Micro Switch 101: Working Principle, Types and Applications

Whether it be the industrial, construction, agriculture, commercial or transportation industry, microswitches have become an integral part of the sensing and control community. There are perhaps trillions of microswitches in use at this very moment in various types of industrial equipment, agriculture or construction machinery, and control systems around the globe. The fact that such a simple device – with normally open or closed contacts coupled with a snap-action switching mechanism – is capable of performing at an enhanced precision level, reliably and safely, in the many harsh environments, is simply astonishing.

What is a Micro Switch?

A micro switch is a snap-action precision changeover switch with a very small contact gap. The snap-action defines a changeover switch whereby contact-making and breaking speed are independent of the operating speed of the actuator. It is also referred to as a miniature snap-action switch.

Compared to conventional switches, micro switches have the following distinctive characteristics:

• Reduced size.
• Reduced operating travel: tenths of a millimeter.
• Increased electrical properties.
• Increased service life.
• Reduced sensitivity to vibrations and high accelerations.

Microswitches are often best suited for presence and absence detection where physical contact with an object is permissible. They also excel in applications requiring simple on-and-off actions. Each switch contains a spring and a set of contacts that convert mechanical energy applied to an actuator/plunger into an electrical output.

These switches are ideal where saving space and weight are important.

How Does a Micro Switch Work?

The microswitch makes physical contact with an object. After that, its contacts change position.

The basic working principle is as follows:

When the plunger is in the released or rest position

• The normally closed circuit can carry current.
• The normally open circuit is electrically insulated.

When the plunger is depressed or switched

• The normally closed circuit is open.
• The normally open circuit is closed.

Construction of a Micro Switch

The construction of a micro switch is really simple. The main parts of the switch are:

1.Plunger (Actuator)

2.Cover

3.Moving piece

4.Support

5.Case

6.NO terminal

7.NC terminal

8.Contact

9.Moving arm

Types of Micro Switches

Microswitches have various types. Some of them are listed below:

1. Pin plunger

Suitable for short direct strokes switch characteristics are used directly and high-precision position detection is possible. The overtravel, however, is the shortest of any actuator requiring a reliable stopper.

2.Spring pin plunger

Overtravel is longer than for a pin plunger but the same application methods are possible. The plunger diameter is slightly larger but off-center loads must be avoided, i.e., the load must be on the center of the plunger.

3.Spring short pin plunger

Overtravel is large, just as for a spring pin plunger. The plunger is short and has a larger diameter to make alignment easier.

4.Panel mount pin plunger

The overtravel is the largest of all plunger types. The switch is mounted to a panel using a hexagonal nut and locknut (the mounting position can be adjusted by adjusting the nuts). The plunger can be operated either manually or mechanically and usage in combination with a low-speed cam is also possible.

5.Panel mounted (cross) roller pin plunger

A roller is attached to a panel mount pin plunger switch and is operated with a cam or dog. The mounting position can be adjusted the same as a panel mount pin plunger switch but the overtravel is slightly smaller. Cross rollers are also available that run parallel to the switch.

6.Leaf spring

The stroke is larger because of a powerful leaf spring, making this actuator suitable for low-speed cams or cylinder drives. The bearing point is fixed, so the overtravel must be maintained within the specifications in the application to prevent damage.

7.Roller leaf spring

A leaf spring actuator with a roller attached. Suitable for cam or dog operation.

8.Leaf lever

A bend in the lever is used to create a large stroke. Suitable for detecting cams or dogs as well as various other moving objects.

9.Simulated roller leaf lever

A leaf lever with the end bent into a curve to enable application as a  simple roller type.

10.Hinge lever

Used with a low-speed, low-torque cam, the shape of the level can be changed to match the operating body. Steel is mainly used as the material for the lever.

11.Simulated roller lever

A hinged lever with the end bent into a curve to enable application as a simple roller type.

12.Hinge roller lever

A hinged lever with a roller attached. Suitable for high-speed cam operation.

