Rotary Switch
A rotary switch is a multi-position selector whose shaft rotates through a fixed set of detents. At each position, a wiper bridges a common terminal to one of N stationary contacts. Multi-deck rotaries stack several wafers on the same shaft to switch many circuits at once.
On each ceramic or phenolic wafer, a metal wiper rides over a circular array of rivet contacts. A spring-loaded ball bearing presses against a star-shaped detent plate on the shaft, latching the wiper at every contact position. Multi-deck stacks share the shaft so each turn switches every wafer in lock-step.
In plain terms
A wagon-wheel: turn the shaft, a wiper sweeps from one fixed terminal to the next, latching with a detent at each spoke.
Why designers use it
- Function selection on multimeters, oscilloscopes, and audio gear.
- Range selection where one shaft position must reroute many signals.
- Stepped attenuators in studio mixing consoles for repeatable level changes.
Best for
- Bench instruments
- Stepped attenuators
- Range selection
Key specifications
- Positions: 2 – 24
- Decks: 1 – 6
- Current rating: 100 mA – 5 A
- Lifetime: 10k – 30k cycles
- Detent angle: 30°, 45°, 60°
When not to use it
- Where infinite rotation is needed — rotary switches have hard stops every position.
- When the function is digital — a rotary encoder + microcontroller is more flexible and resettable.
- In high-shock environments — the wiper can skip detents.
Common mistakes
- Wiring the common pin to the load — it's actually the wiper; the deck pins are the positions.
- Forgetting to limit the deck to the number of positions you need (most are stop-pin adjustable).
- Treating the sealed and shorting/non-shorting variants the same — non-shorting briefly opens the load between detents.
Where you will find it
- A handheld DMM's central dial is a rotary switch with up to 24 positions and 4 decks: each click reroutes the input jacks, current shunt, and ADC range to measure DC voltage, AC voltage, ohms, current, capacitance, or diode drop.
- Pro audio mixing consoles use 11-position make-before-break rotary switches for stepped attenuators — each click is a calibrated 1 dB drop with no zipper noise.
- A vintage AM/FM radio's band selector is a 3-deck rotary that simultaneously switches the antenna network, the IF mixer's local oscillator, and the audio output's de-emphasis filter.
A short history
Rotary switches in their modern form date to the early 20th century; the multi-deck wafer rotary was perfected for telephone-exchange selectors and military radio gear before becoming standard on consumer test equipment in the 1950s. Brands like Centralab, Mallory, Grayhill, and Lorlin each contributed standard wafer geometries. The 'shorting' (make-before-break) versus 'non-shorting' (break-before-make) wiper distinction has been part of the catalog since at least the 1940s and matters when switching impedance-matched RF signals.
Good to know
- Multimeter rotary switches are why your DMM has a satisfying click — the detent ball drops into a notch on every wafer.
- Make-before-break rotaries are mandatory on tube-amp output transformers; break-before-make would float the secondary and arc the windings.
- The Grayhill 56-series rotary has been in continuous production for over 50 years.