How to connect digital sensors to a PLC

How to Connect Digital Sensors on PLC – On occasion This time I will discuss about connecting sensors
as an input device on the PLC. Before connecting the sensor to the PLC, things to know
namely the concept of sinking and sourcing on the input module that is on the PLC. Sourcing and Sinking only applies
on the amount of DC electricity (unidirectional).


Usually the input sourcing module has
common positive,
while in the module
sinking inputs have common negative. For more details can be seen
in the series image below.

In the picture above, connecting the inputs to Group 0 (Figure above) uses the Input Module Type
Sourcing, the input used is Common Positive. While
input connection in Group 1 (Figure below) is Input Module Type
Sinking that uses Common Negative.

As with the input module, sensors are divided into 2 types, namely input sourcing (PNP) and input
sinking (NPN). Sensor with sinking type or commonly called NPN only
can be applied to the input sourcing module, while sensors are of type
sourcing or commonly referred to as PNP can only be applied to the input module
sinking. Some PLCs currently have more than 1 common used
as an input, so that it is able to connect to input sourcing and input sinking in the same system.
However, it is better to use one of these
sourcing or sinking to minimize the occurrence of errors in
connection to PLC.

Sensor with 3 cables

In general, the sensor used as a PLC input has 3 cables. Sensor with 3 wires consists
The 2 cables used as the source are negative (Blue) positive (Brown) while 1 cable is used
as the output signal
(Black). For more details can be seen in the image below

In the picture below is an example
the circuit of the sensor. In the NPN sensor, after the voltage is applied to the cable foot
colored Brown and Blue,
then the output on the cable that is black to blue is 24V even though the sensor
has not detected anything (red circle) and then changes to 0 (zero) when it detects an object
detected by the sensor.
Whereas the PNP type sensor applies the opposite to (blue circle).

Load or commonly called a load is an input terminal pin that will be activated. From the picture you can
seen that the sensor sinking (NPN) in the sensor circuit gets the current flow from the Load,
while on the sensor
source sensor circuit that can provide current to the Load or load. Broadly speaking, sensors
Singking has a working principle similar to an NPN transistor, which is when the sensor detects an object
The main circuit switch will provide Base current, so that an electric current will flow from the side
positive source voltage passes through the load and then is channeled again through
transistor to go to the negative side of the voltage source. In sensor sourcing (PNP), flows from
a positive voltage source will flow through the transistor first, then pass the Load or
load to arrive at
negative side on
voltage source.

With the principle below, the connection of NPN sensors
Common positive is needed at the PLC input, so when the sensor detects an object
that is missed, the current will flow from the positive power supply
towards the common which will flow through the optocoupler input circuit then exit
through the input module port and then through the black cable (Black) into the sensor circuit until
end up on the negative power supply via a Blue cable.

PNP sensor is needed in common negative
at the PLC input, so that when the sensor detects an object, current will flow from the positive power supply through
Brown cable to the sensor circuit. Furthermore, the current that comes out of the sensor will
channeled via cable
Black to the intended PLC input. Current from the battery to the common will
through the optocoupler input circuit then exits through the port
Common and channeled through the Black cable and into the sensor circuit up to
end up on the negative power supply via a Blue cable.

2-Wired Sensor

In addition to sensors that have 3 cables, there are several types of sensors that have 2 cables namely Blue and Brown. Sensor
with 2 pieces of cable
has economic value or a cheaper price compared to the 3-wire sensor.

When using the sinking input module
(common negative), brown cable (Brown)
will be connected to the positive voltage source, the cable is blue
(Blue) on the input module pin terminal
connected to the load (Load). Whereas when using
input sourcing module (common positive), blue cable (Blue) will be connected with negative
voltage source, Then the brown kebel (Brown) on the terminal pin
connected to the module
PLC input (Load) or to load. The connection of the 2-wire sensor to the PLC input module is shown in
the following picture:

Reed Switch

Reed switch is a switch that will be active if it is located
around the magnetic field. This sensor has the main components in the form of sheets
copper leaf as a switch that is sensitive to magnetic fields with 2 legs, namely Brown and Blue. When affected by a magnetic field
A reed switch will be connected to deliver an electric current from Brown's feet to Blue's feet. Picture
from the red switch can be seen in the picture below.

Reed Switch is widely used as a boundary indicator
front and rear boundaries of the pneumatic cylinder. Reed switch connection on
PLC has a similar installation method with 2-wire sensor, in the picture above is a diagram
cable connection with the Common Input Negative, while the bottom picture is the Common Input
Positive, as shown in the following image.

If we look more closely at the picture above, we can see
reed switch has 2 types of resistance, namely the PLC input module circuit resistance
(Load) and resistance in the internal sensor circuit (r).

Never connect
2 foot sensor 2 wires or reed switch directly to a 24 Volt voltage source without overloading
first (input module). Pg
this can cause damage to the sensor due to excessive current
flow in the internal sensor circuit. The right connection will result
current (I1) worth
24V / (Load + r), while the connection as below produces
current (I2) equal to
24V / r. With r small enough, the current I2 will increase significantly compared to I1.

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