PLC - switch input signal and isolating circuit design

PLC controller switch input signal, only two states, compared with analog, the isolation is simpler, coupling can be achieved through light.

below is I used in the circuit design of switch input signal and isolating circuit:

above,

U6 for light coupling,

R32 for current limiting resistor,

R31 to discharge resistance, used to prevent the input disconnected, through the U6 leakage current, make the U6 conduction, lead to error for input PLC closed.

R33 for pull-up resistors, and leds in parallel, it is mainly used for input closing instructions.

X00_IN MCU IO input port is connected to the PLC controller.

dry contact input switch signals and main have polarity NPN input, low voltage and high voltage PNP input.

in order to support the three input at the same time, a bidirectional optical coupling U6 as the isolation device.

SW1 used to test the reference level when the three input

when the input is dry contact or NPN XCOM received 12 v - 24 v power supply input. When the input for the PNP, toggle SW1, making XCOM received a reference to the PLC. When the input disconnect, U6 conduction, X00_IN to microcontroller output level. When the input closed, U6 conduction, X00_IN output low level to the microcontroller, LED lights lit up at the same time.

discharge resistor R32 R31 and current limiting current value is the difficulty of the circuit, PLC beginners often because the values is not rational in different working environment can't work reliable.

in order to guarantee the life of the input switch or button,

PLC controller need when switch or button closure provide 1 ma wetting current, is used to switch closure instant very weak spark, ablation impurities and oxide layer on the surface of the contact.

in humid environment or the switch/button installed in wet locations,

switch/button when disconnected the contact resistance is small, we will contact this kind of circumstance is called water resistance, insulation resistance PLC still need to be able to correct sentences is disconnected.

general water resistance values to a few K o.

comprehensive above consideration, when light coupling of conduction voltage of 1. 2 v - 1. 6 v, we chose the R32 resistance to 3. 3 k o, so that even if the PLC power supply voltage of 10 v, can also provide ( 10 - 1. 6) / 3. 3 = 2. 5 ma of current.

the voltage of power supply and PLC is 24 v, also can ensure the switch/button water resistance, 10 k euro, can reliably sentence to disconnect.

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