isolation safety

[JMooch]

We'd like to use our USB-6009 for measurements of current/voltage (up to 12VDC max) in an electronic load, to provide digital-I/O control of the load and analog voltages to the load to vary the load's "resistance".

The USB-6009  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 regulated, switching "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 (eventually)?  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?  And the desktop PC should have isolation via its switching power supply?

We can opto-isolate the digital I/O to/from the DAQ module but the analog I/O would have to be straight from the DAQ module.  Unless there are ways to isolate a 0.0V-.20V analog output from the USB-6009?

We just wanted to be careful here since we weren't quite sure of what safety issues might be involved.

Thanks!

[JMooch]

Arrgghhhh....sorry.  This should have been posted to the Multifunction DAQ forum.

Message Edited by JMooch on 07-23-2007 03:22 PM

[David L.]

Hi JMooch,

I would not suggest using the USB-6009 for this project, as you are going to be measuring a (possible) 12V signal. The 6009 can only measure +/- 10V. Also, the 6009 is not an isolated card, therefore we cannot isolate AI any more than the computer chassis / ground.

[JMooch]

Thanks for the reply David,

I forgot to mention that we were going to divide the (max.) +/-12VDC signal to the diff. input of the 6009:
                  +---------------+
                  |               |
                  |  6009 Diff In |
                  +---------------+
                      |       |
                      |       |
                  R   |  R/2  |   R
                 ___  |  ___  |  ___
              +-|___|-+-|___|-+-|___|-+
              |                       |
              |                       |
              |                       |
              +--- up to +/-12VDC ----+

But, that still leave any isolation concerns.  We can easily optoisolate the DIO lines, it's the two 0.0VDC-0.2VDC analog inputs and the single 0.0V-0.2V analog output that can't easily be isolated.  If I understand correctly, we're "at the mercy" of whatever isolation the computer has that is attached (via USB cable) to the 6009?  That is, the 6009's ground will always be at the same potential as the computer's chassis?

It appears that the USB-6215 is just about our only step-up to an isolated DAQ?

Thanks for your help!
John

[David L.]

John,

Since the 6009 is not isolated, it will have the same ground as the computer. The 6215 being a bus isolated card, the AI Gnd is not the same as the computer ground. This means that all lines, digital and analog, are isolated from the bus and can handle higher voltages as detailed in the 621X Specifications. The picture below was taken from these specifications, and shows the isolation barrier. Here is also a great article about isolation .

Message Edited by David L. on 07-25-2007 04:54 PM

[AnalogKid2DigitalMan]

Dataforth and Analog Devices have numerous analog isolation modules. Some may be bandwidth limited.

http://www.dataforth.com/

[JMooch]

David,

Though the cost is a lot higher, the isolation that the 6215 provides is something that I feel is necessary in our application.  It's just not worth messing around with this stuff.  🙂

Thanks again for your help!

John

[Edit] Thanks for that article link too!  I missed that one when searching for other safety/certification/isolation articles.

Message Edited by JMooch on 07-26-2007 04:03 AM

[JMooch]

AnalogKid, thanks for that link.

Using one of their analog isolation modules (along with optocoupling for the DIO) might be a good way to go.  We'll probably grab a couple of USB-6215's right away to get started but that will give us some time to check out a USB-6009+optocoupler+analog isolation module solution.

John