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SIMATIC S7-1200

Connected Q0.0 and 3L

Thread Starter: SzTyWnY   Started: 8/5/2012 8:18 PM   Replies: 5

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  8/5/2012, 8:18 PM
SzTyWnY
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Hi, I have a problem with my S7-1200 (1214C DC/DC/DC). I tried to make a fast PWM work, but in some random time (after a weekend when noone touched PLC) I have an 24V always on my output Q0.0. It is very strange, because it is only on this output.
I checked it with my multimeter, and it was buzzing when I toutched Q0.0 and 3L (+24V). It has an resistance about 6Ohm.
Why such situation is happen? Please help.
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  8/6/2012, 2:21 PM
huggy_d1
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SzTyWnY wrote:
Hi, I have a problem with my S7-1200 (1214C DC/DC/DC). I tried to make a fast PWM work, but in some random time (after a weekend when noone touched PLC) I have an 24V always on my output Q0.0. It is very strange, because it is only on this output.
I checked it with my multimeter, and it was buzzing when I toutched Q0.0 and 3L (+24V). It has an resistance about 6Ohm.
Why such situation is happen? Please help.

If you connect an o-scope to the Q0.0 output does it look like a stable 24V or is it actually doing the PWM and the multimeter is simply too slow to react?

What control do you have on the PWM?  100%, 0%, 50%?  Maybe a screenshot of the control logic could help.
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  8/6/2012, 4:11 PM
SzTyWnY
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on the o-scope it is stable 24V. I had a variable value of control and it still was 24V. I think that it should have simply break.
I tested second PWM and it works fine (Q0.2). I tested it on 100kHz.

Can I repair this broken channel by myself? I called today to Siemens Service in Poland and they said, after I gave my PLC number, that they cannot repair my PLC.
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  8/6/2012, 4:27 PM
huggy_d1
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SzTyWnY wrote:
on the o-scope it is stable 24V. I had a variable value of control and it still was 24V. I think that it should have simply break.
I tested second PWM and it works fine (Q0.2). I tested it on 100kHz.

Can I repair this broken channel by myself? I called today to Siemens Service in Poland and they said, after I gave my PLC number, that they cannot repair my PLC.

Is your PLC under warranty? 

If the PLC is not under warranty, you might have some luck locating the failed component(s) on your Q0.0 output to cause it to fail on.  However, it is probably less expensive to simply replace the defective unit and mark the device as 'failed Q0.0 - otherwise ok'.  That gives you a mostly functional PLC for other purposes.

Since the PWM caused the output to fail, maybe there is some inductive load causing excessive spikes on your output reducing the life of the output.  Do you have any type of surge protection on the output?  If not, you might consider adding that before another output fails.
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  8/6/2012, 7:03 PM
SzTyWnY
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Thanks for your help. Maybe you know how looks output in this PLC (schema)? So i could find the damaged part and fix it. 
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  8/6/2012, 8:15 PM
huggy_d1
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SzTyWnY wrote:
Thanks for your help. Maybe you know how looks output in this PLC (schema)? So i could find the damaged part and fix it. 

I have not had the need to disassemble one of these S7-1200 plc's so have no idea what is there.  With the small size, I am guessing there may be multiple outputs per physical IC.  Based solely on the power/common layout, I am guessing there are 2 input ICs 2 output ICs.  These ICs may be proprietary but I would hope not.
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