Way back in the mists of time, I set out on a quest to explain the problems caused by switch bounce, along with the various techniques – both hardware and software – we can use to debounce the signals from our switches. My initial Switch Types column introduced a selection of the little rascals, including toggle switches, rocker switches, slide/slider switches, rotary switches, pushbutton switches, tactile switches, micro and limit switches, knife switches, magnetic switches, reed switches, mercury switches, smart switches, and… the list goes on.
More recently, I penned my Ultimate Guide to Switch Debounce columns (Part 1, Part 2, and Part 3), in which we start to rend the veils asunder. The third “leg on the stool,” as it were, is this column, which provides an introduction to the terminology associated with switches.
What is a Switch?
In the context of electrical engineering, the term “switch” refers to a component that can “make” or “break” an electrical circuit, thereby interrupting the current or diverting it from one conductor to another.
Although we typically tend to think about the switches that we turn on and off ourselves, there are other possibilities. The three main classes are:
Hand Operated: Actuated by human touch (e.g., a toggle switch).
Automatically Operated: Actuated by machine motion (e.g., a limit switch).
Process Operated: Actuated by physical changes in some process (e.g., temperature, pressure, humidity, level, flow, pH level).
Also, switches may be realized using different implementation technologies. The three main technologies are as follows:
Mechanical: All the parts forming the switch are mechanical in nature (e.g., a knife switch).
Electromechanical: A mixture of electrical and mechanical parts (e.g., an electromechanical relay).
Semiconductor: A purely electronic switching device that turns On or Off when a small external voltage is applied to its control terminal(s) (e.g., a solid-state relay (SSR), which may also be called a solid-state switch).
The term “actuator” refers to the part of the switch to which an external force is applied in order to operate the switch; for example, a lever, rocker, knob, or button.
Poles and Throws
The simplest type of switch has only two terminals — one for the electric current to (potentially) go in; the other for the electric current to (potentially) come out.
The term “poles” refers to the number of separate circuits a switch can control. A switch with one pole (“single pole”) can control only one circuit, a switch with two poles (“double pole”) can control two different circuits, and so forth.
The term “throws” refers to the number of terminals to which each of the switch’s poles can be connected. If a switch has one throw, each of its poles can be connected to only one terminal; if a switch has two throws, each of its poles can be connected to two terminals, and so forth.
We tend to think of the poles as being the moving terminals and the throws as being the fixed terminals.
SPST, SPDT, DPST, and DPDT
For the purpose of this column, we will use toggle switches as our examples, but the same terminology applies to the other switch types.
The term “toggle switch” refers to a class of electrical switches that are manually actuated by means of a mechanical lever and that have a positive snap action, often accompanied by an audible click.
Toggle switches come in many different forms, but the four most commonly used types are referred to as single pole, single throw (SPST); single pole, double throw (SPDT); double pole, single throw (DPST); and double pole, double throw (DPDT).
With regard to the image, the dashed lines associated with the double pole version indicate that these switches are “ganged together;” that is, the actuator operates them at the same time (give-or-take, depending on any small mechanical variations in their construction).
One way to think about this is that a single pole, double-throw (SPDT) switch is like having two single pole, single throw (SPST) switches — one with a normally open (NO) terminal and the other with a normally closed (NC) terminal — that share a common terminal and are mechanically operated by the same actuator. Also, it’s perhaps worth noting that you can use a SPDT switch as though it were a SPST switch by only using one of its throws (fixed terminals).
As we increase the number of poles and throws, we start to use numbers to describe the switches. A 6P9T rotary switch, for example, would have six poles, each with nine throws.
Off and On, Open and Closed, Break and Make
Consider the SPST switch from the previous section. When this switch is in its off state, it is said to be “open.” The effect is the same as if the switch were removed from the circuit leaving an “open circuit,” which refers to an electrical circuit that is not complete.
When the switch is in its on state, it is said to be “closed.” The effect is the same as if the switch was replaced by a piece of conducting wire.
As we noted earlier, it’s also common to use the term “make” (i.e. making the connection) to refer to closing the switch, and “break” (i.e., breaking the connection) to refer to opening the switch).
Normally Open (NO), Normally Closed (NC), and Common (COM)
Let’s assume we have an SPST switch that is marked with the terms “On” and “Off.” Let’s further assume that when the switch is in its “Off” position it’s open; that is, there’s no connection between its pole and its throw. In this case, we would refer to the throw terminal as being NO (“Normally Open”), and we could also use the SPST-NO moniker to refer to the switch.
The most common type of SPST switch is normally open, but NC (“Normally Closed”) versions are available. In this case, we can use the SPST-NC moniker to refer to the switch. Of course, if the switch doesn’t have any markings, then you can make a NO switch act like a NC switch by “turning it upside down” (if you see what I mean).
Now, let’s assume we have a SPDT switch. In this case, we may refer to its pole as the COM (“Common”) terminal, while one of its throws will be the NO terminal and the other throw will be the NC terminal.
Break-Before-Make (BBM) and Make-Before-Break (MBB)
In the case of a switch with more than one throw, like a SPDT toggle switch, for example, then the most common type is classed as break-before-make (BBM). This means the moving contact breaks the existing connection with the current throw before making a new connection with the other throw. This is also known as a “non-shorting switch.”
Although not so common, it’s also possible to obtain make-before-break (MBB) switches, in which the moving contact makes connection with the new throw before breaking its connection with the existing throw. This is also known as a “shorting switch.”
SPCO and DPCO
When we think of a SPST switch, we may also think of it as operating in an On-Off manner. In the case of a SPDT switch, we might describe this as operating in an On-On manner.
There are also toggle and rocker switches that are like SPDT switches – including using identical schematic symbols — except they have three positions for the lever (rocker). These are known as single pole, changeover (SPCO) or single pole, center off (SPCO), and we may think of them as operating in an On-Off-On manner.
Similarly, there are double pole, double throw equivalents called double pole, changeover (DPCO) or double pole, center off (SPCO).
All we’ve done in this column is summarize some of the terminology we really need to know in order to have a meaningful discussion about switches. In reality, you could write books about these little rascals.
There are all sorts of topics we haven’t covered, like arcing and quenching and wetting current, but those are beyond the scope of what we want to cover here. On the other hand, if there’s something you think we should add, please make a note of it in the comments below. If I agree, I’ll add it to the main column.