15 Aug
Switch terminology explained : SPST, SPDT, DPDT, DPST

Posted by Admin 

Switch contact configurations can sometimes be confusing. With this article, we hope to clarify some of these ambiguities. Unfortunately, not all switches are created equal. They come in different shapes and forms and for different functions. From limit switches, proximity switches, to toggle switches and many in between, these switches represent critical components of controls system and are used as information gathering. Switches and sensors are the eyes and ears of controls systems. They can be digital inputs of controls. Controls inputs can also be analog and some switches have analog signals as well as digital, but that will not be discussed in this article. Below is a list of general terms to keep in mind while reading this article.

Terminology

Open: The open term means that the circuit is open, in other words, non-conducting or off
Closed: The closed term means that the circuit is closed and that it is conducting electricity
Pole: This refers to the number of switch contact sets
Throw: This refers to the number of single or double conducting positions
Way: This is the number of conducting positions (3 or more)
Momentary: This means that the switch returns to its normal position when no longer actuated
Toggle: Flip in one direction to change state, then flip in the other direction to change the state back
SPST: Single pole, single throw
SPDT: Single pole, double throw
DPST: Double pole, single throw
DPDT: Double pole, double throw
NO: Normally open
NC: Normally closed

SPST Switches

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This is the simplest of all switches and basically consists of an ON and and OFF state. Single Pole, Single Throw switches connect or disconnect one terminal to or from another when the switch it actuated. Typically these switches will have 2 terminals for wire connections and the wires can be swapped without any consequence.

SPDT Switches

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Single Pole, Double Throw switches or SPDT switches can be on in 2 different positions, which can be used for example to operate two separate machines. The two outside terminals are never connected; only one terminal and the common terminal are connected. When the switch is actuated, the common is then connected to L2, then when the switch is switched back, the common will connect back to L1.

DPDT Switches

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Double Pole, Double Throw switches or DPDT switches involve 2 different circuits, two ON and two OFF switches which operate together. This switch can isolate (disconnect) or activate (connect) two different circuits at the same time. The DPDT would be equivalent to two SPDT switches connected together so that they actuate at the same time.

DPST Switches

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Double Pole, Single Throw switches or DPST switches would be equivalent to having two SPST switches connected together so that they actuate at the same time.

Conclusion

These are the most common configurations used, but there are several other switch configurations that are not discussed in this article. These switches also come in momentary, toggle, alternating and many other interface configurations. Please note that these concepts also apply to proximity, limit, and other sensor type switches. Some switches are illuminated while others may have other visual state indicators. When selecting a switch, the current rating and duty cycle are two of the most important characteristics that need to be considered.
Below are a few other configurations that may be of interest to the reader.

Switches configurations