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Hello all,


Recently we performed an installation of 2 PLCs model SM43-J-R20 (both containing the same program and updated to the last OS) to control 2 oil boilers. A few days ago, our customer reported the following problem from only one of them:


Sometimes, one of the PLCs shows a backlighted black screen (as seen in the attached picture). However, the process in the PLC seems not to be affected since it keeps running. Then, if they restart the PLC, it works well. (Note that the PLC has been powered ON for hours when this happens).


We replaced the PLC with a new one with no results. The problem is still the same. 


As additional info, the installation includes one contactor directly driven at 220V by a relay output from the PLC and we think that the problem described happens at the moment when the contactor is actuated (our customer is not sure about it).

But as I said, it doesn't always happen as a regular issue and only one of both PLCs is showing this behavior even though they contain the same program and electrical installation.



Any ideas?





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Individual system response to transient flyback pulses is one of the mysteries of our profession.  Better safe than sorry to put snubbers across ALL inductive loads as a matter of design in an AC controlled system.


There is a pinned post in the M91 section where we talk about this more:



Joe T.

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  • 2 years later...

Going on your diagram, that is a motor somewhere around 3kw.  Each time the contactor kicks in, this would be creating quite a dip on the volts feeding the power supply to the plc, as they are on the same line.  Depending on both the mains power supply setup, and the 24v dc unit itself, this may or may not be an issue.   And I'm assuming the motor is external.

Anything that is an inductive can.......will (!) cause issues to plcs.  The topic Joe refers to  (  http://forum.unitronics.com/index.php?/topic/2735-flyback-diode/  )  is essential reading.  One only has to power an unprotected 24v dc relay on and off, and then do it again with a clamp and you can actually hear the difference.  It sounds so much happier.

The principle of collapsing fields making very high voltage is the way your car puts a spark across the spark plugs, so you can imagine such things being damaging to sensitive devices if not controlled.  So as Joe says, I'd definitely be clamping everything, the contactor, valve and anything else that might be hidden. 

Also, it is not unheard of for end users to add in inductive (and other) devices that play havoc with a normally perfect system.  Denial, denial, damn useless machine, and then when the plc person visits the site they discover the real reason!

cheers, Aus

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Hi Aus,

Many thanks for your link and explanation.

I will change the contactor to one with Surge suppressing.

It is unfortunately not possible for me to go on site so i would have to have the client to re-built the circuit themselves. 

I have also ordered new PLC because i thought it might have taken damage due to this. 


Thank you!

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Don't forget the valve.  Clamping 24dc is easy, and the best method is the diodes mentioned by Walker and Joe at the end of that topic.  But anything in the enclosure that has a coil of some sort needs suppression.  And it would be interesting to measure the line dip and length when the motor starts.  You are going to a lot of trouble, but your first actions may not necessarily be the cure.

cheers, Aus

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