For Siemens S7-200 analog output protection

Hello everyone,

We are currently having problems with one analog output, it seems like it was damaged but we really do no have a clue how this could happen. The output is having 16.5 volts DC output and we are not able to change this. At the output we connected a resistor in parallel but it seems it was not good enough to protect, it was controlling a motor driver. Do you have an idea how this module could be damaged? an also please if any of you know a way how to avoid for this not happen again, we mean a circuit or something else to protect the output.

Thanks in advance for all your help.

Raquel,

normally when an analog output is damaged, this can mean two things:

1. the load on the output has been to high.
   From the datasheets you can learn that the output resistance of the analog voltage output has to be 5kOhms or more.
2. there has been a surge from the outside of the PLC.
   Against this phenomenon you can guard the output by putting two zenerdiodes back-to-back across the output. Since the S7-200 has a maximum output swing of -10V to +10V I would suggest to take 12V zeners. Another possibility is to put a MOV (Metal Oxide Varistor) across the output. I found one type with Harris Semiconductors for 14Vdc (V18ZA1), RS ordernumber 239-220.
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   Maybe there are some other possible reasons for this failure too, but these two are the ones I have already delt with. Hope this info helps.
   
   Kind regards,

Thanks a lot for all your help Jean Pierre, we will try your advice.

Raquel