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Troubleshooting PLC transistor output

Discussion in 'General Electronics Chat' started by Kal_B, Aug 20, 2017.

  1. Kal_B

    Kal_B Member

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    Hello guys


    The title of the thread is somewhat inaccurate as I am troubleshooting the devices connected to the a PLC with transistor outputs and would like to know what would happen if I connect a +24VDC to the output terminals while the PLC is powered up and wired to devices.

    The PLC is powered with 24VDC and the loads are connected just like in the attache pdf, so let's say I wanted to test the device connected to Q4 by connecting a 24DC to Q4 directly to see if the device will turn on. Would that damage the transistors in the PLC?


    Thanks
    Kal
     

    Attached Files:

  2. MaxHeadRoom78

    MaxHeadRoom78 Active Member

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    You cannot connect power directly to an output, it usually consists of a output transistor , IOW, only a suitable load should be connected directly to an output.
    They appear to be sourcing outputs.
    Max.
     
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  3. Kal_B

    Kal_B Member

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    Thanks Max,

    I already did :eek: and smoke came out and the outputs are pretty much fried. I was asking after the fact to understand what technically happened.
    I would like to simulate it again but with components rather that a PLC, is it pretty much like an optocoupler?

    Thanks
    Kal
     
  4. dave

    Dave New Member

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  5. MaxHeadRoom78

    MaxHeadRoom78 Active Member

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    The output transistor/device is opto isolated driven usually, and what you did probably was to apply full power to a device already turned on and shorted the device.
    Max.
     
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  6. Kal_B

    Kal_B Member

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    Actually everything connected to the PLC is working fine. I removed the connector off the PLC and rewired the panel and everything works quite well without those outputs.
    The outputs on the PLC are divided int two groups and the PLC is still working fine with the second group of outputs.

    I don't exactly remember what I did but I think it was as I described above, I connected 24VDC (The same source as on terminal 1 in the diagram) to one or more of the output group in the diagram and smoke immediately came out. I removed the wires that were connected to the outputs and left the 24VDC on terminals 1 and 2 and plugged the connector back in and smoke came out again so I removed the connector and rewired the few devices that were connected to the four outputs.

    Thanks
    Kal
     
  7. MaxHeadRoom78

    MaxHeadRoom78 Active Member

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    V1 connected to an output directly, shorts the output device.
    Not sure I understand? If you moved any outputs over to other terminal out, then the PLC program would need to be changed.
    Max.
     
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  8. Kal_B

    Kal_B Member

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    Thanks Max. I had to think about this a little and I don't get why connecting V1 to let's say Q4 in the above attached diagram would short the device connected to it (Marked L in the diagram). I thought it would short out the PLC output itself not the device as the function of the output is to source 24VDC from V1 to the outputs when they are turned on.

    Thanks Max for your help and patience.

    Kal
     
  9. MaxHeadRoom78

    MaxHeadRoom78 Active Member

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    L= load, relay coil etc, Imagine a transistor used as a switch for a relay coil connected to its collector, now instead of a load, you place the supply directly to the collector, now when the transistor turns on, it is a short circuit which over loads and destroys the output device. in this case the output transistor.
    Max.
     
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  10. N11778

    N11778 Member

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    I don't see how jumping the Plus to the output Q4 could have done any damage to the PLC>
    You must have done something else like putting a ground on Q4
    PLC output.JPG
     
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  11. Kal_B

    Kal_B Member

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    Thanks N11778. Whether it's possible or not is beyond me as I'm still trying to wrap my head around how it would work. Your suggestion that I may have done something else is quite possible though I would have had to be very sloppy to do that.


    Thanks for the schematic by the way, it's quite helpful.
    Kal
     
  12. N11778

    N11778 Member

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    I just thought of something else. Your PLC mite have a 3 state output On Open Off
    If so If you put the +24v on Q4 when it was in the off state It would Smoke.
    The output in the Keyence is Just on Open as in the diagram I attached.

    Looked up yours and its just On or Open

    this is from your manual
    Discrete output logic Positive logic (source)
     
    Last edited: Aug 20, 2017
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  13. MaxHeadRoom78

    MaxHeadRoom78 Active Member

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    The way I interpreted it the Kal_B placed V1- on the output which is a sourced output apparently, and this would have shorted the output transistor, as I already posted..
    Max.
     
    Last edited: Aug 21, 2017
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