Where are limit switches located?

Limit Switch Explained - What it is, How it works & How it is used in Manufacturing

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Beginner

What is the Function of Limit Switch?

A limit switch is an electro-mechanical device used to send an electrical signal based on a physical interaction. Limit Switches are used to detect the presence of objects, thus allowing the system to take desired action.

Limit Switches can be found in a number of everyday devices:

• They are used in microwaves to check that the door is closed before operation is allowed. 
• They are used in elevators to notify the system that the cabin has reached the desired floor. 
• Limit Switches are used in washing machine lids as a permissive to start the wash cycle.

Limit Switches in Manufacturing

Countless limit switches are found in manufacturing. They are used as control devices and safety devices for machinery and personnel. In all cases, the limit switches will send a digital signal to the control system. Based on the hardware and software tied to said switches, the system is able to take appropriate action.

Why is the limit switch important?

Limit switches are an inexpensive way to create a link between the physical and electrical domains. They have been developed a number of decades ago and the mass adoption of their use significantly lowered their cost for the end user. They thus play an important role in manufacturing due to their simplicity and low cost.

Use Cases of Limit Switches

1. Product Detection & Count - As a product pushes against a limit switch, a signal is sent to the control system. Through simple PLC ladder logic, the user can count the number of times the product goes by the limit switch and display the counter for the operator.

Limit Switch - Box Detection on a Conveyor Example

2. Personnel Safety - A limit switch can be used to detect the opening of a safety guard that stops and de-energizes the machine. If the guard is opened during operation, the machine stops. If the guard is opened while the machine is stopped, the limit switch prevents the machine from starting. In both cases, the limit switch is used to safeguard the operator from potential harm.
3. Machine Safety - A limit switch can be used to protect machinery from unintentional damage. This includes components that are part of changeovers (end of arm tools), components that may wear-out over time (motor clutch) and components that may damage others if they fail (gears, shafts, etc.).

Types of Limit Switches

Limit switches can be classified into four main types based on the type of actuator:

• Whisker: Whisker limit switches have a long, thin actuator that can be bent or deflected by even a small amount of force. They are often used in applications where space is limited, such as in robotic arms and grippers.

Limit Switch | Whisker Limit Switch - Example of an Omron Whisker Limit Switch

• Roller: Roller limit switches have a roller-shaped actuator that is rotated by the object being detected. They are often used in applications where the object being detected is moving quickly, such as on conveyor belts and packaging machines.

Limit Switch | Roller Limit Switch - Example of a Renew Roller Limit Switch

• Lever: Lever limit switches have a lever-shaped actuator that is moved by the object being detected. They are often used in applications where the object being detected is large or heavy, such as on machine tools and industrial cranes.

Limit Switch | Lever Limit Switch - Example of a Generic Lever Limit Switch

• Plunger: Plunger limit switches have a plunger-shaped actuator that is pushed in or pulled out by the object being detected. They are often used in applications where the object being detected is small or delicate, such as in electronic assemblies and medical devices.

Limit Switch | Plunger Limit Switch - Example of a COMEPI Plunger Limit Switch

In addition to the type of actuator, limit switches can also be classified by their electrical configuration. The two most common electrical configurations are:

• Normally open (NO): NO limit switches are open when the actuator is not activated and close when the actuator is activated.
• Normally closed (NC): NC limit switches are closed when the actuator is not activated and open when the actuator is activated.

Some limit switches also have a double throw (DPDT) electrical configuration, which means that they have two sets of electrical contacts, one NO and one NC. When the actuator is activated, the NO contacts close and the NC contacts open.

Limit switches are available in a wide variety of sizes, shapes, and materials to meet the specific needs of different applications. They are also available in a variety of electrical ratings, so that they can be used to control a wide range of loads.

Here are some examples of how different types of limit switches are used in real-world applications:

• Whisker limit switches: Whisker limit switches are often used in robotic arms and grippers to detect the presence of objects. They are also used in some types of safety interlocks.
• Roller limit switches: Roller limit switches are often used on conveyor belts and packaging machines to detect the presence of products. They are also used in some types of machine tools to detect the position of the cutting tool.
• Lever limit switches: Lever limit switches are often used on machine tools and industrial cranes to detect the position of the moving parts. They are also used in some types of safety interlocks.
• Plunger limit switches: Plunger limit switches are often used in electronic assemblies and medical devices to detect the position of small or delicate components. They are also used in some types of safety interlocks.

Limit switches are a versatile and reliable way to control and monitor machines and processes. They are used in a wide variety of industries, including manufacturing, construction, agriculture, and transportation.

