Start a car. Ride an elevator. Send an email. Whatever you do in the modern world that has anything to do with electricity you will find switches, and lots of them. The switch is the number one electrical component that consumers around the world interact with on a daily basis. We use it without thinking and pay attention only when it doesn’t work. And all it does is turn things on and off!
In its most basic form, a switch is a mechanical device that, when operated, opens or closes internal contacts in order to complete an electrical circuit or divert current from one circuit to another.
That is the simple definition. In reality, modern switches come in hundreds of variations covering countless applications and offering endless features. If you want to turn something on or off, there is no shortage of ways to make this happen.
Basic switches are binary devices, meaning that they are either on or off. They are made up of contacts and an actuator. The actuator is a push button or lever that transfers external force to an internal mechanism. The contacts are internal metal components that transfer electrical energy when they touch.
Switches can have either normally open (NO) contacts, so the circuit is activated when the switch closes the contacts; or normally closed (NC) contacts, in which the switch is used to open the contacts when a force is applied. Switches can also be listed as changeover (CO), with multiple contacts that are actuated by a central rotating bar, as in rotary switches.
Switches are usually specified based on their voltage rating, or the maximum voltage that can be carried by the switch. It is expressed as VAC or VDC. They can also be specified by current rating, which is the maximum continuous current that a switch can carry for a given load, expressed as amps (Amps) or milliamps (mA). Contact resistance, or the resistance to current flow through the switch when closed can also be specified. This is expressed as Rk and also RC.
Given the broad variety of switches available today, however, many other specifications can also apply, including: tactile feel, color, mechanical operation, illumination, water and solvent resistance, shock and vibration resistance, replacement ease, useful lifecycle, etc. Any of these attributes (or a mixture of several of them) can add to the final operability and user experience of the ultimate end product or system.
The different types of mechanical actions available in switches include:
- Momentary Action - circuit is activated as long as force is applied to the switch, and is de-activated once that force is removed (push button switch).
- Alternating Contact - a circuit is activated by a force on the switch and it remains in that position until another force is applied that closes another set of contacts (rotary switch).
- Latching Action - a force closes the contacts and energizes the circuit, which remains that way until another force is applied to open them (toggle switch).
Switches can also be panel-mounted, pc-board-mounted, wired in-line, wireless, etc. Many products labeled as switches are a combination of a sensor and a switch to control a physical presence or amount, as in a level switch. Here is a quick run-down of some of the many switch types available:
- Push Button - push to turn on and release to turn off, or push once for on and push again for off.
- Rotary - rotate an internal shaft to close contacts involving several circuits.
- Rocker - depress the raised side of the device to open or close a circuit.
- Slide - sliding action closes or opens contacts.
- Toggle - similar to rocker.
- Trigger - when the trigger is pulled the circuit is closed, and opened when it is released.
- Keylock - similar to a rotary switch but lockable at pre-set positions.
- Thumbswitch - operation of a button or wheel engages the contacts.
- Limit - uses an external actuator in a system to limit movement of an object past a predetermined point.
- Level - senses fluid, powder, slurry or solid level and controls amount.
- Keypad - collection of switches in a close configuration.
- Membrane - switch constructed of circuit board-mounted printed plastic, usually grouped as a keypad.
- Pressure - combines a switch with a pressure sensor to close or open a circuit when a pressure value has been reached.
Selecting the right switch for a product design is usually determined by current rating requirements, footprint, mounting needs, lighting necessities, style specifications, tactile requirements, and customization. Certain applications may also need to allow for ease of repair or replacement of the switch.
Switches may also be specified based on resistance to exposure to dirt, dust, gas, moisture, humidity, shock, vibration or current surges. Attention should also be paid to sourcing switches that conform to engineering standards for the final product use. Finally, depending on the application, the failure rate of an individual switch may also be considered and is typically given by the manufacturer.