A switch is used to control the power going to the light so it is not on all the time. If a switch was not needed, you would simply place VAC to one side of the light and attach the other side to neutral or ground to complete the circuit.
A switch between either the line power VAC and the light or ground and the light controls the circuit, enabling it to turn on and off. If the switch is between the power and the light, voltage is sourced to the light, completing the circuit. The light is always attached to the neutral ground line, so when the switch turns on, power is supplied sourced to the light, turning it on. If the switch is between neutral ground and the light, the light sinks to neutral ground , completing the circuit.
The light in this case is always attached to the power line, so when the switch is turned on, the neutral ground is connected sinked to the light, turning it on. Sinking and Sourcing in Industrial Control Circuits. The kitchen light example simplifies the difference between sinking and sourcing circuits. PLCs work a little differently than your kitchen light or a simple control relay, however.
PLCs were developed to automate and simplify control relay circuits, eliminating much of the wiring and adding greater flexibility. PLCs can manipulate control systems programmatically rather than by hard wired control relays.
PLCs use input cards to read the feedback of the control circuits. Reported In shows products that are verified to work for the solution described in this article. This solution might also apply to other similar products or applications. Issue Details I am setting up digital inputs and outputs on my system, and I want to make sure that I have enough current to drive the correct digital level.
Can my data acquisition DAQ device that is a sourcing digital input be utilized with a sinking digital output signal? Because both a voltage source and a ground reference are needed to create a complete circuit, you must have a sourcing input or output connected to a sinking output or input, respectively.
Note: If you wish to connect a sourcing input to a sourcing output or a sinking input to a sinking output, you will need to add an additional resistor. There are a couple of hardware considerations you need to take: You must determine whether your hardware can either be a sink, a source, or both.
Special attention needs to paid to ensure that the current is limited so as not to exceed the maximum current handling capacity of the GPIO pin. Again, check the datasheet as some microcontrollers can sink more current than they can source, but not always. One last note about choosing between a sourcing or sinking current design: your choice of current handling configuration in hardware will have impact on the software. When writing code, or even more importantly when using pre-built software libraries, be sure to review how the logic handles the GPIO pins.
Remember when sourcing current that in order to turn on the load you have to drive the GPIO pin high. The input module will detect either a no-voltage or voltage condition at the input.
These voltage conditions represent the open or closed conditions of the switch. Every input device is connected the same way. There is a current flow, but it is very small and usually under 10 milliamps. Notice in our example that the current is flowing into the PLC module. Depending on the type of PLC input module and how it is connected with the input device, the current could flow in or out of the module. Therefore, we say Device 1 is Sourcing the current, and Device 2 is Sinking the current.
In this case, we say Device 2 is Sourcing the current, and Device 1 is Sinking the current. In every situation where you have a current flow between two devices, one of the devices will be Sourcing and the other will be Sinking. One device is a PLC input module, and the second device is a push button switch.
Based on the connection polarity, the Sinking module will have the current flowing into it from the switch. Based on the connection polarity, the sourcing module will have the current flowing out of it. Current can flow in either direction through a push button switch because it is a passive device and not polarity-dependent. There are many active digital input devices used in industry today that are polarity-dependent.
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