Electronics Switches

- an overview or tutorial giving the basic facts about the different types of mechanical electronics switches for switching applications within electronics circuits.

Switches form a vital element in many electronics circuits. These electronics switches provide a variety of functions. The simplest function is to interrupt a circuit so that it can be switched off and switched on. Other types of electronics switch enable circuits to be routed through to different points. By using switches in electronics circuits in different ways, it is possible to use them to perform a variety of different switching functions.

Types of electronics switches

There is a large variety of different types of electronics switches. Each of these types of switch operates in a slightly different manner and can be used for different applications.

  • Toggle switch:   The toggle switch is usually a two position switch. The internal construction includes a spring such that it is held firmly in the position to which it has been switched. It is widely used for switching items on or off. Occasionally toggle switches are available with a centre "off" position. Many versions are able to withstand switching 250 VAC at current levels of around 1 amp.
  • Push button switch:   Push button switches are used in many electronic circuits. These electronic switches are often used when a push action is required to give a momentary connection. Alternatively they can be used to provide a push on - push off action.
  • Rocker switch:   In many ways rocker switches are similar to toggle switches. They are widely used for mains on-off functions and have a two position capability. Some include an integral neon lamp to indicate when the circuit is on. In view of their intended use, these switches are often able to switch voltages of around 250 VAC and current levels of around 1 amp.
  • Rotary switch:   As the name implies, rotary switches are operated by turning a knob. Selecting the correct position enables the relevant connections to be made. As rotary switches can have many positions, they enable a particular point to be connected to one of a number of other points in the electronics circuit.

there are many types of electronics switches used in circuits and equipment. These types represent some of the major in use, although there are naturally many other types available.

Basic switching terms

The purpose of a switch is to make and break electrical circuits. To achieve this a switch comprises two main sections, namely the contacts and the actuator. The contacts are the fixed part, and the actuator moves over them to make or break the contact.

It is possible to have a number of arrangements for switches. The contacts may be what is termed:

  • Normally closed (NC):   This type of switch has contacts that in the normal position, or biased position of the switch are closed, i.e. the contacts have made contact. Utilising the switch then opens the contacts.
  • Normally open (NO):   This type of switch has contacts that in the normal position, or biased position of the switch are open, i.e. the contacts have made contact. Utilising the switch then closes the contacts.
  • Changeover (CO or C/O):   These types of switch have no form of bias and may have several contacts, the centre or pole piece connecting to any one of the contacts. Rotary switches are generally of the changeover type.

In addition to the contact arrangements for switches, there are a number of other aspects to the way in which they switch. Normally switches move from one position to another and remain there, but other types of switch may operate in a slightly different way:

  • Momentary action:   These switches perform so that when the actuator is moved, (often depressed) to change the contact state, this switch condition only remains for as long as the force is applied to the actuator. This type of switch may also be referred to as "biased".
  • Alternate contact action:   When the switch is actuated, often a push button pressed, the switch will change its state, e.g. from on to off. Once the force on the actuator is removed, the push button or other element of the switch that is pressed or moved by the user will return to its original position, but the contacts will remain in the new state.
  • Latching action:   This type of switch action is very similar to the alternate action. However when the force is removed, the actuator will also remain in the new position. The actuator and contacts will revert to their original position once an operating force is again applied.

Switch parameters and specifications

There are a number of parameters that are important when selecting a particular switch for a given application. Electrical capability along with the physical switching requirements and the mechanical specification for the switch are all important. However to be able to make a choice of the correct switch it is necessary to understand a little about these parameters.

  • Voltage rating:   This is the maximum voltage that the switch can withstand. It is determined by a number of factors including the insulation materials, the contact separation, the rate of separation, and general safety considerations.
  • Current carrying rating:   The rating of the maximum steady current that a switch can carry through a closed contact. This is determined mainly by the heating effect (I^2R) and it is generally greater than the current / power switching rating.
  • Contact resistance:   As the contacts of a switch are not a continuous conductor, but are able to be broken and re-made, there is a contact resistance that is greater than if the conductor were continuous. In view of the fact that power Is dissipated in any resistance, or because even small levels of resistance can be important, it is sometimes necessary to consider the contact resistance of the switch.
  • Number of operations:   As the contacts of a switch move across each other to ensure that the best resistance is obtained, there is always a small amount of wear. Accordingly the performance of a switch will deteriorate after use. To quantify the useful life of a switch, the number of operations is sometimes quoted. This may be many thousands of operations.

Summary

Switches used in electrical and electronics equipment are manufactured and used in their millions. There are also many types of switch to choose from, each having different properties and specifications.



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