Overview of the Rotary Switch

Rotary switches tend to be the switch of choice when the number of circuits or electrical states (values) in a circuit requiring control exceeds the capability of more conventional toggle or slide switches, for example. The range of circuit values covered by a standard slide switch tends to be very limited, while multiple toggle switches would be needed to control a broad range of values or a large number of circuits. The rotary switch solves this problem, allowing control over multiple circuit values or control over multiple circuits from a single switch.

As their name implies, rotary switches function by means of circular rotation around a central rotor, as opposed to, say, pushing or flipping. They can stop in a number of different actuation positions, turning different circuits on or off or increasing or decreasing the value of the circuit. Some are configured to control multiple contacts simultaneously at a single switch position.

Typically, although other possibilities are available, rotary switches are designed with 3-way, 4-way, 6-way and 12-way configurations and may be of ‘make before break’ or ‘break before make’ design.  Some rotary switches have dozens of possible positions, which can be assigned to different circuits or to a broad range of specific radio frequencies, as on a CB radio for example. They can be used domestically to control the dimmer function on a lamp or the speed of an electrical fan, but there are also numerous industrial and manufacturing applications (see below).

 

History

Engineers and inventors are known to have been experimenting with designs for rotary dials (the precursors to the rotary switch) to transmit telegraph messages as early as the 1830s, but the first patented rotary dial did not appear until 1892, when the American undertaker and former school teacher Almon Brown Strowger launched his device for telephones: the telephone dial was born.

Rotary dials on telephones have, of course, subsequently given way to push-button and digital touch-screen alternatives on smartphones, and the once-ubiquitous rotary switch channel selector on television sets has also faded away thanks to the rise of digital technology from the 1970s onwards.

However, while the uptake of electrical and digital alternatives to mechanical rotary switches is growing, the latter are still widely used in many industrial and manufacturing applications primarily because they remain significantly cheaper than their new-generation rivals (see ‘manufacturing uses’ section below).

 

Technical aspects

A spoke projects in a cam-like fashion at right-angles to the electrically conductive rotor or spindle in a rotary switch, functioning as a contact arm. A series of terminals are located radially around the rotor at the circumference of a disc (also known as a wafer or deck); when the spoke makes contact with any one of these terminals during rotation, it forms an electrical connection to one of the multiple circuits which can be wired to the rotor at any or all of these terminal points.

Multiple poles can be incorporated into the switch by means of adding several decks (through which the rotor passes): each deck is effectively a single pole. During rotation, the switch is usually prevented from stalling between contacts by means of a spring-loaded “detent” mechanism which ensures that it clicks cleanly from one activating point to another.

In order to offer actuating positions at defined intervals of 30, 45, 60 and 90 degrees, contemporary mechanical rotary switches employ a “star wheel” design, behind which nylon cams are mounted. During mechanical rotation of the switch dial, sprung contacts can by moved around them, clicking into notches on the cams to complete the electrical circuit.

It is possible to configure some rotary switches to operate a specific number of actuating positions – it isn’t always necessary, for example, to use all of the points on a twelve-position switch when only four are required. The number of positions can be tailored to the operating requirements of the switch by inserting a projection or tooth on a washer into one of several slots on the switch’s holding nut. This will reduce the number of possible actuating positions to only those required for that particular switch’s operational functions.

 

Where the rotary switch is used in manufacturing

Most consumers are familiar with rotary switches: they can be found on most car dashboards where they adjust the speed of the integral air fan and they’re also used to control the speed of desk-top fans. But they are also widely used in a broad range of industrial equipment, although the sheer complexity of switching operations required by modern equipment is driving the development of digital alternatives, such as touch-sensitive screens on computers, tablets and smartphones.

Even so, because computers running complex manufacturing and industrial systems can fail and may pose significant hazards to safety under such circumstances, a need remains for mechanical rotary switches. They are used to control the speed of conveyer belts, to vary electrical values in electronic instruments (for example, the step-control of AC/DC drives or the frequency and time variables on cathode ray oscilloscopes) and are still used in the pilot controls of many modern aircraft.

In addition, rotary switches are frequently found on professional and high-quality audio equipment, in military and aerospace equipment, medical and diagnostic equipment, construction and agricultural equipment, commercial and military communications and in many off-road vehicles.

 

How the rotary switch differs from other switches

As a general rule, rotary switches enable a much larger range of combinations than the span offered by other types of switch, and they permit these options in an easy-to-use fashion. In addition, users are provided with useful tactile feedback so that they are immediately aware when the switch has clicked into position. Finally, in almost all circumstances, rotary switches are appreciably more robust than other types of switch.