What Is a Single Pole Double Throw Switch?

A single pole double throw switch (SPDT switch) is a switch that has one common contact that can make contact with one of two different contacts (throws). The letters “SPDT” represent the topology of the contact points, but they do not indicate if the switch has a center-off position, remains where it was placed, returns to the home position when the actuator is released, overlaps with either path when switching between them, or what specific pin configuration it has.

When designing and determining how to connect to the actual SPDT switch, the schematic symbol may appear very simple, while the physical switch actuator may have a configuration that is not familiar or there may be extra terminals. Start with the one common and two throws and then review the datasheet to determine all other features.

What Is a Single Pole Double Throw Switch?

The SPDT switch is an arrangement of changeover contacts. A single switched common contact connects to one of two fixed contacts. The SPDT glossary by Analog Devices provides a compact definition of this model. Analog Devices and TE Connectivity describe the attributes of poles and throws as features of the contact topology, rather than of the actuator style.

Therefore, the basic contact topology will consist of three electrical contact points – common, throw A and throw B. This does not mean that every SPDT has three physical terminals. Illumination, duplicated terminals, mounting tabs, or other additional electronic functions may all add to the total number of connections without changing the SPDT’s base contact topology.

The contacts are not universally a single input and two outputs. A typical passive mechanical contact may conduct electricity in either permitted direction. For instance, the common may receive a signal and select where it should go. In another situation, either throw may be the source.

What Do “Single Pole” and “Double Throw” Mean?

“Single Pole” and “Double Throw” count different dimensions of the contact topology. The terms do not account for actuator positions or physical terminals. Carling Technologies, in explaining switch poles, treats the counts as separate; TE Connectivity, in guiding switch attributes, applies the same terms across various families of switches.

An OFF position does not constitute a third throw of the switch. An SPDT toggle switch labeled ON-OFF-ON will still have only two throws, while the centre OFF label simply indicates that neither throw is connected to common when the actuator is in this middle (OFF) position.

How Does the Common Contact Switch Between Two Throws?

Terminal numbering varies by part, so identify the two selected connectivity states by the COM-to-throw relationship rather than by assumed pin numbers.

The optional row is not inherent to SPDTs; it is only used on SPDT parts with a center-off position specified by the position table or function notation. Also, labels for COM, normally open, and normally closed will only be relevant when the datasheet indicates that the part has a free or normal position. A maintained selector switch that has two equally valid resting positions may use common and two alternate throws as the labels for the contacts.

How Do You Read an SPDT Switch Symbol?

In a basic SPDT schematic, there is one common node, and the movable/selectable connection connects to one of two alternate throws. The schematic shows the overall contact topology; however, the schematic does not represent or define how the physical terminals are numbered, whether center-off functionality is present, how momentary return functionality operates, or whether the switching is make-before-break or break-before-make.

If there are symbols that have the alternate contacts labeled as NO and NC, then those labels are contingent upon an established free or normal position.

SPDT vs SPST vs DPST vs DPDT

Although the size and shape of the actuator and package will not determine whether a switch is SPST, SPDT, DPST, or DPDT, the actual difference is in the total number of independent poles and available throws.

A Double Pole Double Throw (DPDT) switch can sometimes be used as an SPDT type by utilizing 1 pole and leaving the other pole unused. The use of 1 pole of a DPDT switch is based upon specifications listed within the switch manufacturer’s documented specifications, which define the terminal usage and installation requirements, rating, and timing requirements for that particular pole. The unused terminals on the DPDT should not be interpreted as indicating correct wiring of the switch package; they indicate only that they are unused.

The SPDT can also be used as a simple on/off switch where a user utilizes the Common (COM) terminal, 1 throw, and leaves the other throw unused. In this case, the resting or active state is dependent on the correct switch function notation, such as ON-ON, ON-OFF-ON, or momentary operation. The APEM Switch Topology FAQ further discusses that the pole or throw configuration of a switch does not determine the number of actuation positions or whether a switch will be maintained or momentary.

Does SPDT Mean ON-ON, Center-Off, or Momentary?

The meaning of a generic SPDT switch is one pole with two throws; therefore, the ON-ON, ON-OFF-ON, maintained, and momentary notations describe operational characteristics independent of the pole/throw notation for an SPDT switch. Treating the pole and throw notation identically to the operation type can lead to the correct graphic symbol but incorrect physical operating characteristics.

