Our partial solution was to plug to +12V the PWM signal wire
Might as well take the 12V DC and use a step-down (DC to DC) converter (buck converter) of suitable power capability to bring it down to 10V or whatever you want...... and then use that to drive the fan.This is assuming that the "PWM signal wire" that you mentioned is simply the fan's DC voltage supply input.
My first choice was arduino. But it need to be cost effective. We have 1000 telecom sites around the country and almost all have an average of 3 cabinets. I cannot buy 3000 arduinos to do the task.
It´s not. The input it´s not analog, it´s digital. Already tried a voltage divider and it did nothing.
A cooling fan that overheats when running at full speed ? - something not quite right there.
Designing a retrofit hardware device to solve this problem is of course a fun thing to do and there are many alternative solutions to look at, but ultimately if the bought in system is inadequate (in this case cooling capacity is insufficient) then I'd tend to think of that as a problem for the manufacturer to solve.You have to consider that any modification you make could be rendered invalid if there is a software or other upgrade on the ELTEK systems. You also have to think of what happens if, say, one of these things catches fire and it is discovered that some unapproved modifications have been made. It might not look too good.
Also, it would not be practical to make one IC chip run 3 cabinets, since some 3G frequencys have more load than others and some overheat and some not. Would not want to have a cool cabinet running with more duty cicle because his neighboor is hot