relay. The contact configuration tells you what the relay can switch.
| Relay Contact Type | Typical Terminals | Uso comune |
|---|---|---|
| SPST-NO | COM and NO | Turns a circuit ON when coil energizes |
| SPST-NC | COM and NC | Turns a circuit OFF when coil energizes |
| SPDT | COM, NO, NC | Changes one circuit from NC to NO |
| DPDT | Two sets of COM, NO, NC | Switches two changeover contact sets together |
Timer relays, control relays, and interface relays often use SPDT or DPDT contact arrangements because they allow both normally open and normally closed logic from the same device.
How These Terms Apply to Selector, Cam, and Changeover Switches
For industrial switches, pole and throw terms describe what happens inside the contact block or cam mechanism.
In a selector switch, one handle position may close one contact while another position closes a different contact. In a cam switch, the cam profile determines which contact pairs open or close at each position. In a changeover switch, the device transfers a circuit from one path to another.
These products may not always be labeled only as SPST or DPDT in the catalog. They may be described by:
- numero di poli
- number of positions
- contact sequence
- maintained or momentary operation
- ON-OFF, ON-ON, or ON-OFF-ON function
- contact rating and utilization category
- panel mounting size
For VIOX applications, these terms are most useful when comparing changeover switches, cam switches, selector switches, and relay outputs.
Errori comuni di selezione
Mistake 1: Confusing Pole Count With Terminal Count
Counting terminals is not always enough. Some devices have extra terminals for lamps, auxiliary contacts, or internal links. Always check the contact diagram.
Mistake 2: Choosing SPST When You Need Changeover Logic
If the circuit must switch between two paths, SPST is not enough. SPDT is usually needed for one changeover circuit, while DPDT is used when two changeover circuits must operate together.
Mistake 3: Using DPDT for Motor Reversal Without Proper Protection
DPDT contact logic can reverse polarity in some control circuits, but motor reversing circuits may require contactors, mechanical or electrical interlocking, overload protection, and correct utilization ratings. Do not treat a small DPDT switch as a universal motor reversing solution.
Mistake 4: Ignoring Contact Ratings
The contact arrangement only tells you the circuit logic. It does not tell you whether the device can safely switch the load. Check rated voltage, rated current, load type, AC or DC duty, and utilization category.
Mistake 5: Mixing Relay Coil Voltage With Contact Configuration
A 24V relay can have SPST, SPDT, or DPDT contacts. A 230V relay can also have different contact arrangements. Coil voltage and contact configuration are separate specifications.
Guida alla selezione
| If You Need To… | Common Choice | Perché |
|---|---|---|
| Turn one control circuit ON/OFF | SPST | Simple single-circuit switching |
| Switch one signal between two outputs | SPDT | One common terminal changes between NO and NC |
| Switch two circuits ON/OFF together | Bipolare a una via (DPST) | Two independent poles operate together |
| Switch two circuits between two paths | DPDT | Two changeover contact sets operate together |
| Get both NO and NC relay logic | SPDT relay contact | Provides changeover function |
| Reverse polarity in a suitable DC control circuit | DPDT | Can cross-connect two conductors when rated correctly |
| Select between manual and automatic control | SPDT or selector switch contact block | Provides alternate control path |
FAQ
What is the difference between SPST and SPDT?
SPST controls one circuit ON/OFF. SPDT switches one common terminal between two output paths, usually described as NO and NC contacts in relay circuits.
What is the difference between DPST and DPDT?
DPST switches two circuits ON/OFF at the same time. DPDT switches two circuits between two output paths at the same time, like two SPDT contacts operated together.
SPDT è la stessa cosa di un contatto di scambio?
Yes, in many relay and control panel contexts, an SPDT contact is called a changeover contact because it changes the common terminal from one throw to another.
Is DPDT the same as two SPDT switches?
Functionally, DPDT behaves like two SPDT switches operated by one actuator or relay mechanism. The two poles switch together, but they are separate contact paths.
Which is better, SPST or SPDT?
Neither is universally better. SPST is better for simple ON/OFF control. SPDT is better when one circuit must switch between two paths or provide both normally open and normally closed logic.
Can a DPDT switch reverse motor direction?
DPDT contacts can reverse polarity in some suitable DC circuits, but motor direction control must consider load rating, contact type, protection, interlocking, and the correct motor control method. For industrial motors, contactors or dedicated reversing starters are often more appropriate.
How do I know which contact configuration my relay has?
Check the relay wiring diagram or terminal marking. SPDT relays usually show COM, NO, and NC. DPDT relays show two separate sets of COM, NO, and NC terminals.
Conclusione
SPST, SPDT, DPST, and DPDT are not just switch abbreviations. They describe the internal contact logic of switches, relays, selector switches, cam switches, and changeover devices.
The fastest way to choose is to ask two questions: how many independent circuits must be controlled, and does each circuit need simple ON/OFF switching or changeover switching? Once those answers are clear, the correct pole and throw configuration becomes much easier to select.
