Solid-State Devices Used in Industrial Logic Circuits: TYPICAL LOGIC CIRCUITS: Identifying Inputs and Logic in the Circuit

The condition that allows the coil of CR1 to energize is that both LS1 on the first door AND LS2 on the second door must both be closed AND the start switch must be turned to the on position. The word AND means the logic condition AND. If either door is opened prior to starting the oven, or if they are opened while the oven is running, the coil to CR1 will no longer be energized. Turning the start button to the off position will also dc-energize the coil of relay CR1. The condition of the coil of CR1 (energized or dc-energized) will also be part of the logic for the fan and the electric heating coil.

The NO contacts of CR1 must be picked (closed) to pass power to the fan motor. Since the contacts of CR1 can only be closed if the coil of CR1 is energized, notice that the condition of the start switch, LS1, and LS2 will also affect the fan motor. If the start switch is in the off position, or if either door is opened, the contacts of CR1 will drop (open) and the fan will turn off.

The electric heating element is controlled by the logic of the flow switch. If the fan is operating and air is blowing against the flow switch, it will close and pass power to the heating element, lithe start switch is in the off position or if either of the doors are opened, LS1 or LS2 will open and dc-energize the coil to CR1. The contacts of CR1 will then drop and turn off the fan. This will cause the air flow from the fan to stop and the flow switch will open and turn off power to the heating element.

This example circuit shows how switches such as LS1 and LS2 can be inputs to the circuit and also be an integral part of the logic that operates the system. In most circuits, the logic is determined by safety and operational conditions. For instance, someone may get burned if he or she opened the door while the oven is operating, so the door limit switches will dc-energize both the fan and the heating element to prevent this from happening. Sometimes logic is used in the circuit to protect the equipment. For instance, the heating element of this oven will burn out if sufficient air is not passing over it when it's energized. The flow switch protects the heating element by interrupting current to it if the fan is not moving air.