AVR-Chat

Problem:

Industrial contactors make such a high noise (surge) that occasionally
cause these symptoms on AVR MCU :

1- LCD blink,  unrecognized characters

2- Flash rom bitmap change

3- EEPROM change or erasure

Explanation:

My atmega16, controls 3-phase contactors
<http://en.wikipedia.org/wiki/Contactor>   through 5V relays which are
using ULN2803. As chokes,  I wound about 10 turn of  wire over a 8mm
diameter ferrite rod taken from AM radio antenna.  So, MCU commands to
ULN, it activates 5V relays, and after the rod choke, contactors (in
another metal case) are trigged.  I used the same (one) rod for all
input/output lines in controller box. This made the situation much
better, yet I get the same problem after 100-200 cycle of contactor work
(for exaple after a 5 hours of work).

My Understaing:

  I guess when 220V AC sine wave gets it peack, and contactors is
released at the same moment, the maximum spark is created and it
reflects back through the output wire to MCU. Maybe some radiowave is
also made, so I put the whole 220V contactors in a seperate metal case.

Question:

1- What is the real frequency of this noise/spark/surge, and its
voltage? If I know it, I can manage to attenuate it to tolerable level.

2- How can I improve the situation? Maybe a low-pass filter do the job
(or complete the isolation)?

3- Is there a web link to information regarding contactor surge
reduction? I couldn't find any.      I probably can not find commercial
surge killers, so please teach me how to make one with electronic parts.



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