Switch Basics

The switch is a universally employed and highly useful device because of its elegance, functionality, and simplicity. It is easily designed as a basic control in millions of products, systems, homes, factories, cars, airliners, and on and on. From humble beginnings as a push button device in 1884, through the first toggle switches in 1917, to the modern membrane and touch-sensitive models of today, the switch is something we often use without thinking, frequently noticed only when not working, and could not do without.

From a design standpoint, switches are binary devices. Meaning they are either on or off. In their most basic form, switches simply move an internal part that either opens or closes contacts to complete an electrical circuit or divert current from one conductor to another.

Switches are composed of contacts and an actuator. When the contacts in the device are normally open (NO), the circuit is activated when the switch closes the contacts after an applied force. When the contacts in the device are normally closed (NC), the switch is used to open the contacts when a force is applied. In changeover (CO) switches, there are no normally open or closed states, but multiple contacts that are actuated through the rotation of a central rotating bar, as in a rotary switch.

Despite their simplicity of operation (or perhaps because of it) there are literally hundreds of different types of switches available on the market designed to add value to specific applications. Finding the right one for your design not only depends on operation and performance, but also on the other physical attributes the switch adds to your product. Physical attributes include appearance, color, tactile feel, mechanical operation, sound, durability, resistance to water, solvents and cleaners, ease of replacement, etc. The list goes on, and demonstrates just why there are so many switch types and styles available today.

There are also different types of mechanical action employed in switches. In a momentary action device, the circuit is activated as long as force is applied to the actuator, as in a push button switch, and is de-activated once that force is removed.

In an alternating contact device there are usually multiple contacts. A force activates a circuit and the switch stays in that configuration until another force is applied, closing an alternate set of contacts, as in a rotary switch.

In a latching action device, a force closes the contacts, which remain in that configuration until another force is applied to open them.

When selecting a specific switch that will perform correctly in a particular application, certain basic technical factors need to be considered in how a device will perform. These factors include:

• Voltage Rating - the maximum voltage that the switch can withstand in the circuit in which it is used. It is usually a function of contact separation, insulation materials, size and type of actuator and contacts, etc. Usually expressed as VAC or VDC.
• Current Rating – the maximum continuous current that a switch is designed to carry for a given load through a closed contact, expressed as amps (Amps) or milliamps (mA).
• Contact Resistance – resistance to current flow that exists in the contacts of a switch. Expressed as Rĸ.
• Useful Life – average life expectancy, expressed in cycles, for a device, if voltage and current ratings are not exceeded. It is useful to remember that any electrical device can be compromised if it is subjected to events that exceed its tested ratings. Switches are no different. Contact sticking, bounce and arcing can occur and degrade the action of the switch. Since many switches are electromechanical, other individual components can experience wear and eventual failure.

As stated, there are a considerable number of switch types and styles available today for both the original equipment and MRO markets. Many products labeled as switches are really a combination of a sensor and a switch to control a physical presence or quantity. Here is a run-down of some of the basic operating types that you will want to consider for your own design or replacement use. While extensive, this list is not allinclusive.

• Push Button – simple mechanism that either remains closed until released (non-latching), or remains closed until pushed again (latching)
• Rocker – simple on/off action operated by depressing the raised side of the device to either open or close a circuit.
• Rotary – operated by rotation of an internal rod that closes multiple internal contacts to control several circuits.
• Slide – uses a slider to move contacts to either an open or closed position.
• Toggle – incorporates a rocker device (lever) that opens or closes a circuit when moved.
• Trigger – uses an operator-pulled trigger to close a circuit, which is then opened when the trigger is released.
• Keylock – activated by a security key turned in a circle and capable of stopping at pre-set positions to close contacts.
• Thumbswitch – operated by thumb movement of a button or wheel to engage a circuit.
• Limit – switch with an actuator linked to internal contacts that prevents travel of an outside object past a predetermined point.

• Level – combines a switch with a sensor to detect and control the level of liquids, powders, solids or slurries.
• Keypad – a set of mechanical switches situated in a block or pad with symbols or letters to differentiate individual key function.
• Membrane – similar to a keypad but the switches are usually custom designed circuit board-based printed plastic, usually ganged as a keypad or keyboard.
• DIP Switch – multiple switches arranged in a dual, in-line package used to select the operating mode of a device.
• Pushwheel – see Thumbswitch.
• Snap Action – switch operated by minimal physical force that can open or close a circuit at rapid speed.
• Joystick – a lever-controlled device that communicates its position across 360° to the device it is controlling.
• Detect/Interlock – responds to the position or presence of objects to control a circuit.
• Capacitive – operates by measuring a change in capacitance across a surface, usually from a manual touch.
• Touch Switch – see Capacitive Switch.
• Piezo – uses a piezoelectric element to generate a charge when activated that is used to in turn activate a circuit, usually by means of a field effect transistor (FET).
• Proximity – device that can sense the approach of something metallic and activate a monitoring circuit.
• Pressure – combines a switch with a sensor to close an electrical circuit when a predetermined fluid pressure has been reached.

Selecting the appropriate switches for your OEM design or MRO needs requires more than a review of individual current ratings, footprint, design and life expectancy. It also necessitates selecting appropriate parts that conform to one or more of the various international engineering standards that address the quality of materials and methods used to build products. Several of the key organizations supplying standards, testing and certification in the area of electronic and electrical switches include:

• IEEE (Institute of Electrical and Electronics Engineers) – develop and maintain global standards relating to many electronic and electrical products.
• UL (Underwriters Laboratories) – develop and maintain standards that assess product safety across many industries, including electronics and technology.
• NEMA (National Electrical Manufacturers Association) – provides standards for electrical products and housing to protect products.
• CSA (Canadian Standards Association) – the CSA Group publishes standards and provides product certification and testing services worldwide, including electrical products and systems.
• ASTM (American Society for Testing and Materials) – provides standards for use in the development and testing of materials used in the manufacturing of components, including switches.
• IEC (International Electrotechnical Commission) – sets international standards for electrical and electronic products and systems.

Standards from additional organizations may also apply, as in the cases of harsh environments or mil-spec. As with most other products, applicable standards can also vary from country to country.

Continuous technology development by modern suppliers has led to advances in switch functionality, lifecycle, quality, and cost. Switches can also program, alert, display, detect and control. No longer are they just on or off devices.