Instrumentation

Limit Switch

In this lesson, we will study the limit switch.

A limit switch is a switch operated by the motion of a machine part or presence of an object.

  •  Limit Switches are electro-mechanical devices that consist of an actuator mechanically linked to a set of contacts.
  • When an object comes into contact with the actuator, the device operates the contacts to make or break an electrical connection.
  • Limit switches work in a variety of applications and environments because of their ruggedness, simple visible operation, easy installation, and reliable operation.
  • They can determine the presence or absence, passing, positioning, and end of travel of an object. They were first used to define the limit of travel of an object; hence the name “Limit Switch”.
  • Limit switches may be directly mechanically operated by the motion of the operating lever.
  • A  limit switch begins operating when a moving machine or a moving component of a machine makes contact with an actuator or operating lever that activates the switch Limit Switch Contacts

The most common contact symbols show whether the device has normally open or normally closed limit switch contacts.

Limit Switches are of 2 types

1.Single contact

In this type of limit switch, one pair of contacts is available. When limit switch operates this contact becomes NO  to NC or NC to NO.

2.Double contact

In this type of limit switch, two pairs of contacts are available. When Limit Switch operates  NC becomes NO and NO becomes NC.

Fig A    limit switch is in normal or non operated position.

Fig B    limit switch is in an operating position.

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