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Old May 2nd, 2003, 05:23 AM   #1
rajendrad
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Siemens Analog input card

IN THE RECENT COMMISSIONING I HAD FACED SIEMENS MAKE ANALOG OUTPUT CARD FAILURES.WHILE CHECKING THE SYSTEM I NOTICED THAT 24VDC INPUT TO THE CARD IS VARYING FROM 19VDC-25VDC.THIS IS BECAUSE SOLENOID LOAD IS ALSO ON THE SAME POWER SUPPLY.IS SUCH POWER SUPPLY WILL CAUSE FAILUARE OF ANALOG OUTPUT CARDS ???????????
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Old May 2nd, 2003, 09:14 PM   #2
RRobbins
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Please, turn off the capitals. WE CAN HEAR YOU!!

Now, you were using a digital meter, I'm guessing. The voltage was dropping lower, but you meter averages for short times and misses the peaks, up and down. The cards are not designed to have fluctuating power and puts a huge load on the solid state componets. The filters can't filter out everything and I guess a spike wiped you out. Seperate the power supplies.
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Old May 5th, 2003, 08:56 AM   #3
icky812
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What RRobins said is certainly accurate. A lot of times what happens is that the DC solenoids do not have surge suppression on them. So when they are turned off the reverse voltage spike travels back up the line and starts destroying the weakest device on the line. In this case the Analog Output cards.

Transorb diodes or MOV's placed across the coil are very cheap insurance against this type of failure. DC coils are the worst, but AC coils should have surge suppression as well. I talk with people everyday that have destroyed a high dollar plc, touchscreen, or other piece of equipment from not using surge suppression.
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Old May 5th, 2003, 09:41 AM   #4
Fritz_frederix
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Rajendrad,

A litlle while ago I also had kind of the same problem, what RRobin
is saying is certainly true but it also can be another thing.
Earthing, make sure that the earth of your plc is properly
connected. Also be sure that one site of the powersupply is
earthed. It's a typical Siemens problem.

Best regards Rudi
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