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F-DO and coil diodes

Created by: zombit at: 4/2/2013 9:31 AM (8 Replies)

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4/2/2013 9:31 AM	Rate (0)
zombit Posts: 16 Rating: (0)	Hello all, We are using an 4 F-DO 24vdc digital output card to energise safety contactors, the safety contactors are 24vdc coils and capable of 43amps on the main poles, therefore these units are quite large. Typical electrical design will incorparate diodes across the coil to supress arcs, but if I use a diode across the coil and try to pull the contactor in, the system lights up red and my card diagnostics reads power source / short circuit failure (I have disabled "wire break" diagnostics). I would like to know if it is common practice to not use diodes on contactors with this specfic output card. (card I am using is a 6ES7 138-4FB03-0AB0) I am wondering if they have internal diodes or other suppersion, but I could not find anything in the datasheet. Can someone please help.
zombit Posts: 16 Rating: (0)
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4/2/2013 2:21 PM	Rate (0)
Jazz_M Posts: 1121 Rating: (86)	Hi, What does the help topics tell you about the fault. Did you check that. I would advice you to check with a local Siemens Person for assistance.
Jazz_M Posts: 1121 Rating: (86)
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4/5/2013 7:40 AM	Rate (0)
zombit Posts: 16 Rating: (0)	My question is simple, am I able to use supperssion diodes with F-DO cards?
zombit Posts: 16 Rating: (0)
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4/6/2013 12:08 AM	Rate (0)
Marko Bursic Posts: 1410 Rating: (154)	I dont think that f-do is capable to detect a diode or that diode has anything to do with your problem. perhaps you have connected in wrong way, the coil - reversed and diode is conducting while the power is applied to the coil?
Marko Bursic Posts: 1410 Rating: (154)	Last edited by: Marko Bursic at: 4/6/2013 12:18 AM
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4/12/2013 2:05 AM	Rate (0)
zombit Posts: 16 Rating: (0)	Hi, The safety contactors will pull in normally without the diode modules. If I place the diode modules on the contractors, the fault comes up staright away, there is no way the diode module can be placed in the wrong way as it is he actual manufacturer diode.
zombit Posts: 16 Rating: (0)	Last edited by: zombit at: 4/12/2013 2:06 AM
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4/12/2013 8:56 AM	Rate (0)
Marko Bursic Posts: 1410 Rating: (154)	I agree, but if you swaped A1 and A2 contact on coil (wrong polarity) then it is possible.
Marko Bursic Posts: 1410 Rating: (154)
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4/17/2013 7:54 AM	Rate (0)
zombit Posts: 16 Rating: (0)	I have been doing more tests, basically the F-DO is fine without diode and faults with diode suppression, we also tried inverting the diode and inverting the voltage with no success. I have a screen shot of the actual fault in card diagnostics. Attachment diode error.JPG (258 Downloads)
zombit Posts: 16 Rating: (0)
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4/18/2013 11:33 AM	Rate (0)
Marko Bursic Posts: 1410 Rating: (154)	I don't have any more explanation, the system indicates problem with load supply. with my intuitive guessing I would make one logical conclusion: - flywheel diode shall be polarized in reverse mode, so no current can flow trough, that means there is impossible to detect such diode from F-DO module. - there isn't a short circuit, module would report short circuit instead has reported low voltage - if you twist, you get the same error, there should be a short circuit error if diode goes in conductive mode (turned) - there is no diode at all, suppressor circuit isn't a flywheel diode, most likely is a varistor. It is rated for lower voltage like 12V, when voltage rises to 24V varistor is conducting current, lowering the voltage but current is not such high to trip short circuit error. Find some diode and wire it accros the coil, to see if it still this error. Put numbers of contactor and suppressor, to see what kind is it.
Marko Bursic Posts: 1410 Rating: (154)
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4/30/2013 7:44 AM	Rate (0)
zombit Posts: 16 Rating: (0)	Thank you very muchfor your assistance Marko. I have found a solution to the problem, it was a power supply issue, the inrush current of the contactors was slightly below the allowable on the power supply. I noticed this on powering up another exactly the same system, I was getting the same fault. Upgrading the power supply and placing diodes on the contactors didnt return any errors.
zombit Posts: 16 Rating: (0)
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