Dear all, as maxHeadRoom pointed, today I purchased two 0.22uf 275v caps and a 100ohm resistor,wired all in series and tested the project it runs 90 complete cycle without max232 latch and no mcu reset. I didnt run this much correct amount before therefore it mean the snubber across the coil gave me solution for me. Infact I have upgraded the code such that it disconnect max232 before a contactor switched off.

Thank you everyone!

If I made confusion. There is no doubt the snubbers made the solution beside that I modified the code in such a way to handle itself if the noise appear for example if the snubber were absent

 

You may want to use an opto isolation chip. This will prevent noise on the microprocessor side.

 

You may want to use an opto isolation chip. This will prevent noise on the microprocessor side.

Infact the noise is avoided using snubber circuit sugggested by maxHeadRoom but it is better for to know more. But I didnt understand the mexhanism how opto protect radiated noise

 

Infact the noise is avoided using snubber circuit sugggested by maxHeadRoom but it is better for to know more. But I didnt understand the mexhanism how opto protect radiated noise

Often, adding opto isolation breaks the current loops that may exist and so the induced voltages produced by them are eliminated.
It is a bit like the stopping of hum in audio amplifiers by correct shield earthing practices.

 

Often, adding opto isolation breaks the current loops that may exist and so the induced voltages produced by them are eliminated.
It is a bit like the stopping of hum in audio amplifiers by correct shield earthing practices.

I am sorry for being very late. I am trying to understand it in the future work I am sure I also include optoisolater thank you dendad

 

As well as the manuf such as Electrocube, many suppliers of contactors have ready made snubbers for their line of contactors.
A good starting point for a DIY version is 220ohm res in series with .5uf cap.
Max.

Thanks for the mention MaxHeadRoom.

To prevent both conducted and radiated EMI from the coil and contacts (also helps with contact protection), snubbers should be attached at both locations. 0.5uf and 220 ohm is a starting place, he needs to obtain a number of resistors and capacitors, and using a storage oscilloscope, tune the value for optimum attenuation.

This guide may help - https://www.electrocube.com/pages/rc-networks-substitution-box-engineering-guide-data-sheet