What is Limit Switch : Working & Its Applications

Detecting the presence or absence of an object using a contact or noncontact detecting device is possible using a simple switch. Once the object is detected then these devices will generate an electrical signal that is used to control different types of equipment and also processes in different industries. One such is a limit switch. It has also become an essential part of the control and sensing community of different sectors like construction, industrial, commercial, agriculture, and transportation. So thousands of limit switches are used in different types of construction or agriculture machinery, industrial equipment & control systems around the world. So this article discusses an overview of a limit switch – working with applications.

What is Limit Switch?

Limit switch definition is an electromechanical switch that operates by any physical force or the movement of a machine. These switches are very helpful in detecting the absence or presence of an object, counting, detecting speed, detecting movement range, travel limit, positioning, etc. These switches include three terminals NO (Normally Open), NC (Normally Open) & Common.

The controlling of this switch can be done by different factors like temperature, position, and pressure. This switch is mainly designed to operate only once a fixed limit is achieved, and it is generally activated through contact by using an object like a cam. The limit switch symbol is shown below.

Limit Switch Construction

The construction of a limit switch can be done by using different components like an actuator, cover, built-in switch, and a switch case.

Actuator

The actuator is an essential part of the limit switch which comes in contact physically with the object. The connection of an actuator within this switch can be done by connecting to an operating head which translates a linear, rotary otherwise perpendicular motion to control the electrical contacts of the switch.

Cover

Connect the cover once you have connected the outside connection cable to the built-in switch terminal. To seal the connection between the switch case and the cover, rubber packaging is used.

Built-in Switch

The built-in switch can directly switch ON/OFF by using its snap-action mechanism. The exterior force reverses the variable spring & shifts the moving contact from the upper NO contact & to the lower NC contact.

Switch Case

The switch case of the limiter switch encloses for safety. These cases are designed with resin or metal to enhance mechanical strength. At the bottom of this switch, a special connector can be connected to the opening of the conduit to protect the outside cable connection into position and to form a protecting seal for the inside components.

A limit switch can be made out of many different materials depending on its application. For example, limit switches used in elevators are usually made out of stainless steel so they won’t rust when exposed to water from humidity in an enclosed space like an elevator shaft or shaftway between floors. Limit switches used in industrial applications where there might be hazardous chemicals may also be made out of stainless steel with copper contacts (instead of silver) because copper doesn’t corrode like silver does when exposed.

Limit Switch Working Principle

A limit switch working principle is similar to a sensor to identify the presence & absence of an object. This switch can be triggered mechanically by communicating with other substances. When the object contacts the actuator of the switch, then it moves eventually to the boundary of the actuator wherever the location of the contact changes. Most of these switches are mechanical in their operation & include heavy-duty contacts which are capable of switching high current.

This switch can be arranged in your furnace’s supply plenum. So this switch reads the plenum’s temperature & waits until it gets hot properly. Once the plenum achieves a specific temperature, then this switch transmits a signal to the air handler to start running & transmitting warm air to your home. The limit switch commonly operates at an upper limit of 160°F and a lower limit of 130°F.

Pressure switches mainly rely on the pressure of water to perform all the work. The change in the pressure will trigger the switch. The water pressure will move up through the diaphragm which pushes against a spring & piston, which in turn contacts will be opened or closed.

Limit switches utilize the actuator plunger’s mechanical movement to change or control the condition of the electrical switch.

Types of limit switches:

In general, there are Two Types of Limit switches

Mechanical – These switches are controlled by a physical movement (typically using a lever). This often makes them suitable for delicate equipment because they don’t require any power to operate. However, they can be more prone to wear and tear than electronic versions.

Electronic – Electronic limit switches are powered by batteries or mains electricity and use sensors to detect when an object crosses a certain point.

A limit switch can be activated by several types of mechanisms: magnetic, mechanical and optical. Magnetic limit switches are activated by magnets located near them. Mechanical limit switches are activated by physical contact with a lever or other object that moves when the actuator comes into close proximity with it. Optical limit switches use light-emitting diodes (LEDs) and photoelectric cells to detect when an object has come within a certain distance of them

Limit Switch Circuit Diagram

The circuit diagram of the limit switch is shown below. This circuit can be built with a 12V battery, 3.5V, 10mA of green color LED, 3.5V, 10mA of red color LED, and 850 ohms resistor. The connections of this circuit can be done like the following.

The positive terminal of the battery is connected to the common terminal of the switch through a resistor. Here resistor is connected in series with the common terminal and the battery’s positive terminal.

Both the LED’s negative terminals are connected directly to the battery negative terminal. Connect the green color LED’s positive terminal to the NO terminal of the switch and the red color LED’s negative terminal to the NC terminal of the switch.

Circuit Working

The above switch works in two conditions which are discussed below like when the switch is OFF and the switch is ON.

Once the switch is turned OFF, the NC terminal of the switch is connected to the common terminal whereas the NO terminal of the switch is detached from the common terminal. As a result, the RED color LED gets the supply & it will turn ON whereas the green color LED will be turned OFF.

