101 Guide to Switches
Understanding The Basic Types and Functions of Mechanical Switches
A Switch is a simple device that is used to break a circuit and disturb the flow of electrons.
A switch can be either closed or open. A closed switch is 'On' and will activate the circuit whereas an open switch is 'Off' and will interrupt or break the circuit.
Switches can be either Mechanical or Electronic. A Mechanical Switch is activated with a physical action; pressing, moving or touching it's contacts. An Electronic Switch is activated differently and does not need physical contact; instead it is activated by semi conductors such as transistors. This guide will focus on Mechanical Style Switches, we will create a guide to Electronic Switches in due course.......
Many Mechanical Style Switches are categorised by their number of poles, throws and style of operation. Poles & Throws are simply denoted by letters; SPST, SPDT, DPDT for example.
POLES: Tell you how many seperate circuits a switch can control. A switch with one pole can direct one circuit, two poles can direct two circuits, and so on.
THROWS: How many positions can be activated by the switch. A single throw can close a circuit at one position whereas a double throw switch can close a circuit in two positions.
An SPST (Singe Pole Single Throw) switch is the simplest form of mechanical switch. It is a sinmple On - Off Switch with one input contact and one output contact. It switches a single circuit and can either make (On) or break (Off).
An SPDT (Singe Pole Double Throw) switch has three terminals, one more than the simple SPST switch. It has one input contact and two output contacts. This means that an SPDT switch has two ON positions and one OFF position. Most slide switches have an SPDT configuration and are used to changeover between two outputs.
A DPDT (Double Pole Double Throw) switch has six terminals, it is simply two SPDT switches. The switch has two input contacts and four output contacts. Commonly used as On-On or On-Off-On Switches.
A good visual example for a DPDT Switch is controlling car turning indicators. Activating the switch up would turn on both the front and back car indicator in one direction; activating the switch down would turn on both the fron and back indicatorsin in the opposite direction.
Momentary vs Latching
A Momentary (Double Pole Double Throw) switch requires the switch to be pressed/held continuously for the operation to work, when it is released the switch will stop. For example the switches on your keyboard! When they are released they no longer operate. A Latching Switch will stay on (or off) until it is pressed again, for example a light switch.
Normally Open (NO) Vs Normally Closed (NC)
Simply: A Closed Switch allows current flow and is therefore ON. An Open Switch does not allow current flow and is therefore OFF.
A Normally Open (NO) Switch has an OFF resting position - when the switch is pressed it becomes a closed circuit and is therefore ON..
A Normally Closed Switch has an ON resting position - when the switch is pressed it becomes an open circuit and is therefore OFF.
Type of Switches
Types of switch operations include Toggle Switches, Push Button Switches, Rotary Switches and Slide Switches to name a few. This section will look at the most common types of Mechanical Switches.
- Toggle Switches
- Push Button Switches
- Rocker Switches
- Rotary Switches
- Slide Switches
1) Toggle Switches
A toggle switch is actuated by a lever that can be 'flicked' into one, two more positions to actuate the switch. Most commonly described using the pole and throw notation described in the previous section. A momentary toggle switch will spring back to the original position when pressure is released.
2) Push Button Switches
The most common type of two position switch - Come in a variety of shapes, styles and materials. Pushing the button will either make (On) or break (off) a circuit. The action can either be latching or momentary. A latching switch will stay On or off until it is pressed again.
3) Rocker Switches
A Rock Switch has a 'seesaw' type action whereby pressing one side will rock the button in that direction and close the contacts (ON). Rocking the swithc back to the alternate side will open the contacts (OFF). Many styles and shapes are available in single pole and double pole configurations for example.
4) Rotary Switches
A rotary switch is turned in order to open or close the contacts of the switch. It consists of a moving contact which is turned in order to connect with the stationary contacts. A visual example would be the dial on a cooker.
5) Slide Switches
Commonly have a small knob that can 'slide' in order to open or close the switch contacts. They are very compact and easy to operate making them ideal for small scale projects or circuits where changeovers are required, for example within model railway.
Microswitches have either a small push button or lever in order to actuate the switch. They require very little physical force in order to operate and are ideal for small scale projects or applications. They use a tipping point mechanism or 'over centre' mechanism, sensitive to small actions.
A common practical example of a microswitch in action would be within a microwave door; if the door is opened whilst the microwave is operating the switch lever is released and the contacts are open, stopping the machine.
This is intended as a general guide to Mechanical Switches - We will be looking to update this guide to provide additional details in the future. If you have any further questions please do not hesitate to get in touch!