• NKK Model M2012LL1G45, which is identified as SPDT with an On-On circuit.
• C&K Model U13P1DZGE, which is identified as SPDT with On-Off-On switching.
• C&K Model MDS6500AL02PS, which is identified as SPDT with an On-Mom On function (one side has a momentary function).

Maintained switches remain in their chosen activated position after the actuator is released, while momentary actions return or change state when the applied force is removed, according to the defined operation of that specific product. Center-Off defines that the middle position of the actuator opens both circuits; none of these characteristics can be derived from the SPDT labelling.

What Forms Can an SPDT Switch Take?

The SPDT marking indicates the configuration of the contacts and not the appearance of the actuator or housing. The electronic implementation of SPDT using analog-switch integrated circuits (ICs) employs semiconductor paths to provide the same 1-of-2 path-selection function.

The MAX4901 – MAX4905 family of devices by Analog Devices includes various single-SPDT analog-switch models that operate within a supply voltage range of +1.8 V to +5.5 V with a typical on-resistance of 0.6 ohm, which is specific to semiconductor signal-switching characteristics and should not be compared directly with mechanical contact current ratings.

Why Does Contact Timing Need Its Own Datasheet Check?

Contact timing describes the process that occurs during the transition between two throws, rather than the two settled states. An example of contact timing is “break before make,” or “nonshorting,” where the first path has been completely opened before the second path is closed. Conversely, “make before break,” or “shorting,” is characterized by a momentary connection of the newly established path immediately before the first path is opened.

The SPDT designation does not determine the timing method. The NKK M2 rocker switch catalogue identifies which of the appropriate switches for that family’s product line are nonshorting, whereas C&K’s ELUM Series datasheets identify which of those switches for that product family are make-before-break.

There are cases where contact timing is critical when a circuit must avoid overlapping contacts, maintain uninterrupted continuity, or provide controlled sequencing, but those circuit requirements do not create a universally applicable rule. Review the timing notation and the associated switching table for your specific part’s ordering code; do not infer any timing characteristics based on the switch symbol.

Timing information may be provided graphically, in tables, or through codes. Follow the movement of each blade through all labeled detent positions or extreme positions. Be sure to carefully cross-reference footnotes regarding shorting contacts, nonshorting contacts, travel, release, bounce, and simultaneous-contact limitations.

How Do You Identify an SPDT Switch in a Datasheet?

The package shape, toggle count, or number of pins isn’t a dependable indicator for determination.

Before searching a catalog, photograph any faintly molded letters using angled light. A partial suffix can help you distinguish between the following variants of the same family of products: panel mount, circuit board mount, illuminated, sealed, and alternative actuators.

After removing the power from the switch and before testing continuity, isolate the switch from any other circuit paths. Thus, when using a continuity/ohmmeter test for identifying the common terminal, the same terminal will alternately connect to each of the two throw terminals as you change the state of the actuator. Lastly, a center-off part will show open to both throws in the center position, while illuminated and other electronic devices may have additional connections that are not throw terminals.

Follow this method for identifying switches:

1. Match every character of the ordering code, including suffixes.
2. Locate in the contact diagram where the common terminal and both of the throw terminals are shown.
3. Review the position table that lists ON, OFF, maintained, and momentary states.
4. Confirm that the timing on the device is either break-before-make, make-before-break, or another timing parameter that may have been specified by the manufacturer.
5. Confirm that the voltage, current, load type, life, and environmental ratings match those of the circuit.
6. Confirm that the terminal location and footprint match those shown in the manufacturer’s package drawing.

Electrical ratings are part-specific, not a generic SPDT property. NKK’s M2012LL1G45 is rated 0.4 VA at 28 V AC/DC, while C&K’s U13P1DZGE is listed at 5 A at 120 VAC or 28 VDC; those two SPDT examples alone show why a current or voltage number cannot be transferred from one switch to another.

Do not carry an AC current rating over to DC, or a resistive-load rating over to another load type, unless the exact datasheet explicitly allows it.

