 Welcome to Interlocking Ladder Diagrams. The ladder circuit consists of two power connections, L1 and L2, that have different voltage potentials. These vertical lines are called rails. Between the vertical rails, a horizontal circuit is connected. This horizontal circuit consists of a normally open push button and a coil that drives a motor, M1. This horizontal circuit is called a rung. When the push button is pressed, its contacts close, providing a current path that energizes the motor. The disadvantage of this circuit is that it is necessary for the operator to keep a finger pressed on the push button to run the motor. The interlocking ladder circuit shown will keep the motor running after the push button has been only momentarily pressed. Observe the current path in rung 1 when the push button is pressed. Current flows from L1 to L2 through the normally closed NC push button, the NO start button, and coil C1. Coil C1 consists of a coil and contacts. Observe the operation of C1. When the coil becomes energized by the current flowing through it, a magnetic field forms around the coil and pulls the NO contact closed. When the coil becomes de-energized, the NO contacts open, because the magnetic field is no longer present. When current flows from L1 to L2 while the start button is pressed, the current also flows through contact C1, which is parallel with the push button. This causes output coil C1 to remain energized after a momentary closure of the start push button. This kind of circuit operation is referred to as latching. The output coil also closes a second contact in rung 2. When the contact is closed, a current path is provided to run the motor. When the normally closed push button is pressed, the current path in rung 1 opens, de-energizing coil 1 and opening the contacts in both rungs, causing the motor to stop. Congratulations! You have completed this learning object interlocking ladder diagrams.