Limit Switch Types & Usage

Limit switches are devices used to detect the presence or absence of an object or to monitor and indicate whether the movement limits of that object have been exceeded. They are commonly used in industrial control systems and various mechanical applications to control machinery by turning it on or off when an object passes a certain point. This article explores the various types of limit switches and their applications.

Types of limit switches

Limit switches can be categorized based on actuation type and contact configuration.

Based on the actuation type

Mechanical

• Plunger actuated: Plunger limit switches have a plunger or push button depressed by the moving object to activate the switch. They are ideal for simple on-off position detection. Their direct contact nature requires precise alignment, and they are not suitable for applications with high cycle rates due to wear on the actuator.
• Roller lever actuated: Equipped with a roller at the end of a lever, these switches are actuated by the movement of an object that pushes the roller. The roller reduces wear and allows for smoother actuation with less friction. They're versatile, with adjustable levers for different actuation points, but they can be sensitive to misalignment, and the lever can be damaged by excessive force.
• Rod lever actuated: These switches are actuated by a rod or lever pushed by an object. They are typically more robust than roller lever switches and can withstand higher actuation forces but require more activation force.
• Cable pull: Activated by a pulled cable, these limit switches are great for perimeter guarding or monitoring large areas. They offer flexibility in switch placement relative to the moving object but require proper cable tension and can have less precise actuation than direct contact switches.
• Rotary actuated: Rotary limit switches are activated by a rotating cam or wheel that depresses the switch at certain positions. They are precise for rotational movements but require careful setup and can be complex to adjust.
• Whisker actuated: Whisker-type limit switches are sensitive tactile sensors that use a thin metal wire or "whisker" to detect the presence or position of objects through physical contact. They are valued for their high sensitivity and simple design, making them suitable for applications in industrial automation, robotics, and safety mechanisms where precise parts detection is necessary. However, they can be susceptible to environmental factors, prone to whisker fatigue and breakage, and require more maintenance.

Non-contact

These limit switches do not require physical contact with the object, which reduces wear and can be beneficial in harsh environments.

• Magnetic limit switches: Utilizing a magnetic field to detect the presence of a magnet or a ferrous object, these switches are durable and immune to dust, dirt, and moisture. However, their sensing range is limited, and they can be prone to false triggering in environments with stray magnetic fields.
• Inductive limit switches: These switches detect metallic objects through an electromagnetic field. They are robust against dust and moisture and are suitable for high-speed applications. However, they can only detect metal objects and have a relatively short sensing range.
• Capacitive limit switches: Capable of detecting both metallic and non-metallic objects by sensing changes in capacitance, these switches are versatile and can even detect materials through thin barriers. However, they can be sensitive to environmental changes like temperature and humidity.
• Optical limit switches: With a light beam, often infrared, to detect objects, these switches can offer high-speed actuation and are not affected by the object's material. They can be sensitive to alignment, dirt, and dust on the sensor, and their performance can vary with the object's surface and color.

Based on the contact configuration

1. Normally open (NO): The contacts in this configuration are open when the switch is resting and close when the switch is activated.
2. Normally closed (NC): The contacts are closed in the resting state and open when the switch is actuated.
3. Changeover or single-pole, double-throw (SPDT): These have a common terminal and two other contacts where the switch can either close the circuit with the normally open contact or open the circuit with the normally closed contact.

Other types

In addition to mechanical and non-contact actuation, specialized limit switches are designed for specific applications or environments.

• Pressure limit switches: Pressure limit switches are actuated by a change in pressure and are often used in fluid power systems. They are sensitive to pressure variations and can be used to monitor flow rates, but they require calibration and can be affected by temperature fluctuations.
• Thermal limit switches: Thermal limit switches respond to temperature changes and are used in heating and cooling systems. They provide reliable operation within certain temperature ranges but may not have the precision of dedicated temperature sensors like thermocouples or RTDs.

Choosing a limit switch

• When choosing a limit switch, consider the application's specific needs, such as actuation method, environmental conditions, required precision, and potential for wear and tear.
• Mechanical switches are typically chosen for their reliability and simplicity in applications where direct contact is acceptable.
• Non-contact switches are preferred when contact with the object is undesirable or could lead to damage or in environments where the switch might be exposed to contaminants that could affect its operation.
• Specialized limit switches are chosen based on the specific parameters they are designed to monitor, such as pressure or temperature.