A method for comparing two switches before CAD work is to create a pencil sketch showing the electrical connectivity of each component. Create a single sketch to show the common terminal, two destinations for the throws, and a separate note as needed for each lamp, shield, duplicate lug, or mounting feature associated with each switch. Compare the pencil sketches with the function table provided by the manufacturer; this step shows that the mechanical properties of a switch are separate from the electrical properties and make an ambiguous footprint easier to identify.

Examine the mechanical drawings of the switches independently. Solder tails, bushings, keyways, panel mounting hardware, and support stakes may affect assembly; however, they do not contain any information regarding the contact form of the switch. The legends printed beside the switch lever provide only a description of the physical orientation and user interface of the component and not necessarily the electrical continuity of the component. The manufacturer’s circuit code is what correlates these two properties.

OMRON®’s A3A® lighted pushbutton switch has an SPDT contact configuration; however, it is supplied with terminals labelled “NC”, “NO”, and “C” plus two terminals for the illumination LEDs. Installation can therefore require five holes on the printed circuit board (PCB), because the two illumination terminals are not part of the actual switching; they are solely associated with the illumination function.

SPDT alone cannot determine pin numbers, electrical ratings, transition behavior, or a safe wiring method; these require the exact manufacturer’s datasheet and circuit information. Once the device behavior is established, verify the schematic symbol, PCB footprint, and electrical continuity separately rather than assuming a correct footprint proves a correct switching state.

Where Is an SPDT Switch Used?

Signal and Mode Selection

An SPDT can route a single low-level signal from one source to one of two circuits, select operating modes, or choose paths for indicators. For mechanical SPDTs, contact resistance, bounce, suitability for minimum load, and timing (whether make-before-break or break-before-make) can often be as important as or even more important than rated current capacity in signal applications. For example, Analog Devices’ MAX4901 through MAX4905 provide a family of analog switches with SPDT variants having a typical on-resistance of 0.6 ohm. This can help limit the switch’s added error in a suitable low-level signal path; however, this only describes the contribution from the semiconductor. Other signal-switching characteristics also need to be verified from the same datasheet before the part can be trusted for use with a specific signal.

Power Source Selection

To select between two power sources using an SPDT, the switch must be specifically rated for the source voltage, current, and load type. If the two power sources are not to be connected in parallel (i.e., the two power sources must be completely isolated), then either the break-before-make feature must be used, or some other method to guarantee non-overlap during transfer must be utilized. An SPDT switch by itself does not guarantee non-overlap during transfer. In addition, when using a shorting (make-before-break) type SPDT to select between two power sources, the two power sources will be momentarily connected together through the common terminal during the switch transfer time. This may create a problem if the selected power sources are at different voltages, or if either of the selected power sources is not intended to tolerate being backfed. Prior to selecting an SPDT in this configuration, one should verify the shorting or nonshorting behavior from the datasheet and not solely the voltage and current ratings.

Two-Location Lighting

Electrical terminology for residential wiring in North America may lead to confusion regarding the actual topology of a lighting circuit. The method for controlling one lighting load from two separate locations is accomplished by using a pair of devices called 3-way or three-way switches; each switch has one common terminal and two traveler terminals. An explanatory write-up on how Leviton 3-way switches function can be found at the Leviton website, while the electrical manual at BCcampus identifies that 3-way light-switch devices reflect the underlying SPDT arrangement. In UK product literature, a device with a similar function to the 3-way switch is referred to as a two-way switch.

The names used for switches reflect the application and local terminology and do not include every SPDT product. Also, these names are not installation instructions for the switches. Installation of mains voltage devices should be done using the appropriate devices as specified by local codes and standards, ensuring safe isolation and qualified execution. For a factory-assembled low-voltage control board, powered functional testing should verify the functionality of each commanded control path and, if specified, center or momentary behavior.

DC Motor Reversal

A single SPDT switch is not a universal method for reversing the direction of two-wire DC motors because reversing the polarity of the motor requires the reversal of both leads; thus, a DPDT switch configuration or an H-bridge is a common method of accomplishing DC motor reversal, as described in the bidirectional motor-control tutorial from DigiKey.

Conclusion: Read the Contact Diagram, Not Just the Acronym

The remainder of the SPDT specifications — i.e., center-off functionality, return action, overlap timing, ratings, terminal numbering, and physical pin count — are all separate specifications.