current Sink vs Source

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current Sink vs Source

Postby om_tech_support_JT » Mon Jun 03, 2013 4:39 pm

Q: I’ve looked through your manual and you show different connection diagrams for SINK or SOURCE logic, could you explain what the difference is between these two connection methods?

A: SINK and SOURCE logic are commonly associated with PLC I/O signals. First of all, SINK and SOURCE only applies to DC circuits. DC power source are labeled with “+” and “-” signs indicating the direction of current flow, and this signal current should flow in one direction only when we are connecting I/O terminals. Therefore, we need to make sure the current flows in the right direction in order to operate the circuits properly. This direction of current flow is referred as SINK and SOURCE logic.

In order for a signal to activate or deactivate, signal current needs to enter a terminal, to the internal circuit, and then exit out from another terminal to ground to complete the circuit. On a PLC/controller, each individual I/O signal should provide 2 terminals for connection: one for current going in, and one for going out. In order to save space, sometimes terminals are grouped together and thus called COMMON, This COMMON could either be the POSISTIVE VOLTAGE, or COMMON GROUND.

In general, SINK means that the device internal circuits will provide the path to ground, while SOURCE means the circuits will supply the voltage/current. Let’s look at the example for Current SINK: the Input/Output signal terminals are drawing current from an external device (PLC, Controller, power supply etc), and the internal circuit will provide a path for this current to go to ground.

sink vs source.png
Sinking Logic
sink vs source.png (67.74 KiB) Viewed 4959 times

The diagram above shows a Current Sink for the AR series driver. The SCX11 controller is on the left, and AR driver is on the right. From the diagram, a 5VDC is internally supplying a current to the controller circuit, then this current travels to the input signal terminal on the AR driver, through driver’s photocoupler circuit, to ground. Essentially, the input terminal on the AR draws signal current from controller, and provides a path to ground by closing the internal photocoupler circuit. This is the current SINK input.

On the other hand, when using current SOURCE logic, instead of providing the path for signal current to go to ground, the I/O terminal will supply the current to control device. Please review the diagram below:

Sourcing Logic
source.png (64.3 KiB) Viewed 4959 times

In this diagram, 5VDC goes directly into the driver first, (terminal 13 on the left), then the negative side of the input signal on the AR driver is connected to the device. Therefore, current goes into the driver internal circuit first, then goes to ground through the controller circuit. In this SOURCE configuration, the input terminal is sourcing signal current to the control device, and by closing the control device circuit, signal current will have a path to ground to complete the circuit.

To summarize, when following current SINK logic, the control device will supply the current through its internal circuit first. This signal current then travels to driver I/O terminals and to ground. With current SOURCE logic, driver I/O terminals will supply the current to controller device terminal and then to ground.

If there’s any questions, please feel free to contact our Tech Support at or 1800-468-3982.

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