2 Important Factors about Sinking and Sourcing Circuits?

Sinking and Sourcing, You might think what is this topic about? Right.

When we choose the external I/O Cards for PLC system, then this concept Sinking and Sourcing is a primary. You must have a clear understanding of what is sinking? and what is sourcing? first.

To understand this concept let’s first check the concept of current flow in basic DC circuit.

sinking and sourcing

Basic DC Circuit

The above diagram shows a current flow in the basic DC circuit. Current will flow only in one direction from positive to negative terminal through the load.

To control the load(ON and OFF the load), we can place the switch either between the positive terminal of the source and load or between the negative terminal of the source and load.

sinking and sourcing

Sinking Switch vs Sourcing Switch

Sinking and Sourcing Circuits

Sinking and Sourcing often create confusion for those people who are newly introduced to the concept, especially those who are not familiar with the electronics field. But when concepts are cleared there is a new question: Which concept is useful for which application?

Programmable Logic Controller(PLC) is designed to simplify the complex control circuits. PLC reduces the wiring of inputs and outputs greatly and reduces troubleshooting time and thus provide flexibility. PLC can modify the process control very easily just by changing the user program which is mind-boggling in hard-wired relay control systems.

PLC systems use Input/Output cards to read the signal or send the signal to the field devices. These Input/Output cards are either sinking or sourcing cards.


Sinking Input and Output

“Sinking means internally connected with the common (-ve terminal)”

sinking and sourcing
Sinking Input and Sinking Output

An input or output card wired internally to common is typically regarded as a sinking input or sinking output card.

A sinking input card requires power to be sourced to the input to turn it “ON”. The same way a sinking output card requires power to be sourced to the load to turn it “ON”.


Sourcing Input and Output

“Sourcing means internally connected with the source (+ve terminal)”

sinking and sourcing
Sourcing Input and Sourcing Output

An input or output card connected directly to power, it’s typically called as sourcing input or sourcing output card.

A sourcing input card required a ground connection to the input to turn it “ON”, same way a sourcing output card requires a ground connection to the load to turn it “ON”.


Let’s take an example wiring diagram of PLC input card

sinking and sourcing

PLC Input Card Wiring Diagram

In the above input card Byte, 1 is connected as sinking input where all the inputs Ch-1 to Ch-8 are internally connected with common negative supply and positive supply is coming back from the field devices input.

Byte-2 is connected as sourcing input where all the inputs Ch-1 to Ch-8 are internally connected with common positive supply and negative supply is returning back from the field devices input.


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