Limit Switch Explained | Working Principles

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.

Styles of limit switches

The actuator is the part of the switch that directly interacts with the moving object.

There are 4 general styles of limit switch actuators:

1. Whisker

2. Roller

3. Lever

4. Plunger

Depending on the application, a limit switch may be a combination of 2 of the general styles such as roller-lever.

What are limit switches?

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

Limit switches are used to detect the presence or absence of an object.

These switches were originally used to define the limit of travel of an object, and as a result, they were named Limit Switch.

For example, a roller limit switch can be used to detect an objext moving past a specific point on a conveyor belt.

Limit switches applications

When you open the fridge door, a light comes on inside. How does that happen? Yes…. you guessed it! A limit switch is used to detect if the fridge door is open or closed.

Let’s look at another application of a limit switch that you may encounter at home. On many overhead garage doors, there is a limit switch that stops the movement of the door when it reaches its fully opened position.

Limit Switch Working Principle

Alright….now that we’ve looked at a couple of limit switch applications where you might see them in action at home, let’s have a closer look at the device itself.

A limit switch detects an object's physical movement or presence, typically using a mechanical actuator such as a lever or plunger.

Limit switches are electromechanical devices 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.

This change signals that a component or part has reached a predefined point or position.

Configurations of limit switches

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

Symbols of limit switches

Depending on the origin of the electrical schematic, you may see limit switches drawn in different ways.

The International Electrotechnical Commission (IEC) and the National Electrical Manufacturers Association (NEMA) have slightly different symbols.

Microswitch

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.

Be aware that not all microswitches are configured as a SPDT Limit Switch.   Some are configured as a Single Pole Single Throw (SPST) limit switch, which has only two terminals.

Microswitch simple circuit

Alright, let’s connect the microswitch to a lamp circuit. We’ll look at a limit switch wiring diagram where we use 12 volts to operate two different coloured lamps. 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.

Limit switches in action

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 limit switches operated by a container on an assembly line, or operated by a rotating machine part or by any number of other moving mechanical objects.

Limit switches could be used to count passing objects, or determining the position of a hydraulic cylinder.

Proximity sensor vs. limit switch

Limit switches are slowly starting to disappear from many industrial applications. They are being replaced by proximity sensors.

Unlike a limit switch, a limit 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.

You might want to review one of our other articles:

What is a Sensor? Different Types of Sensors, Applications

Summary

OK, let’s review…

– There are 4 general types of limit switches: whisker, roller, lever, and plunger.

– Limit switches are electromechanical devices operated by a physical force applied to it by an object.

– A limit switch is an electromechanical device consisting of an actuator mechanically linked to an electrical switch.

– 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.
– Limit switches are being replaced by proximity sensors in many applications.

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The RealPars Team

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