A “limit switch” is a mechanically operated electrical switch that changes its Electrical contact’s state when its actuator lever crosses the defined position.
“Have you ever notices the light inside the refrigerator ? “
When we open the door, Light get on. While when we close the door, Light get off. These all, is happening due to the “limit switch” connected behind the door of refrigerator.
Further more, In automation system, The physical movement can not be directly measured by the microprocessor or microcontroller. These little limit switches actually gives signal about the movement, to them. And then microprocessor or microcontroller works.
What is a limit switch ?
Limit switch is a device or a machine, Which converts physical input generated from physical quantity like force or movement, into an Electrical signal. Signal can be Ac or Dc, Depends upon the circuit.
As name suggest, This device operates or ignite the electrical switch when its actuator lever crosses the defined limit !
Why Is a Limit Switch Used?
- To set the limit.
- To get an electrical signal from physical movement.
Type of limit switch
- Roller lever
- Adjustable roller lever
- Plunger
- Rotary
- Whisker/rod type
Working of limit switch
Limit switch is having two main components :
- Actuator lever
- Electrical contact or changeover contact
Actuator lever
Actuator lever is the part which comes first in physical contact either movement or force. Normally, It stays at its normal position, wherever it is arranged, due to spring tension.
Due to force, This actuator lever starts moving with the force. This actuator lever further pushes the electrical contacts when certain level is crossed by it. Electrical contacts are also fitted with spring arrangement.
Under the removal of force or movement, Actuator lever moves back to its normal position due to spring tension. In parallel with actuator lever, Electrical contacts also get back to respective normal state.
Electrical contact or changeover contact
Trigger from actuator lever, Triggers the electrical contacts. Electrical contacts changes its state depends whether it is “NO “or “NC “.
So there are 2 states of limit switch : Normal state and forced state.
| Contact type | Normal state | Forced state | Remarks |
| NO contact | Contacts are opened. | Contacts are closed. | Forced state is actuation state Where actuator lever is pressed due to force or movement |
| NC contact | Contacts are closed. | Contacts are opened. | Normal state is the default state due mechanical spring tension. |
Limit Switch Wiring
Wiring of limit switch depend upon the type of switch (“changeover “or “normal contact“) and application (Where it is implemented)
Limit switch has its voltage and ampere rating. Wiring must be done accordingly. As shown in aņ image below : The rating of 1 NO +1 NC contact is AC 15 240V 6 A. Rated insulation voltage is 400 V. AC 15 defines the utilization factor of the switch, Which can be used in switching of electromagnetic coils.
Before Limit switch wiring, We should have to know about “NO “, “NC “and “changeover relay“.
- NO – NO stands for “normally open“. Contact stays open at its normal position/state. Contact get closed when force or actuation is applied. Contact get back open again in removal of force or actuation.
- NC – NC stands for “normally closed” . Contact stays closed at its normal position/state. Contact get open when force or actuation is applied. Contact get back closed again in removal of force or actuation.
Both NO and NC contacts are shown through a symbol below.
Changeover contacts or changeover Relay
“Changeover relay” is a type of electromechanical switch. Which contains three terminal “common“, “NC “and NO .
- Normal position/state, Common contact is connected with NC contact due to spring tension. Common contact do not have any link with NO contact..
- Actuation or forced state, Common contact get connected to NO contact from NC contact. Common contact do not have any link with NC contact.
A schematic diagram of “solenoid actuated changeover relay” is shown below. When solenoid get energized, Common contact get connected to NO contact. While in de-energized state, Common contact stays connected to NC contact.
Limit switch Example 1
To understand the limit switch wiring, Lets take a simple example of light of refrigerator. Which get “ON “when refrigerator door get open. Get “OFF” when refrigerator door get closed. A limit switch is fitted behind the door. Switch get pressed when door get shut.
NC contact of limit switch is used in this circuit. Which makes circuit closed in normal open door situation. While contact get open, When door is get closed.
Circuit for this, illustrate in a GIF below. The negative of a battery connected directly to the bulb. While Positive is passes through the NC contact, to Bulb.
Limit switch Example 2
Here, We are taking an Example of “lift with 1 floor“. Lift is running in between 2 floors with Induction motor rated 3.7KW-1440 RPM.
The arrangement is like; for first floor from basement, motor needs to run in clock wise direction. While; for basement from 1st floor; motor need to run anti-clock wise direction.
For Stoppage; On both the sides; limit switches are connected. Both the floor level contains a switch to start the movement.
Let suppose, Lift at ground floor. Need to press the switch for the movement towards first floor. On reaching the first floor, Due to limit switch, lift get stop. At first floor, switch is get pressed to move back to the ground floor. At ground floor, There is another limit switch which stops the lift.
Working is shows through a GIF below :
Circuit Diagram
- Motors rating 3.7KW/1440 RPM.
- Motor needs to run in both the directions forward and reverse.
- Both the levels has 2 limit switches.
- Both the levels have 2 switches to start the movement.
- Need to press the switch only once lift moves towards the opposite side until it reaches the limit switch.
- When switch is pressed at ground level. Motor get hold and starts rotating in clockwise, Lift moves upward. It get stop when limit switch 2 is get pressed.
- When switch is pressed at first floor level. Motor get hold and starts rotating in anti-clockwise direction, Lift moves downward. It get stop when limit switch 1 is get pressed.
- For this circuit, 2 contactor with coil voltage of 230V AC are used.
- MPCB current ranges from 6 to 8.5 Amps. Current is selected at 7.5 Amps.
