Limit Switch Application

mechanical level switch

What is a limit switch and how is it used in a control process?

Limit Switches are simply switches that are commonly used to detect the presence of a moving object when the object reaches a desired location. When the limit switch is triggered, it will operate the contacts linked to it and will close or open an electrical connection.

Limit switches are either normally open (N.O.) or normally closed (N.C.) contacts.

One type of limit switch is a mechanical limit switch. It is used to detect the presence of an object when physical contact is made between the object and the switch.

a Mechanical limit switch has a lever or a plunger that triggers the switch. When the moving object touches and moves the lever, a set of electrical contacts are forced either open or closed.

Each symbol indicates the position of the switch contacts in their normal state when physical contact is not present. This is referred to as the switch’s unoperated condition.

 

When an object makes physical contact with the switch, the switch is in its operated condition.

The contacts of an N.O. limit switch closed and the contacts of an N.C switch opens when physical contact is made..

Mechanical limit switch are commonly used in automatic garage door openers. When a peg attached to the garage door makes contact with a lever on the the limit switch, the motor shuts off and the door stops opening.

It is also used in the closing of the refrigerator door. when the door closes and triggers the limit switch attached to the body of the fridge. the lights turns off.
In addition to mechanical limit switch, other types of limit switch includes pressure , flow, level and temperature switches.

Pressure switches are commonly used in pumps, air, compressors, and lubrication systems.

Flow switches are used to detect the flow of liquids in a pipe or duct.
Level switches are used to detect the level of liquids, slurries or solids, powder or paste.

Temperature switches are used to detect the temperature of a solid, liquid or gas.
Some limit switches have both N.O and N.C contacts.
The broken line indicates that the contacts are mechanically joined together.
when the bottom switch is forced open. the top switch closes.