13.Reverse operation hinge lever

Used with a low-speed, low-torque cam, the shape of the level can be changed to match the operating body. Steel is mainly used as the material for the lever.

14.Reverse operation hinge roller lever

A reverse operation hinge lever with a roller attached. Suitable for cam operation.

15.Rotating operation

A low-torque actuator with a rotating operation. Suitable for detecting lightweight objects, such as coins and paper currency.

16.Flexible rod

Can be operated in any of 360° (but not in the direction running along the center of the rod). The operating force is small. Effective for detecting when the direction or shape is not consistent. The overtravel is absorbed by the actuator, providing a large leeway in operation object play.

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Examples of actuators

Definitions of operating characteristics

The definitions below explain the meaning of operating characteristics. Sketches show how characteristics are measured for in-line plunger actuation. Linear dimensions for in-line actuation are from the top of the plunger to a reference line, usually the center of the mounting holes.

Differential Travel (D.T.)

The plunger or actuator travels from the point where contacts ‘‘snap-over’’ to the point where they ‘‘snapback.

Free Position (F.P.)

Position of switch plunger or actuator when no external force is applied (other than gravity).

Full Overtravel Force

Force required to attain full overtravel of the actuator.

Operating Position (O.P.)

Position of switch plunger or actuator at which point contacts snap from normal to operating position. Note that in the case of flexible or adjustable actuators, the operating position is measured from the end of the lever or its maximum length.

Operating Force (O.F.)

Amount of force applied to switch plunger or actuator to cause contact ‘‘snap-over.’’ Note in the case of adjustable actuators, the force is measured from the maximum length position of the lever.

Overtravel (O.T.)

Plunger or actuator travel safely available beyond the operating position.

Pretravel (P.T.)

Distance or angle traveled in moving plunger or actuator from free position to operating position.

Release Force (R.F.)

Amount of force still applied to switch plunger or actuator at moment contacts snap from operated position to unoperated position.

Total Travel (T.T.)

Distance from actuator free position to overtravel limit position.

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Actuating force – movement – diagram

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Contact force – movement – diagram

Terms related to durability

Mechanical Durability

The switching durability is when a switch is operated at a specified frequency and specified overtravel (OT) without the contacts energized.

Electrical Durability

The switching durability is when a switch is operated at a specified frequency and specified overtravel (OT) under the rated load.

Maintenance of Micro Switches

Microswitches are not user-maintainable but they should be kept in a reasonably clean, paint-free condition, especially in the actuator area. Regular checks should be made on mounting security and on the actuating medium to switch actuator relationships. Lubrication or the use of aqueous or chemical cleaning fluids is not recommended.

Do not use switches in atmospheres containing combustible or explosive gases. Arc or heat generated by switching may cause fires or explosions. Do not drop or disassemble switches. Not only will characteristics be jeopardized, but also damage, electric shock, or burning may result.

Mounting of Micro Switches

Side mounting switches should be mounted on smooth, firm, flat surfaces using the recommended screw size. Avoid over-tightening the screws. For added security, they should be locked using epoxy resin. Do not attempt to enlarge switch mounting holes and avoid over-stressing the switch. Use insulating material between the switch and metallic plates to increase clearance on switches with open terminals.

When soldering, overheating of the switch insulation must be avoided. In certain circumstances, it may be advisable to use a heat shunt. For optimum mechanical strength, the conductor should be wrapped around the tip of the terminal taking care to avoid loose strands of wire.

Micro Switch Applications

Micro switches can be used for a broad range of electrical loads and operating forces in high cost-of-failure potential applications such as:

• Appliances, timing devices, office equipment, computer/ business equipment, test instruments, manually operated devices and valves.
• Vending machines, water heaters and industrial controls.
• HVAC wall controls, automotive, truck and boat wire harnesses, lock modules for tail-gate/ trunk, sub-assemblies for convertible roofs and foot pedal controls.
• Aerospace, instrumentation, office equipment, medical/dental equipment and more.