There are 2 circuits shown below. On left, Control circuit is shown. While on right, Power circuit is shown.
“Power circuit” side Both the contactors are connected to MPCB. While motor connections are connected across both the contactors; in parallel.
“Control circuit” side, Neutral is directly connected to both the contactors. Phase taken from a control MCB single pole. Both the contactors are controlled through this phase.
Control to contactor 1 for clock wise movement. Phase passes through L2(1st floor Level limit switch). Than phase passes through ground floor start switch (NO ). To hold the circuit, NO contact of contactor 1; is connected across the start switch in parallel. At last, Supply to contactor C1, is connected through the NC contact of contactor C2.
Control to contactor 2 for anti-clock wise movement. Phase passes through L1(Ground level limit switch). Than phase passes through 1st floor start switch (NO ). To hold the circuit, NO contact of contactor 2; is connected across the start switch in parallel. At last, Supply to contactor C2, is connected through the NC contact of contactor C1.
Troubleshoot of limit switch
| Limit switch faults | Causes | Remedies |
| Contacts (NO or NC ) not working | Contacts get damaged Contacts get corroded Contacts get stuck Material ingress | Change the contacts Clean the contacts At last, Change the limit switch |
| Lever get damage | Due to heavy strike or continue strike (Long exposure) | Change the lever |
| Lever get stuck | Due to heavy strike | Change the limit switch |
Drawbacks of limit switch
- Direct physical contact leads to reduced limit switch life.
- IP rating matters, Wrong selection leads to damage to limit switch due to material ingress.
- Over the time, due to continuous use, contact get corroded or contacts get stuck.
- Long exposure to high current leads to weld in between the contacts
Limit switch vs proximity switch
| PARTICULARS | LIMIT SWITCH | PROXIMITY SWITCH |
| Physical contact | Happening | Not happening |
| Useful life | Comparatively low | Comparatively high |
| Current rating | higher comparatively | Lower comparatively |
| Auxiliary supply | not required | depend upon the model |
How to test a limit switch with multimeter
There are multiple method to check the limit switch, Depends whether limit switch is energized or not. Testing method is different for both the scenarios that’s why next, Both the scenarios are discussed !
When limit switch is energized
| PARAMETERS | Limit switch in normal state | Limit switch in forced state | Remarks |
| Check Potential difference across the contacts (NO or NC ) | NO contact : Potential difference exist. NC contact : No Potential difference. | NO contact : no Potential difference. NC contact : Potential difference exist. | Power type(AC or DC), Voltage level – Important to consider ! |
| Check line current | NO contact : No current (circuit open) NC contact : current exist if further load is connected and circuit is closed. | NO contact : Current exist if further circuit is closed. NC contact : No current. | Power type(AC or DC) and current level Important to consider ! |
When limit switch is not energized
| PARAMETERS | Limit switch in normal state | Limit switch in forced state | Remarks |
| Visual inspection | NO contact : There is gap in between the contacts. NC contact : Closed contacts | NO contacts : Closed NC contacts : Opened | Contacts must be clean, no carbon. No sticking, having a clear contact gap in between the contacts. |
| Continuity across the contacts (NO or NC ) | NO contact : No continuity (open circuit) NC contact : having continuity (Closed circuit) | NO contact : having continuity (closed circuit) NC contact : No continuity (as circuit is open) | Contacts must not be energized |
Applications
- Door limit switch
- float switch
- linear limit
- Rotational limit
- Conveyor systems
- Cranes and hoists
- Machine tools
- Doors/gates
- Actuators
- Material-handling equipment
Frequently Asked Questions About Limit Switches
1. What is a limit switch?
A limit switch is an electromechanical switch used to detect the position or movement of a machine part. When the moving part operates the switch actuator, its internal contacts change state and can be used to stop, start, or control a circuit.
2. How does a limit switch work?
A limit switch works by converting mechanical movement into an electrical switching signal. When a machine part contacts the actuator, the internal contacts change from their normal state. The resulting signal can be used in a control circuit, relay circuit, contactor circuit, or PLC input.
3. What is NO and NC in a limit switch?
NO means Normally Open, while NC means Normally Closed. In the normal, unactuated condition, an NO contact is open and an NC contact is closed. When the actuator operates, the contact states change. Many industrial limit switches provide both NO and NC contacts through a common terminal.
4. How do I test a limit switch with a multimeter?
First isolate the switch from the electrical circuit and use the multimeter’s continuity or resistance function. Check the Common (COM) to NC and COM to NO terminals in the normal position, then operate the actuator and check again. A working contact should change its state when the actuator is operated.
5. How do I know if a limit switch is faulty?
A limit switch may be faulty if its contacts do not change state when the actuator is operated, the actuator does not move or return properly, or the electrical readings are intermittent or abnormal. Mechanical inspection and continuity testing can help identify the problem.
6. Where are limit switches used?
Limit switches are commonly used in machines and equipment where the position or travel of a moving part needs to be detected. Typical applications include lifts, conveyors, machine tools, doors, material-handling equipment, and industrial automation systems.
7. What is the difference between a limit switch and a proximity sensor?
A limit switch normally detects position through physical mechanical contact with an actuator such as a lever, roller, or plunger. A proximity sensor detects an object without requiring physical contact. The choice depends on the machine design, environment, required sensing distance, and application.
8. Can a limit switch be used to control a motor?
Yes. A limit switch can be incorporated into a motor control circuit to detect an end position and change or interrupt the control circuit. For example, limit switches can be used in forward/reverse mechanisms to prevent a moving machine part from travelling beyond its intended position.