Current Sink and Source in PLC.PLC Sinking and Sourcing wiring. NPN PNP sensor input to PLC

A Three-wire proximity sensor which consists of  three wires  in which the
Brown wire is connected to the positive voltage,
Blue wire is connected to the ground and the
Black wire is the signal output wire.

In a 3 wire PNP proximity sensor we know that a PNP transistor is used for positive switching.
Which means that a PNP transistor is installed between the positive voltage and the sensor signal output wire.(Already discussed in the previous chapter)

Sensors are used in a control system to provide information’s to the controller such as a PLC. So, the black wire which is a signal output wire of the proximity sensor is connected to the Input Card of the PLC.

PNP sensors give a high voltage as output, when an object is detected

and low voltage as output when no object is detected.

But how will a PLC detect the output voltage from the sensor, this can be done by connecting the output of the proximity sensor in a closed circuit with the PLC input card.

Now when we closely watch this circuit, we can understand that the connection between the PNP proximity sensor and the PLC input card can be considered as a simple circuit which has a power source, a PNP transistor switch and a PLC as the load.

In this circuit for the current to flow one thing is missing that is the “ground connection”.

PLC input output card is of two types

1. The one in which the Input of the card is internally connected to the positive voltage is known as the SOURCING CARD and

2.The one in which the input of the card is internally connected to the ground is known as SINKING CARD

The Sinking Card already has a connection to the ground while the Sourcing Card already has the connection to the POSITIVE VOLTAGE

In a PNP Sensor the sensor output is internally connected to the positive voltage and to make the circuit complete the PLC input card must be connected to the ground.

As we need input card which is internally connected to the ground to complete the circuit, we select sinking input card.

When the PNP sensor detects an object the PNP transistor switch closes and so the circuit gets closed and current flows from the positive terminal of the sensor to the ground of the input card.

Now the sensor can be thought of as giving power to the input card, which means that PNP sensor sources power to the PLC input card. So PNP sensor is known as sourcing sensor.

Now let’s discuss the case of 3-wire NPN proximity sensor.

Three-wire NPN proximity sensor.

As discussed earlier a 3-wire proximity sensor consist of three wires in which the brown wire is used to connect the positive voltage, blue wire is used to connect the ground and the black wire is the signal output wire.

In a Three-way NPN Proximity Sensor we know that NPN transistor is used for negative switching, which means an NPN transistor is installed between the negative voltage and the sensor signal output wire.

As you know the output of the center is connected to the input card of the PLC. Now when we closely watch the connection between NPN proximity sensor output and PLC input card we can consider it as a simple circuit which has a NPN transistor switch, PLC card as the load and a ground connection.

Now only one thing is missing in the circuit for the current to flow, that is the positive voltage connection in the circuit. So, we connect a sourcing input card in which the input of the card is connected internally to the POSITIVE VOLTAGE.

As you know NPN sensors are ON naturally, that means it gives high output voltage when no metal object is detected and goes OFF or low, when an object is detected.

When NPN sensor does not detect an object, the NPN transistors switch closes and so the circuit gets closed and as a result current can flow from the positive terminal of the input card through the ground of the sensor.

The sensor can be thought of as, receiving input power from the sourcing input card of PLC, which means that NPN sensor sinks the power from the sourcing input card to the ground and so NPN sensor is known as sinking sensor.

From the circuits discussed above we can find that sinking and sourcing concept describes a current flow relationship between field input and output devices in a control

system and their power supply. A device sending current out of its control terminal to some other device is known as current sourcing device, while a device accepting current into its control terminal is said to be current sinking device. 

PNP sensors are current sourcing devices and NPN sensors are current sinking devices.

As both voltage source and a ground reference are needed in order to create a complete circuit a current sourcing sensor must be connected to a current sinking input likewise a current sinking sensor must be connected to our current sourcing input.

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.