Flush swith; Raised switch; Recessed switch

Images credits: Automation Direct; Digi-Key; Schurter

 

Pushbutton switches are mechanical switches defined by the method used to activate the switch. The activation method is typically in the form of a plunger that is pushed down to open or close the switch. 

Switching Mechanism

There are several pole and throw configurations for pushbutton switches. The number of switch contact sets used is known as the number of poles and the number of conducting positions (single or double) is referred to as the throw. Switching mechanisms function differently by type; the five types of switches are described below:

  • Single pole single throw (SPST) is a switch that makes or breaks the connection of a single conductor in a single branch circuit. This switch typically has two terminals. It is commonly referred to as asimple on-off switch and can be used to switch the power supply to a circuit.

Circuit symbol pushbutton switch 

 On-Off switch circuit symbol. Image Credit: The Electronics Club

 

SPST switches can also work as "push-to-make" on, where when the button is released it returns to its normally open (off) position or vice-versa. 

Selecting SPST push-to-make switch

(On)-Off switch circuit symbol. Image Credit: The Electronics Club

  • Single pole double throw (SPDT) is a switch that makes or breaks the connection of a single conductor with either of two other single conductors. This switch typically has 3 terminals, and is commonly used in pairs and called a "Three-Way" switch.The switch can be in both on/off positions, switching on a separate device in each case. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in another position. Special versions can have a third switch position which turns both circuits off.

On-On switch circuit. Image Credit: The Electronics Club

  • Double pole single throw (DPST) is a switch that makes or breaks the connection of two circuit conductors in a single branch circuit. This switch typically has four terminals. The pair of on-off switches operate together and it is often used to switch the main electricity because it can isolate both the live and neutral connections.

On-Off switch circuit. Image Credit: The Electronics Club

  • Double pole double throw (DPDT) is a switch that makes or breaks the connection of two conductors to two separate circuits. This switch typically has six terminals and is available in both momentary and maintained contact versions.It can be wired up as a reversing switch for a motor and special versions can include a center off position.

DPDT switch       

On-On switch circuit; Wiring for Reversing Switch.

Image Credit: The Electronics Club

  • Solid-state switching mechanism includes transistors and piezoelectric materials.

Configuration

A normally open (NO) switch has contacts that are open or disconnected in their unactuated (normal) position.  A normally closed (NC) switch has contacts that are closed or connected in their unactuated (normal) position.

Actuator Style

The actuator in pushbutton switches is the part of the switch to which an external force is applied to operate the switch. It can come in several styles including,

  • Recessed- A recessed pushbutton actuator is within base, this is useful to protect against accidental actuation.
  • Flush-A flush pushbutton is flush with the base; typical example would be a doorbell.
  • Raised- A raised pushbutton resembles a plunger; typical example would be a pedestrian cross walk button.

Switch Type Function

  • In a maintained contact switch an actuator stays in thrown position. This includes on-off, but it also includes Three-Position - (Center-off) and Three Position - (No Center-off) switches where the switch remains in its actuated position.
  • Alternating action such as push on, push off characterizes an alternating contact.
  • In a momentary contact switch the switch must be held in position; it reverts to normal position when actuating force is removed.

Specifications

Important switch specifications to consider when searching for pushbutton switches include,

  • Maximum current
  • Maximum AC voltage
  • Maximum DC voltage
  • Maximum power

Terminal Type

Choices for terminal types for pushbutton switches include,

  • Feed-through style is an in-line switch that can be attached at any point on a length of flexible cord to provide switching control of attached equipment.
  • Quick connect or blade, sometimes referred to as "spade" terminals, are designed to be inserted/removed rapidly without the need for soldering.

Blade Terminal. Image Credit: Digi-Key

  • Right angle PC pins are printed circuit board type, angled pins.
  • Screw terminalsare terminals requiring a screw (clamped) for a positive electrical connection.
  • Side PC pins are a printed circuit board type withside exit pins.
  • Solder terminalsare terminals requiring solder for a positive electrical connection.
  • Straight PC pins are a printed circuit board type with no angled pins.
  • Surface mount technologyisa method of soldering packages directly onto the surface of printed circuit boards without insertion of leads through a board.
  • Wire leads means the switch has integral wire leads for electrical connection.

Materials

Construction materials for the base or housing and the pushbutton include thermoplastics or metals. 

Features

Features common to pushbutton switches include:

  • CE certification is for the International commission on rules for the approval of electrical equipment. This directive ensures that electrical equipment does not get placed on the market if, when installed and maintained, it endangers the safety of persons, domestic animals, or property.
  • CSA certification stands forthe Canadian Standards Association.
  • UL listing stands for the Underwriters Laboratory, which is a safety science company that advises on product safety.
  • Dustproof means the device is resistant to dust which could interfere with the switch connection.
  • High inrush current compatible (>100A) devices are capable of handling a high inrush current. Inrush current is a momentary surge in current that can occur when an electrical device starts. The surge is very brief but it can trip circuit breakers or blow fuses. A compatible device can handle high inrush current without overloading.
  • Illuminated switch has an integral lamp that illuminates when the switch is connected to an energized circuit. This makes locating the switch and identifying the on/off position easier.

Illuminated switch

Illuminated switch. Image Credit: Schurter

  • Imprinted markings include a variety of imprinted indicators, such as ON/OFF, HI/LOW, etc.
  • Locking switchesare equipped with a locking mechanism that requires a key to operate the switching functions
  • Pilot light are used to indicate when the circuit is active so 'on' switches can be identified at a glance.
  • Tape and reel packagingmeans switches come in a reel that can be fed into an automation process.
  • Time delayfeatures use a switch with an integral mechanism or electronic circuit that will automatically switch a load OFF at a predetermined time interval.
  • Vandal proof switcheshave rugged housings, break-resistant type shielding and tamper-proof screws.
  • Wiping contacts are self-cleaning contact type switches. They usually provide a low resistance in circuits where contact resistance is critical.
  • Weather resistant or waterproof switches are hermetically sealed against moisture.

Resources

Pushbutton Switches

Summaries of EU Legislation

Guide to Specifying High-Frequency Electronic Ballasts

Switches


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