Electrical switches are always drawn in schematic diagrams in their “normal” statuses, regardless of their application. Another way to think of the “normal” status is to think in terms of the switch being at rest.įor a momentary-contact pushbutton switch, this would be the status of the switch contact when it is not being pressed. The “normal” status for a switch is the status its electrical contacts are in during a condition of no physical stimulation. The answer is not complex, but is often misunderstood due to the ambiguous nature of the word normal. But what exactly defines “normal” for a switch? The next part of this article looks at some of the many critical switch parameters and implementation issues.Perhaps the most confusing aspect of discrete sensors is the definition of a sensor’s normal status.Įlectrical switch contacts are typically classified as either normally-open or normally-closed, referring to the open or closed status of the contacts under “normal” conditions. 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 article does not look at pushbutton (momentary contact) switches. However, this introduces some possible ambiguity because “toggle” also refers to a specific type of latching switch, as we will see in Part 4. These non-momentary switches are sometimes referred to generically as “toggle” switches since their action latches. These two broad classes have many similarities but also some differences. 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. (Image: Edrawsoft)Īll of these switches are latching devices that stay at one position until moved to the alternate on/off position by the user. (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.

On off switch symbols