Operation of a Limit Switch

As discussed above, a limit switch is used to translate a change in mechanical state into an electrical signal. How does the device actually accomplish that?

A limit switch is a simple device. The lever is used to close a contact within the switch. When the contact is closed, it allows the electrical signal to go through. The mechanical operation is similar to that of a light switch. However, a spring is added to return the limit switch into its original position.

The critical parameter of a limit switch is at which point it closes the contact within. This parameter will vary between switch types as well as brands. It is important to verify the datasheet to confirm this position. It will influence the system design that will utilize the limit switch.

Limit Switch - Electrical Signal Configurations

As discussed previously, once the actuator is displaced a certain distance, the switch is made. At this point, it needs to transmit a signal to an electrical device which may be a Programmable Logic Controller (PLC), relay, control circuit, or an additional limit switch. A limit switch is similar to a relay in the sense that it can be purchased in one of the following configurations:

• Normally Open | The switch will allow the current to pass through once it is actuated. In resting state, no current will pass through the switch.
• Normal Closed | The switch will stop the current once it is actuated. In resting state, current will flow freely.
• Combination of Two | The switch will provide a set of contacts that can be used in Normally Open (NO) and Normally Closed (NC) circuits.

There is an argument to be made for either configuration. However, when it comes to limit switches being used for safety purposes, it&#;s always advised to have current circulating in &#;resting state&#;. This is important as during a problem in the circuit, the safety should trigger. Should it be set to no-power in the &#;resting state&#;, the circuit may fail to prevent injury or damage.

Limit Switch - Real Applications & Datasheet

In this section, we&#;re looking at the Allen Bradley 801 General Purpose Limit Switch configurations. Below, you&#;ll find the &#;Contact Arrangement&#; provided by the manufacturer. Let&#;s review each section in detail

Section 1

A - Normally Open (NO) limit switch symbol.

B - Normally Closed (NC) limit switch symbol.

AC - Alternating Current Ratings for light duty limit switches at different voltage levels. It is important to remember that although your application may require a switch to operate within a 24VDC circuit, it&#;s possible to use these across multiple industries in which voltage level standards are different.

DC - Direct Current Ratings for light duty limit switches.

Section 2

C - Normally Open (NO) and Normally Closed (NC) set of contacts on a single limit switch.

H - Dual Normally Open (NO) and Normally Closed (NC) set of contacts on a single limit switch.

N - Dual Normally Open (NO) set of contacts on a single limit switch.

O - Dual Normally Closed (NC) set of contacts on a single limit switch.

Section 3 - Dual Position Limit Switch

In the first example of a limit switch we had reviewed, there was a single position at which the switch would trigger the signal. In this case, there are two points at which the signals will be different. These signals can be picked up in discrete logic, but most commonly, this would be done using a PLC. A PLC programmer can utilize such a limit switch to detect a difference between a partial and a full limit switch closure.

Position of Lever = OFF

• Contact 1 = Closed
• Contact 2 = Open

Position of Lever = Partially Closed

• Contact 1 = Open
• Contact 2 = Open

Position of Lever = Fully Closed

• Contact 1 = Open
• Contact 2 = Closed

Section 3 - Normally Open (NO) Differential Location Limit Switch

Until now, we&#;ve talked about limit switches that would actuate the same one way or the other. This is the case for most general purpose limit switches. However, it is possible to detect if the switch was pushed in one way or the other through the interlock present in the differential location limit switch. The operation will be as follows.

Position of Lever = OFF

• Contact 1 = Open
• Contact 2 = Open

Position of Lever = Closed to the Left

• Contact 1 = Open
• Contact 2 = Closed

Position of Lever = Closed to the Right

• Contact 1 = Closed
• Contact 2 = Open

Section 4 - Normally Closed (NC) Differential Location Limit Switch

The Normally Closed (NC) variation of the previous switch will operate as follows. The only difference is the state of the contacts in the &#;Off&#; position.

Position of Lever = OFF

• Contact 1 = Closed
• Contact 2 = Closed

Position of Lever = Closed to the Left

• Contact 1 = Open
• Contact 2 = Closed

Position of Lever = Closed to the Right

• Contact 1 = Closed
• Contact 2 = Open

Section 5 - Dual Normally Closed (NC) & Normally Open (NO) Differential Location Limit Switch

This variation of a limit switch features four contacts. Two of these contacts can be used as Normally Open (NO) connections and two other as Normally Closed (NC).

Section 6

Current ratings for regular duty limit switches. Note that these ratings are higher than the ones found in Section 1.

