Switch : Basics, Types, Structure

In our day-to-day lives, how often do we encounter switches? Switches can be seen everywhere. For electronic and electrical system, they act as a physical interface to the real world. Be it a key of a keyboard or mobile or be it a knob on an electrical appliance or be it a miniature button used on the PCBs of embedded systems or be it a circuit breaker used on the power lines- all these are switches. We are in continuous interaction with switches without bothering about the manner it functions behind the scene?

Switches are of different types, of different specifications and are selected and used in a particular application according to specific requirements. Subsequent sections will discuss the switches in detail.

WHAT IS A SWITCH?

What we may all be aware about the switches is that, they make or break the contacts. When we press switch of a bulb, bulb glows. On depressing it, bulb glow off. Similar mechanism takes place with other electrical/ electronic appliances/ gadgets, etc.

Literal meaning of a switch is “change of state”. In electrical sense, ON and OFF are the two states and switch helps to change the state of an electrical appliance from ON to OFF or vice versa. Strictly speaking, switch doesn’t turn on or off the appliance; it just makes or breaks the contact.

Fig. 1: Image Explaining The Need For Switches

The circuit symbol of a ON-OFF switch is shown below:

Fig. 2: Circuit Symbol of ON-Off Switch

Apart from the simplest switches wherein actuator make or breaks the contact, there are some complex switches which use some other physical stimulus. There are switches which changes their state by sensing light/ magnetic field/temperature/pressure, etc.. When the parameter being sensed is above some threshold value, switch changes its state.

Switch – Stage

Switches have two states – ON and OFF or CLOSED/MAKE and OPEN/CLOSE. Normally, it is in either of the two states, the states are referred to as Normally OPEN (NO) position and Normally CLOSED (NC) position. “Normally” is prefixed to the OPEN and CLOSED states to refer to the state of a switch when it has not been actuated.

Fig. 3: Picture Showing States of a Switch

Normally OPEN (NO) switches are those which remain in OFF state. On pressing the switch, they gets closed(or becomes ON).

Exactly opposite Normally CLOSED (NC) switches are those which remain in ON state. On pressing the switch, they gets opened(or becomes OFF).

Switches having alternate states are available in two configurations – Latchdown & No Latchdown.

Fig. 4: Image Showing Alternate States of Switches

In No Latchdown configuration, pressing of a switch toggles its state from one to another. However, the actuator returns to its original state once the operating force is removed, though the contacts remain in new state. On pressing the switch again, switch again toggles its state.

In Latchdown configuration, pressing of a switch toggles its state from one to another and also, the actuator doesn’t return to its original state; it remains in new position even after removal of operating force. On applying the operating force again, switch toggles its state and so does the actuator.

Other than Normally OPEN or Normally CLOSED states, some switches have one more state – momentary state. Momentary switches moves to other state and remains there as long as the switch is pressed (or operating force is applied). On release of the switch, they return to their ‘Normally OPEN’ or ‘Normally OPEN’ states. They are expressed as “(ON) – OFF”, where the brackets indicate the momentary state. This is often described as “Biased”.

Fig. 5: Momentary State of a Switch

Switch – Parts

The panel mount toggle switch is depicted in the following picture.

Fig. 6: Diagrammatical Image Showing Various Parts of A Switch

Important parts of the switch are

· Actuator type

Actuator is the one which provides interface to the real world. Movement of actuator changes the state of the switch. There is a wide variety of actuators used in the switches. The list is tabulated below

S No	Type of Actuator	Sample picture
1	Toggles
2	Rockers
3	Paddles
4	Push Buttons
5	Slides
6	Rotaries
7	Keylocks
8	DIP

Mounting Arrangement

· Mounting Arrangement

Switches are used in different kinds of applications and are required to be mounted on the equipment/appliance/circuit board differently. The switches are available with different mounting options as tabulated below

S No	Type of mounting arrangement	Sample picture
1	Panel Mount
2	Front panel mount
3	Subpanel mount
4	Snap-in mount
5	Through-hole PCB-mount
6	Surface mount PCB-mount

Terminal type

· Terminal type

Terminals provide an electrical interface of the switch to the electrical circuit in an equipment/appliance. Due to the wide variety of interfacing options in use, switches are available with different type of terminals. The list of the terminals is given in the following table

S No	Terminal type	Sample picture
1	SMD
2	Solder Lug
3	Wire Wrap
4	Straight PC
5	Right angle PC
6	Vertical PC
7	Screw Lug
8	Quick Connect
9	Solder Lug

Switch – Terminologies

SWITCH – TERMINOLOGIES

1. Contact state

A switch has internally two set of metallic plates which exist in closed (in contact) or opened (not in contact) state. Normally among the two set of metallic plates, one of them is able to move and can make contact with the other one which cannot move.

Switches are sometimes described by its normal state, which is the state in which it exists before someone try to change the state.

