Limit Switch Explained | Working Principles

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In this video, we’re going to introduce you to a device called a Limit Switch.

There’s probably not a day that goes by where you don’t use or encounter a limit switch at home, or at your workplace.

There are 4 general types of limit switches: whisker, roller, lever, and plunger.
Depending on the application, a limit switch may be a combination of 2 of the general types such as roller-lever.

A limit switch is an electromechanical device operated by a physical force applied to it by an object.

The limit switch is used to detect the presence or absence of an object.
These switches originally used to define the limit of travel of an object, and as a result, they were named Limit Switch.

When you open the fridge door, a light comes on inside. A limit switch is used to detect if the fridge door is open or closed.

On many overhead garage doors, there is a limit switch that stops the movement of the door when it reaches its fully opened position.

A limit switch is an electromechanical device consisting of an actuator mechanically linked to an electrical switch.
When an object contacts the actuator, the switch will operate causing an electrical connection to make or break.

Limit switches are available in several switch configurations: Normally Open, Normally Closed, or one of each.

Depending on the origin of the electrical schematic, you may see limit switches drawn in different ways.
The International Electrotechnical Commission (or IEC) and the National Electrical Manufacturers Association (or NEMA) have slightly different symbols.

Let’s have a look inside a microswitch that is a type of limit switch.

A microswitch has 2 limit switches operating together and sharing a common terminal. One limit switch is normally open and the other is normally closed.

To be technically correct, the switch configuration is Single Pole Double Throw, or commonly referred to as SPDT.
The dashed line indicates that both switches are mechanically connected and will operate at the same time.

Alright, let’s connect the microswitch to a lamp circuit. In the inactive state, the Red lamp is on as the device is not being operated by an object pushing on the trigger.
When the Trigger is pushed the device will activate, and the Green lamp will come on.

Now that you’ve seen the limit switch in action you are probably thinking about some of the applications where you have seen them in action.

For example, you might see a limit switch operated by a container on an assembly line, or operated by a rotating machine part or by any number of other moving mechanical objects.

A limit switch could be used to count passing objects, or determining the position of a hydraulic cylinder.

Limit switches are slowly starting to disappear from many industrial applications. They are being replaced by proximity sensors.
Unlike a limit switch, a proximity sensor has no mechanical moving parts.
A proximity sensor performs the switching action with electronic switches.

Limit switches will not completely disappear any time soon as they outshine their proximity switch counterpart in their ruggedness and reliable operation in difficult environments.
Generally speaking, limit switches are capable of handling much higher current values than proximity sensors.

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You might want to review one of our other articles:
What is a Sensor? https://realpars.com/types-of-sensors/

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