What is a Limit Switch & How Does a Limit Switch Work

A limit switch is an electromechanical device that consists of an actuator mechanically linked to an electrical switch. When the actuator is moved by an object, it causes the electrical switch to open or close, depending on the type of limit switch.

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There are two main types of limit switches: normally open (NO) and normally closed (NC). An NO limit switch is open when the actuator is not activated, and closes when it is activated. An NC limit switch is closed when the actuator is not activated, and opens when it is activated.

Limit Switch | How Does a Limit Switch Work - Example of a Mechanical Limit Switch

Limit switches are used in a wide variety of applications, such as:

• To control the movement of machinery, such as conveyor belts and robot arms
• To detect the presence or absence of objects
• To signal the start or end of a process
• To trigger safety alarms

Here is a simplified explanation of how a limit switch works:

1. The actuator is attached to a moving object or part of a machine.
2. When the object or machine part moves, it activates the actuator.
3. The actuator causes the electrical switch to open or close, depending on the type of limit switch.
4. The open or closed electrical switch sends a signal to a controller, such as a PLC or relay.
5. The controller then performs the desired action, such as stopping the machine or triggering an alarm.

Here is an example of how a limit switch is used in a real-world application:

A conveyor belt is used to transport products from one area of a factory to another. A limit switch is installed at the end of the conveyor belt to detect the presence of products. When a product reaches the end of the conveyor belt, it activates the limit switch. The limit switch then sends a signal to a controller, which stops the conveyor belt. This prevents the product from falling off the conveyor belt.

Limit Switches - Other Types & Applications

A limit switch used in manufacturing is rated for hundreds of thousands of cycles. However, there are various flavors of this device that aren&#;t as robust for other everyday applications.

Limit Switches in Elevator Systems

Elevator systems commonly require limit switches for a variety of purposes by building codes. They are used for position detection, safety and operational state of the elevator. The types used in this application are rated for an even greater number of cycles as they constantly experience actuator movement through the life of the system.

Limit Switches in Household Systems

Limit Switches are used in everyday appliances: fridges, tea kettles, mixers, washers, dryers and more. The limit switches in these appliances are the same in principle, but different in reliability. In other words, they operate the same, but may fail much faster then their manufacturing counterparts.

How do I know if my limit switch is bad?

Using a basic ohmmeter or Digital Multimeter (DMM), it&#;s possible to determine whether or not a limit switch is bad. Disconnect the limit switch from the system and apply the ohmmeter leads to each terminal. If the limit switch is normally open (NO), the resistance should be very high. If it&#;s normally closed (NC), the resistance should be close to zero. Toggle the limit switch into the active position and measure the resistance. It should be the opposite in this setting. If the transition does not occur, the limit switch is bad.

Conclusion on Limit Switches

Limit Switches are used in a wide array of applications across the production floor as well as our daily lives. A limit switch is one of the most basic mechanical / electronics components on a production floor. They are used for object detection and machine / personnel safety. The limit switch will send a signal to a control circuit once it is in a certain position. Although the function is straight-forward, a wide range of flavors of such switches exist in order to provide flexibility to the end user. In a real-world example, we&#;ve looked at the datasheet that specifies multiple functions for general purpose allen bradley limit switches.

Does your furnace keep shutting off unexpectedly or running in shorter cycles? If so, you might have a problem with your furnace limit switch.

Also known as a high-limit switch, this small (but critical) component helps keep your home safe by automatically shutting off the system when your furnace gets too hot. In this guide, we&#;ll explain how your furnace high limit switch works, how it impacts your home, and solutions for when it malfunctions.

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What is a furnace limit switch?

A furnace limit switch is a device within your furnace that detects when the internal temperature is too high and turns the unit off as a safety measure. You may also hear it referred to as a fan limit switch or a limit switch.

When your furnace is working correctly, you shouldn&#;t have to worry about your furnace limit switch. But when it starts to trip or malfunction, you may need to know about this part and how to fix it.

What does a limit switch do?

Let&#;s get into a more technical explanation of what your furnace limit switch does in the context of the entire heating system. When you turn up the heat on your thermostat, the burners will activate and begin heating up air inside the plenum.

When the unit is at an appropriate temperature, the limit switch in the furnace allows for burner operation and the furnace will continue to send out heat. If the temperature gets too high within the furnace, the switch will open, causing the furnace to stop operating. The burners will shut down and the fan will continue to operate, cooling down the internal components of the furnace.

When the furnace&#;s internal temperature decreases, the limit switch will close, allowing the furnace to start back up for normal operation.

