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DAQ module isolation safety reality check

Discussion in 'Electronic Basics' started by John, Jul 23, 2007.

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  1. John

    John Guest

    We'd like to use a National Instruments USB-6009 DAQ module for
    measurements of current/voltage in an electronic load and to provide
    digital-I/O control of the load and analog voltages to the load to
    vary the load's "resistance".
    http://www.ni.com/pdf/products/us/20043762301101dlr.pdf

    The DAQ module is operated from a Dell desktop system (120VAC, North
    America) via a USB cable directly to the system (no hub).

    The electronic load is powered via a Meanwell ES25U "wall-wart" 12V
    power supply (http://www.meanwellusa.com/search/es25u/default.htm).

    Are there any safety issues here since both devices are attached to
    the 120VAC mains? The wall-wart mentions an Isolation Resistance
    spec, "I/P-O/P:100M Ohms / 500VDC/ 25/ 70% RH", which sounds like it's
    fully isolated up to 500V?

    We can opto-isolate the digital I/O to/from the DAQ module but the
    analog I/O would be straight from the DAQ module (which does not
    mention any isolation at all). We just wanted to be careful here
    since we weren't quite sure of what safety issues might be involved.

    Thanks!

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  2. Phil Allison

    Phil Allison Guest

    "John"

    ** Amazing how the one spec you need to see is always missing !!!

    In this case, the input overvoltage protection is claimed to be +/- 35
    volts ??

    Does that mean the unit with be damaged by more ??

    What about brief spikes of +/- 300 volts ??



    ** That thing is a SMPS - so will almost certainly have Y-caps from the AC
    side to the output common.

    Again, the makers have omitted to spec the AC leakage current and AC voltage
    on the output when floating - cos no-one makes them do it. But you can
    check it with any decent DMM, if you like.

    Expect to see 60 volts AC and about 250 uA of AC current when the DMM is
    linked to ground on one lead.



    ** The only foreseeable "safety" issue ( aside from a catastrophic failure
    of the SMPS wall wart) is possible damage to the input circuit of your DAQ
    device.

    Play safe, ground the common 12 volt line from the SMPS to AC supply earth
    with a dedicated wire link - direct to ie the desk PC's case.

    This way no nasty spike voltages from a " floating " the SMPS can * zap *
    the input of your DAQ and bugger it .




    ........ Phil
     
  3. John

    John Guest

    ** That thing is a SMPS - so will almost certainly have Y-caps from the AC
    Thanks Phil.
    With the output of the SMPS floating, I measured 51.4VAC from the AC
    GND to either terminal of the SMPS' output.

    LOL, this is normal? What's the use of having an "isolated" supply?
    Just to keep things from frying ONLY if all is connected properly and
    working OK?



    OK, I think I understand....
    Adding a low impedance ground directly to the host PC's case (i.e., AC
    GND) will help keep current from flowing "back" thru the DAQ module,
    up through the USB cable and onward to AC ground? If the SMPS ever
    floats, that is.

    Thanks again for your help Phil.

    John
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  4. Phil Allison

    Phil Allison Guest

    "John"

    ** As I predicted.


    ** Don't as me - I just live here on planet earth .......



    ** See this article:

    http://sound.westhost.com/articles/external-psu.htm#kil

    Particularly under these sub headings:

    http://sound.westhost.com/articles/external-psu.htm#kil


    http://sound.westhost.com/articles/external-psu.htm#ded

    Figure 12 was taken from my scope screen.

    SMPS wall warts are my PET HATE !!




    ** It floats alright

    - just like a Zeppelin full of damn hydrogen gas !!




    ........ Phil
     
  5. Phil Allison

    Phil Allison Guest

  6. John

    John Guest

    What's the use of having an "isolated" supply?
    Thanks! I'll check those links out.




    Ahhh, I misunderstood. I thought "floating" meant that the output of
    the SMPS was to be left unconnected. I just tested the SMPS as it was
    powering the load module (fans and logic) and I still got that high
    voltage to AC GND. Now I know what you meant by floating!

    John
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