How to read a control circuit (Control Logic Diagram)?


Learn to read Control Circuit

Control circuit can be re represented pictorially in various ways.One of the common approach is to use control logic diagram which use common symbols which represents control components.

Input device Symbols

Input device - Control logic diagram

Decision device Symbols

Decision devices are important components of a properly operating control system.One type of this device is the push button.

There are two types of push buttons .Momentary contact and Maintained contact.

The contacts of a momentary pushbutton change state open to close or vice versa when the button is pressed.They returned to their normal state as soon as the button released.

Maintain pushbutton latches into place when pressed the it must be unlatched to about to return to it normal state.

Pilot light - It is used to indicate specific condition of a circuit.

Contacts - Contacts can be in the form of Instant operating contacts or timed contacts.
Letter designation N and O or N and C describe the status of contacts when they are not energized.
Letter designation T and C or T and O describe their operation as being based on a stipulated amount of elapsed time.

How to read Control circuit - decision device symbols

Contact Symbols

Various device incorporate contacts to control the flow of current to other control components. In an operation a contact may be either open resulting in a condition that blocks current flow or they can be closed resulting in a condition that allows current flow.

Control logic diagram however cannot show the dynamic operation of contacts and instead they show contact is either normally open with designation NO or normally closed with designation NC.

Control Logic Diagram - Contact symbols

Control logic diagram show actuating devices in their de-energized (off) state.

Coil Symbols

Motor starters,Contacts and relays are examples of devices that open and close contacts electromagnetically.The electro magnet in these devices called coil.

The coil symbolized a circle with letter or a number inside of it.The letter often represents the type of the device such as letter M for motor starter.

Control Logic Diagram - Coil Symbol