What are the symbols for electromechanical on/off switches?

Despite wide use of “soft” on/off switching, the traditional electromechanical switch is still often required or preferred and is available in countless versions.

Switches are represented by their schematic diagram symbols, which correspond to the number and arrangement of their contacts.

Schematic symbols tell part of the story
In an electromechanical hardwired on/off switch, the flow of current is interrupted by the mechanical movement of conducting elements. The schematic symbols for such switches are simple, widely used, and well known (note that there are several styles in common use), beginning with the basic single-pole, single-throw (SPST) switch, with one conducting path (called the pole) and a single on/off position (the throw) (Figure 1).

Fig 1: The single-pole, single-throw switch is basic, as seen in its schematic symbol. (Image source: Edrawsoft)

There are many variations on this basic symbol covering specialized versions of switches used in diverse applications. There are at least 100 schematic symbols for different types of “simple” SPST variations, such as a symbol for the old-fashioned on/off telegraph key or one used as a limit switch.

Hardwired EM switches are also widely available with multiple poles and throws beyond SPST, such as the single-pole, double-throw (SPDT) (Figure 2), the double-pole single-throw (DPST) (Figure 3), and the double-pole, double-throw (DPDT) (Figure 4). The DPDT is often chosen even if all the throws and poles are not needed, as it gives the engineer a “just in case” set of extra contacts.

Fig 2: The single-pole, double-throw switch adds another current path on the output side. (Image: Edrawsoft)

Fig 3: The double-pole, double-throw switch supports two parallel current paths which are switched simultaneously (Image: Edrawsoft)

Fig 4: The double-pole, the double-through switch offers two separate but linked inputs, each with two outputs. (Image: Edrawsoft)

All of these switches are latching devices that stay at one position until moved to the alternate on/off position by the user. There are also electrically identical switches which are momentary contact devices where the action does not latch but returns to its resting position when the pressure (usually but not always from a finger) is removed. These two broad classes have many similarities but also some differences.

These non-momentary switches are sometimes referred to generically as “toggle” switches since their action latches. However, this introduces some possible ambiguity because “toggle” also refers to a specific type of latching switch, as we will see in Part 4. The article does not look at pushbutton (momentary contact) switches. While they, too, play a significant place in electronic products and applications of all types, they are simply outside the scope of this article.

The next part of this article looks at some of the many critical switch parameters and implementation issues.

Related EE World Content

Switches: The complete guide
Miniature Toggle Switches
Dual Seal Waterproof All-in-One Toggle Switch
In defense of the toggle switch
Panel Mount Toggle Switches Feature One Through Four Pole Configurations

References