Limit switch applications

Industrial use

In industries, a limit switch provides position feedback for machinery. A limit switch can be used to determine when a mechanical part has reached the end of its travel on a production line. This is crucial in automated systems where precise control of movement is necessary. For example, limit switches are installed in conveyor belt systems at the end of the path to signal when an item has reached its destination. When the product touches the actuator of the limit switch, the contacts change state, sending a signal to the control system. This signal can stop the conveyor belt, prevent items from falling off the end, or trigger the start of another process, such as sorting, packaging, or stacking.

Furnace limit switch

The furnace limit switch measures the temperature of the air passing over the heat exchanger and will shut off the furnace if the system overheats. This prevents damage to the furnace and the possible release of dangerous gasses.

A fan limit switch is a control switch in the furnace control system. Its function is discussed below:

1. Temperature sensing: Continuously monitor the air temperature within the furnace or air handler.
2. Fan activation: Activate the blower fan when the temperature reaches a preset fan-on setting (typically around 49 °C - 65 °C or 120 °F - 140 °F).
3. Fan operation: Keep the fan running until the temperature drops below a fan-off setting (usually around 32 °C - 38 °C or 90 °F - 100 °F).
4. Overheat protection: Act as a safety device, shutting off the heating system entirely if the temperature exceeds a high-limit setting (usually around 93 °C or 200 °F) to prevent overheating and potential fire hazards.

These switches are typically inside the furnace's access panel, often mounted on or near the heat exchanger or blower assembly.

Garage door limit switch

Garage door limit switches control the stopping points of the garage door during opening and closing. These switches are usually located at the track or motor unit and are adjusted to ensure the door stops accurately at the open and closed positions. They are typically located on the opener track or motor assembly. Two switches are used:

• Up limit switch: Signals the opener to stop when the door reaches its fully open position.
• Down limit switch: Signals the opener to stop when the door is closed.

HVAC limit switch

An HVAC limit switch is similar to a furnace limit switch. It monitors temperatures within heating and cooling systems and will interrupt power if temperatures exceed safe limits. This protects the system from overheating or freezing. Applications of limit switches in HVAC systems are:

• Overheat protection: Prevent overheating the furnace or heat pump and shut off the system if the temperature exceeds a safe threshold (usually around 93 °C or 200 °F).
• Blower control: Limit switches control the fan to manage airflow and temperature distribution. They combine a temperature sensor and a switch within a single housing. The limit switch activates the blower when the furnace reaches a preset temperature and continues running until the temperature drops below a lower limit.
• Damper and valve control: Roller limit switches are used for position sensing in dampers, valves, and other moving components. They are activated when a roller contacts a cam or lever, indicating a specific position has been reached.
• Condensate overflow prevention: Protect against water damage.
• Refrigerant pressure control: Pressure switches monitor the system's air or refrigerant pressure and trigger control actions to protect compressors and maintain proper system operation.

Water heater limit switch

A water heater limit switch is a safety device designed to shut off the power to the heating element if the water temperature exceeds a safe level. They are typically located near the top heating element or thermostat within the water heater's access panel.

Elevator limit switch

Elevator limit switches control the positions of the elevator car. Applications of limit switches in elevators are summarized below:

• Prevent overtravel at the top and bottom of the hoistway.
• Initiate speed reduction as the elevator approaches a floor.
• Control door opening and closing sequences.
• Restrict access to certain levels or areas.

FAQs

Where is the limit switch on a furnace?

Typically, the limit switch in a furnace is located near the burner assembly or within the blower compartment.

What is a high-temperature limit switch?

A high-temperature limit switch is a safety device that shuts off a furnace if internal temperatures exceed safe limits, preventing overheating and potential damage.

What does a proximity limit switch/sensor do?

A proximity limit switch/sensor detects the presence or absence of objects nearby without physical contact, often using inductive, capacitive, magnetic, or optical technology to trigger a response when an object enters its sensing field.

What is a pneumatic actuated limit switch?

Pneumatic limit switches rely on air pressure change caused by the approaching object, which then actuates the switch.

What is a pressure limit switch?

Pressure limit switches respond to fluid pressure. They ensure safe pressure levels in pipelines and activate alarms in the event of overpressure conditions.