The two normal states can be

– Normally open (NO)

– Normally closed (NC)

Normally opened switches are switches in which the metal plates are normally in open state, means doesn’t touch each other. It can be converted to closed circuit whenever required.

Normally closed switches are switches in which the metal plates contact is normally in closed state. It can be converted to open circuit whenever required.

2. Pole

Among the two set of metallic plates, one of them is able to move and can make contact with the other one which cannot move. A pole is the external connector point of a switch which is internally connected to the movable metallic plates.

Number of poles simply means the number of movable metallic contact plates in a switch. The number of poles is actually the possible number of separate circuits that can be made closed with the switch.

3. Throw

The term throw simply means the number of positions in which one can throw the movable metallic contact plate. In other words, it is the number of possible directions into which the movable metallic plates can be put to make a contact with the fixed metallic plates.

Hence it is the number of circuits which a pole can make contact with.

A switch can be defined using the above three terms. The contact states, number of poles and number of throw varies in different switches. Now let’s go through some of the common NO (Normally open) switch configurations.

4. Break

The break number is the number of contacts which the switch breaks when it is turned off.

To break a circuit, breaking at a single point is enough. But certain switches use double breaking mechanism.

Fig. 7: Image Showing Switch Breaks

· Way

For more than three conducting positions, ‘Way’ is used instead of ‘Throw’.

· Current/Power Switching Rating

Mentioned as Contact Rating on the datasheets, it is the maximum steady current/power that can be interrupted or made with purely resistive loads. Often manufacturers quote two ratings- one for resistive loads and one for inductive loads.

· Current Carrying Rating

It is the maximum steady current through a closed switch. The limit is defined by the conductive losses and is usually greator than current/power Switching Rating.

· Voltage Rating

It is the maximum working voltage, assuming resistive loads, for a switch. It is governed by insulation materials, contact separation, rate of separation and by safety considerations.

· Rest Position

The state of a switch in which no force is applied on the actuator is known as rest position.

· Pre-travel

Pretravel is the distance covered by the actuator from its rest position to the operating point.

· Operating point

Operating point is the position of the actuator at the initial switch contact transfer.

· Overtravel

Overtravel is the possible movement of the actuator after intial contact transfer is made.

· Movement Differential

Movement Differential is the movement required between the operating point and the reset point.

· Reset Point

Reset point is the point of actuation at which return contact transfer takes place; the contacts return to their original position.

· Reset Travel

Reset travel is the continued actuator movement back from reset point to rest position.

Various phases and related terminologies during the movement of actuator from one state to another are indicated in the following figure.

Fig. 8: Figure Showing Actuator Movement From One State to Another

Basic switch configurations

1. SPST

Single Pole Single Throw (SPST) is the most commonly used switch configuration. It has only a single pole and only a single throw is possible to open or close the contacts. There is only one moving metal plate which can make contact with a metal plate.

The circuit symbol of an SPST switch is shown below

Fig. 9: Image Showing Single Pole Single Throw Switch Configuration

The Pole and throw direction of a SPST switch is shown in the above figure.

2. SPDT

Single Pole Double Throw (SPDT) switch has a single pole as in the case of an SPST. This single pole can make contact with two metal plates, but only one at a time. This is made possible by moving the pole metallic plate to two different directions to make contact with the metallic plates, with one in each direction. When the pole makes a contact with one of the metallic plate, it breaks the contact with the other one simultaneously.

The advantage is that we can make one device on and other device off simultaneously.

Fig. 10: Picture Showing Single Pole Double Throw Switch Configuration

Switch configurations Contd..

3. DPST

Double Pole Single throw (DPST) has two separate poles but only single throw direction. There are two movable metallic plates which are operated together. They can be moved together in a single direction to make contact with two corresponding fixed metal plates.

The advantage is that two separate devices can be connected via the switch and both can be operated together, means both can be made on or off together by the single throw.

Fig. 11: Double Pole Single Throw Switch Configuration

4. DPDT

Double Pole Double throw (DPDT) has two separate poles and two throw directions as well. There are two movable metallic plates which are operated together. They can be moved together in two separate directions to make contact with two set of corresponding fixed metal plates.

The advantage is that two separate devices can be connected via the switch and both can be operated together. We can make one set of device on and other set of device off simultaneously.

Fig. 12: Image Showing Double Pole Double Throw Switch Configuration

Switch – Different Types of Switches

Switches have been classified primarily upon the type of actuator it uses. Following are the variety of switches available in the market

· Toggle

One of the most commonly used switches is a toggle switch. It can be seen on numerous lab equipments.

It is one of the classes of switches which use mechanical action lever or a handle as an actuator. They are available in different configurations –different actuators and different contact arrangement and styles.

Following table presents the type of actuators available in toggle switches.

S No	Type of Toggle switch	Picture
1	Standard
2	Flat
3	Locking Lever
4	Cone
5	Large Bat

Toggle switches are available in both configurations –Alternate Contact Latchdown and Momentary. They find their applications in medium power, bench-size devices, measurement equipments, etc.