Once the switch is turned ON, the NO terminal of the switch is connected to the common terminal whereas the NC terminal of the switch is detached from the common terminal. As a result, the Green color LED gets the supply & it will be turned ON whereas the red color LED will be turned OFF.

Difference between Limit Switch Vs Proximity Switch

The difference between limit switch and proximity switch includes the following.

Limit Switch	Proximity Switch
The limit switch works by using a physical response to function as ON and OFF.	The proximity switch works under the responses of the sensor.
This switch needs either a machine or a manually operated switch to change the current flow.	In this switch, the sensor detects the object in the specified range to change the current flow.
These are contact switches.	These are non-contact switches.
Its working life is shorter because of the internal contacts.	Its working life is longer.
These switches are used for the detection of positions in only one direction.	These switches are used to detect two-way positions,
These switches use two wires.	These switches use more wires as compared to the limit switch.

Types of Limit Switches

There are different types of limit switches available in the market which are discussed below.

Roller Lever

These switches include a roller on the tip of the lever. Once an object is there the roller will activate radially. Here the main use of a roller is to remove a high level of friction & reduce the power required to control the limit switch. Similarly, when there is no object then the lever will come back to its original position because of a spring mechanism fixed within the switch’s body. These switches are most frequently used in the industrial sector.

Adjustable Roller Lever

These switches are similar to a typical standard roller level switch except for one function which is the capacity to alter the roller’s height. So this gives adjustment once the limit switch is fixed within the place. Here, the adjustment range can be changed through the limit switch when it is 2 millimeters to 6 centimeters distance. Generally, these switches are applicable where the object they are sensing may move slightly.

Roller Plunger

These switches are available in different sizes and shapes. So, the most frequently used type is the roller type. This switch is mainly used whenever the pressure is not applied directly to the head of plungers at 90 degrees. The Roller plunger changes some of the pressure from non-perpendicular to perpendicular after that it can be used to trigger or disable the plunger. These switches will trigger once something pushes downward on the plunger’s end.

Rounded Plunger

This switch includes a rounded tip which is triggered once it gets perpendicular energy that is provided to the end part of the plunger. In this switch, the main function of the rounded tip is to provide a larger detection region on the head of the switch. Here head types can come in different sizes & shapes.

Rod Lever

These switches include a thin and long rod that is triggered once it notices presence. These switches have a spring mechanism inside so when the object is detached then the lever will come back to its original position. Here, the rod length ranges from 1 centimeter to above 10 centimeters. These switches are normally applicable in non-heavy-duty applications.

Flexible Rod

These switches are approximately the same as typical rod lever limit switches but they can trigger in a 360º region. They seem practically identical in appearance to rod lever type.

Rotative Axis

These limit switches are applicable where the application features an axis or hinge. Once an axis or hinge turns, the switch will activate. These types of switches are used on door hinges & also on any application that includes a turning axis.

Cable Pull

These types of limit switches are available with a lanyard or hook on the tip of the body of the switch. A pull cord or cable is connected to the hook & triggers the switch once the cable is pulled. This switch includes a spring-loaded lever that activates when it is pulled.

Generally, these switches are mainly used on security devices like a pull cord on a conveyor. When somebody needs to stop the conveyor in an emergency then they can drag the pull cord. These are normally very simple to install & set up.

Failures of Limit Switch

There are many reasons to fail limit switch including the following.

• Cracks because of straight sunlight.
• Extreme stroke.
• The operating speed is too slow.
• Corrosion.
• The actuator’s operating direction is not correct.
• Contact weld or short-circuit because of contact migration.
• Contact weld because of wrongly connected power source.
• The incorrect switch has been chosen.
• The sealing element has not been correctly tightened.

Advantages and Disadvantages

The advantages of the limit switch include the following.

• Limit switch installation is simple.
• Its size is compact.
• It controls several loads.
• It consumes less electrical energy.
• It is applicable in many industries.
• It has accuracy, repeatability & high precision.
• It works in the toughest environmental conditions.
• The maximum current it can switch is upto 10A.

The disadvantages of the limit switch include the following.

• To activate this switch, it should make physical contact with an object.
• The mechanical component can exhaust.
• Usually, it is restricted to equipment that operates at comparatively low speeds.

Applications

The applications of the limit switch include the following.

• The limit switch is used to detect the position of production lines.
• It is used to detect the position of drive units and also industrial devices & equipment.
• Limit switches are used to detect doors & covers on semiconductor manufacturing devices and machine tools.
• These switches are used in machine tools like metal fabrication devices, transfer lines, presses, etc.
• These types of switches are used in material handling equipment like elevators, conveyors, hoists, and cranes.
• These are used in textile machinery, process equipment, and packaging machinery.
• These are applicable in lift trucks, vehicles, equipment, construction machinery, etc.
• These switches are used in control cabinets and garage doors.

Thus, this is all about an overview of a limit switch with working, types, and its applications. Some of the industries which use limit switches are; manufacturing, material handling, packing and distribution of food and beverage, metals, control cabinets, commercial, and many more. Here is a question for you, what is a proximity switch?