What Is The Difference Between Sink And Source

As a technical support person, I am often asked what the difference between sink and source is. I will explain the main difference between sink and source in this post.

The Difference Between Sink And Source

To explain the difference between sink and source when it comes to 4-20mA loop wiring, first, we need to understand basic electricity.

Let’s examine the two diagrams below. Both are basic, and they have a power source, a toggle switch, and a bulb. The build will turn on if you turn the toggle switch in both.

The Difference Between Sink And Source

Even though the two diagrams above have the same components, they have one key difference.

In the first one, the toggle switch is connected to the power source, and the current moves from the switch to the bulb, while in the second diagram, the switch is connected to the earth and the current moves from the bulb to the switch.

In instrumentation, we say that in the first diagram, the switch is the source, i.e., connected to the power source, and the bulb is sinking, i.e., connected to the earth. In contrast, in the second diagram, we say that the switch is sinking and the bulb is sourcing.

I know this is a simplified explanation, but it will help you to understand the difference between source and sink.

Now, replace the toggle switch with a transmitter and the bulb with a controller or a PLC.

So, the difference between sink and source is that in the direction in which the 4-20 mA flows, the unit sending the current is the source, and the one receiving the current is the sink.

What determines if the transmitter is a sink or source?

Usually, the manufacturer will decide if the transmitter or the PLC is a sink or source.

They do this by adding either an NPN transistor to make the transmitter sink or they will add a PNP transistor to make the transmitter source.

Five main differences between sink and source

There are five main differences between sink and source; understanding these differences will help you to determine which sensors and transmitters you use during the buying process, and it will help you to understand the wiring of the system.

Current Flow

In the sink configuration, the transistor is NPN, and the current flows from the sensor into the PLC and then to the ground.

The transistor in the source configuration is PNP and the current flows from the PLC to the ground through the sensor.

Wiring

The sensor is typically connected between the input terminal and the ground in the sink configuration.

In the source configuration, typically, the sensor is connected between the input terminal and the positive supply voltage.

Polarity

Sink devices are connected to negative common (ground).

Source devices are connected to positive common (positive supply voltage).

Device Compatibility

Ensure the field devices (sensors, switches) are compatible with the PLC input/output configuration (sink or source).

For instance, NPN sensors are typically used with sinking inputs, and PNP sensors are used with sourcing inputs.

Safety and Noise Considerations

Sink: Often preferred for its simplicity and fewer wiring errors in negative common setups.

Source: Sometimes preferred in applications where grounding issues or noise immunity are a concern.

How do you know if the transmitter or a PLC is a sink or source?

There are three ways to know if your transmitter or a PLC is a sink or source.

Checking the PLC input card

Most PLCs come with a card that shows whether the card is in a sinking or sourcing. Regarding transmitters, they usually have a +mA signal for the source and a -mA signal for the sink.

Check the user manual.

All PLCs and transmitters will have a user guide and a wiring section. Usually, the wiring section will tell you if the device is a sink or source and how to wire it.

Call technical support

Most controllers and transmitter manufacturers have a technical support department. You can call them, and they should be able to tell you if the device is sink or source.

FAQ: What Is The Difference Between Sink And Source

What is sink and source in PLC?

A Sourcing PLC output module will flow electric current from its control terminal into the connected load.
A Sourcing digital PLC output module provides a positive voltage to the connected load, whereas a Sinking digital PLC output module provides a ground connection.

What is sinking and sourcing?

Sinking and Sourcing define the direction of conventional current flow between 2 devices. In a Sinking module, the current flows from the load.

What are Sinking and Sourcing Outputs?

Sinking Output – The device’s output signal does not provide power. It must be connected to a device that provides power for the output signal or a sinking input with a loop power supply in the circuit.

Sourcing Output – The device’s output signal powers the output circuit. It must be connected to a receiving device that provides no power and acts as a resistive load, such as a 2-wire passive transmitter.

Definition of Source and Sink

The words themselves are simple to define.

Sourcing: Providing the (+) voltage supply.

Sinking: Providing the (-) voltage return path.

Is Sinking PNP or NPN?

PNP sensors are sometimes called “sourcing sensors” because they source positive power to the output. NPN sensors are sometimes called “sinking sensors” because they sink ground to the output.

Key Takeaways

The main difference between sink and source is the direction of conventional current flow between 2 devices.

In a sink configuration, the current from the controller or PLC goes to the sensor and then to the ground, while in the source, the current flows from the sensor to the controller and then to the ground.

To figure out if your sensor, transmitter, or controller is a source or sink, you can check that in the device manual in the wiring section or you can call the manufacturer and they should be able to tell you the configuration of the device.

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