Paddle & Rocker Switch

· Paddle Switch

One more type of switch in the same class and also very much similar to toggle switch is the paddle switch. Some of them are shown below

Fig. 13: Paddle Switch

· Rocker Switch

Fig. 14: Picture Showing A Regular Rocket Switch

Another type of switch in the same class is the rocker. A rocker switch is one that rocks when pressed; one side of the switch gets raised, the other side gets depressed somewhat similar to a rocking horse which rocks back and forth.

A rocker switch typically has a circle on one side and a horizontal line on the other to signify the state of the switch (and the device).

Actuators available in rocker switches are shown in the following figure

Fig. 16: Figure Showing Various Actuators Available for Rocket Switches

Rocker switches are used in surge protectors, power supplies, test and measuring equipments, display monitors and in many other applications.

Slide & Push Button

· Slide Switch

This type of switch is also very similar to rocker switches; the difference is in the actuator employed.

Fig. 16: A Typical Slide and Push Button

It is used for low power, on-off applications in digital circuits, consumer electronics, toys, measurement equipments, etc. Read more on how slide switch works

· Push Button Switch

Push Button use a button as an actuator along with a internal spring mechanism. Push buttons are available in all configurations – momentary, alternate contact with no latchdown and alternate contact with latchdown.

Fig. 16: A Regular Push Button Switch

They are typically used in bench-top equipments, test and measuring equipments, toys, etc. Insight – how push button switch works

Rotary & Key Lock

· Rotary Switch

Rotary switches are typically used as selector switches and generally have two or more position to select from.

Fig. 17: Rotary Switch

Different types of knobs or levers are used as actuators in rotary switches

Fig. 18: Various Knobs Present in Rotary Switch Actuators

They are also used for low power switching applications, measurement equipments, etc. generally for applications where one out of many positions is needed to be selected. Insight – how rotary switch works

· Key Locks

Key locks are switches which operate with a key; only the person with a key can operate the switch and hence the equipment. It is used in safety-critical applications.

Fig. 19: Key Lock Switch

These are used for high power, on-off switching applications in consumer and industrial-safety critical applications

DIPs & Membrane Switches

· DIPs

DIP is an acronym for Dual In-line Parallel. DIP switches are basically an array of miniature SPST switches packages in same size as the DIP package of integrated circuits.

Fig. 20: Dual in-line Parallel Switch

It is used in electronic circuits for setting some code, for selecting a particular setup based on the status on the DIP switches. Insight – how DIP switch works

· Membrane Switches

Membrane switches are typically used in momentary configuration. They are of two types- tactile and non-tactile.

Non-tactile membrane switches are designed with a wide range of actuation forces. Tactile membrane switches use a metal dome to achieve the desired tactile response.

Fig. 21: Image Showing Array of Membrane Switches

These are used in low power, miniature devices, for digital switching in PDAs, mobiles, etc.

Micro & Reed Switches

· Micro Switches

Fig. 22: Pictures Showing Micro Switches

Micro-switches are designed to switch between the two states in responses to small movements. The lever is pushed by the machine part. They typically are available in momentary configurations and have internal spring mechanism for level to return to its rest state.

Micro-switches are available with multiple actuator styles including pin plunger, hinge lever or roller lever

Fig. 23: Various Kinds of Actuators Available For Micro Switches

They are used in industrial applications where applied force by the machine part is used to toggle the state of the switch. Insight – how micro switch works

· Reed Switches

Fig. 24: Diagrammatical Figure of a Reed Swtich

A reed switch consists of two ferromagnetic nickel-iron wires and specially shaped contact blades (reeds) positioned in a hermetically sealed glass capsule with a gap between them and in a protective atmosphere. The plated contact surfaces are isolated from the outside environment, which protects the contacts from contamination.

Fig. 25: Image Showing A Figuratical Image of Reed Switch and Its Parts

Reed switches can be operated using a magnetic field generated by either a permanent magnet or current-carrying coil.

Fig. 26: Image Showing Working of a Reed Switch

Reed switches offer advantages of compact size, low weight, quick response time, long life and low cost. Common applications of reed switches are in reed relays, automotive sensors, security systems, robotics, toys and games. Know details of How Reed Switch works

Switch – Selection

SWITCH – SELECTION

Important specifications to be considered while selecting a switch are

1. No of Contacts – Pole and Throw/Way

This is solely dependent upon the specific application where switch is to be used.

2. Contact Rating

Contact Rating defines the maximum current and voltage a switch can safely handle. This is again application dependant.

3. Contact type

Choice of contact type – solder/screw lug or PC type- should be made considering the end application

4. Mounting Arrangement

Switch should be selected for application specific mounting; a switch with same functionality but with different mounting may not suitable for your application

5. Actuator Type

Choice of actuator is mostly based on ergonomics and user friendliness of the product.

Also read article on Specification of a switch