(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. Fig 2: The single-pole, double-throw switch adds another current path on the output side.
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. 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).
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. There are many variations on this basic symbol covering specialized versions of switches used in diverse applications. Fig 1: The single-pole, single-throw switch is basic, as seen in its schematic symbol. 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). In an electromechanical hardwired on/off switch, the flow of current is interrupted by the mechanical movement of conducting elements. S chematic symbols tell part of the story Switches are represented by their schematic diagram symbols, which correspond to the number and arrangement of their contacts.
Despite wide use of “soft” on/off switching, the traditional electromechanical switch is still often required or preferred and is available in countless versions.
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