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.
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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.
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.
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.
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.
Electrical 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 (IEC) and the National Electrical Manufacturers Association (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.
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.
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.
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.
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Limit switches 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 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.
What is a Sensor? Different Types of Sensors, Applications
OK, let’s review…
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The RealPars Team
A limit switch is an electromechanical device that is operated by a physical force applied to it by an object. Limit switches are used to detect the presence or absence of an object, and these switches were originally used to define the limits of travel of an object, hence their name.
A limit switch consists of an actuator with an operating head, a switch body mechanism and a series of electrical terminals used to connect the switch to the circuit it controls. The operating head is the part of the limit switch that is in contact with the target.The actuator contains a linear, vertical or rotary motion connected to the operating head which is then translated by the actuator to close or open the switch. The switch body contains the switch contact mechanism, the state of which is controlled by the actuator. Electrical terminals are connected to the switch contacts and enable wires to be connected to the switch via terminal screws.
Industrial machinery that performs automatic operations often requires control switches that are activated based on the motion involved in the performance of the machine.For repeated use, the accuracy of electrical switches needs to be reliable and their response time should be fast. Due to the mechanical specifications and performance parameters of different machines, factors such as size, operating force, mounting method, and travel rate are important characteristics for limit switch installation and maintenance. In addition, the electrical rating of the limit switches should be matched to the mechanical system load that will be controlled to avoid instrumentation failure.
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When I open the refrigerator door, the light inside comes on. How did this happen? Yes! A limit switch is used to detect whether the refrigerator door is open or closed. Let's look at another application for your home. On many overhead garage doors, there is a limit switch that stops the door from moving when it reaches the fully open position. In most cases, the limit switch begins to operate when the moving machine or a moving part of the machine makes contact with the actuator or operating lever that activates the switch. The limit switch then regulates the circuitry that controls the machine and its moving parts. These switches can be used as guides for the control circuits of magnetic starters, allowing them to start, stop, decelerate or accelerate the function of the motor. Most switches are either hold contact or momentary contact models.
Limit switches offer several advantages inherent to their design.
Simple and straightforward design
Works in virtually any industrial environment
High accuracy and repeatability
Low power consumption device
Can be used to switch multiple loads
Easy to install, robust and reliable
Can be used to directly switch higher levels of current without the need for auxiliary relay control
T4N-4A31R Manual Reset Safety Limit Switch
Limit switches also have a number of limitations, which means they may not be suitable for all applications.
Because they rely on mechanical action, they are typically used in equipment that operates at relatively low speeds
They are contact sensors, which means they must be in physical contact with the target to operate
They are subject to mechanical wear or fatigue over time and will eventually need to be replaced