How a limit switch impacts your safety and comfort

Since your furnace uses gas to heat your home, it must have safety features in place to prevent it from overheating or breaking. If those safety components fail, necessary parts of the furnace, like the heat exchanger, can break more easily.

Fortunately, your switch acts as a safety device to control these variables and keep your family safe. It does this by shutting off power to the gas valve when the furnace exceeds the manufacturer&#;s temperature limit. If a gas furnace warms your home, install a carbon monoxide alarm to detect the odorless gas in case of a leak.

What does a furnace limit switch look like?

A furnace limit switch has two components. The first is a small button-shaped mounting plate that&#;s fastened outside of the furnace plenum. It attaches to a long temperature probe that extends through the furnace housing.

Where is the limit switch on a furnace?

In most cases, the limit switch will be located inside the cover panel of your furnace. (You might need a screwdriver to remove this panel, depending on the model of your system.) On older furnaces, however, it may be set up on the outside of your furnace.

Limit switch tripping

Because the limit switch is designed to detect and shut down issues within your furnace, there are several reasons that it might trip. For example, if your furnace keeps shutting off soon after you fire up your thermostat, it could be indicative of issues such as:

• Airflow problems inside your furnace (including dirty air filters), which cause overheating
• Dirty temperature sensor on your furnace limit switch
• Defective high limit switch

In order to avoid these issues, it&#;s important to keep up with the recommended maintenance schedule from your furnace manufacturer. For example, many manufacturers advise you to change your furnace filter every three months. 

When necessary, you should also clean the temperature sensor on your limit switch to keep it in good working condition and prevent tripping. If your switch continues tripping after you&#;ve replaced your filter and cleaned the sensor probe, you may have a defective limit switch (more on that below).

When your switch repeatedly trips, it may enter a &#;lockout mode,&#; where the furnace will completely shut down. This can only be reversed by servicing and resetting the furnace, which should be done by a professional.

How do I know if my limit switch is bad?

As we&#;ve covered, your limit switch can signal other issues with your furnace that aren&#;t necessarily related to the switch itself. So how do you know if it&#;s your switch that&#;s the problem? Here are some warning signs.

• Furnace blowers don&#;t shut off
• Air coming out of your system is cool
• System repeatedly turns on and off (called &#;short cycling&#;)

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How do you reset a furnace limit switch?

If your furnace shuts down several times in a row, it may enter a hard lockout mode that will require you to reset your switch or the entire furnace system. This happens as a safety mechanism to prevent your furnace from cracking and leaking carbon monoxide into your home.

You may be able to reset the system yourself, depending on the age and model of your furnace. If you have a newer model, you should be able to turn off the power for about 30 seconds and then turn it back on.

If you can&#;t reset it yourself or if it repeatedly goes into hard shutdown, it&#;s a good idea to contact a specialist and have them perform a safety check. A faulty furnace can have life-threatening consequences for your family, so it&#;s essential to confirm that everything is operating correctly.

Can I replace a furnace limit switch myself?

Yes, it&#;s possible to replace a furnace limit switch yourself. To do it, you&#;ll first need to use a multimeter to test the switch. If it&#;s faulty, you&#;ll also need a new switch that matches your old one. Here&#;s how to test the furnace limit switch and replace it if necessary.

1. Shut off the power and gas to your furnace.
2. Locate the limit switch and remove it from your furnace.
3. Place the multimeter probes on the switch terminals and check for continuity.
4. If there&#;s no continuity, replace the switch with a new model.

Before trying to complete any work yourself, keep in mind that furnaces can be dangerous. Not only do they emit hot air, but they also contain flammable gas and run on high-voltage electricity. For those reasons, you shouldn&#;t try to complete any complicated repairs on your fan limit switch or your furnace unless you know what you&#;re doing. If not, call in an HVAC specialist for help.

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Furnace limit switch repair and replacement costs

If you&#;re the DIY type, you&#;ll save quite a bit of money by replacing your furnace limit switch yourself (assuming you already have a multimeter at home). You can find new switches for less than $20 at your local hardware store. However, if replacing the switch doesn&#;t solve your issue, you should contact an HVAC expert.

The cost of professionally repairing your furnace will vary depending on the specific issue. Generally speaking, you should expect to pay between $100 and $1,000 per furnace repair visit, with an average of $270 per repair.

When to hire furnace repair specialist

If you&#;re still having problems with your furnace after replacing the switch, changing the air filter, and cleaning the temperature sensor, it may be time to replace the entire furnace. When you receive a quote for a new furnace, QuoteScore from HVAC.com can analyze it to determine if it&#;s a fair deal.
 

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