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DAQ module isolation safety reality check

J

John

Jan 1, 1970
0
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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P

Phil Allison

Jan 1, 1970
0
"John"
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


** 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 ??


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?


** 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.


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.


** 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
 
J

John

Jan 1, 1970
0
** 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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P

Phil Allison

Jan 1, 1970
0
"John"
Thanks Phil.
With the output of the SMPS floating, I measured 51.4VAC from the AC
GND to either terminal of the SMPS' output.


** As I predicted.

LOL, this is normal?


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


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


** 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 !!



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.


** It floats alright

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




........ Phil
 
J

John

Jan 1, 1970
0
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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