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DAQ grounding

Hi,

 

I am not sure if I should post this question here or not. It's about grounding of my DAQ.

 

I am doing some test on electric motor (with high voltage ground ~200V) and I use some voltage transducer to get 10-10V signal to feed to DAQ (NI USB 6009). My question is if anybody knows what happens to the DAQ ground? Does it connect to computer ground and is it optically connected; so isolated or it's just wired? Because if it is just wired to PC ground which is my chassis ground (high voltage ground) then my high voltage and low voltage grounds are connected. 

 

Will the problem be solved if I use differential inputs in my DAQ?

 

Also, is this the same for other DAQs? Like 6210 and 6211?

 

 

Thanks.

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Hi,

 

This would fit better at the hardware forums.

It depends on your 'transducer' or the circuit which gives you the +/- 10V signal. If this one is isolated then you can use differential measurement.

However, for such an application I would always use an channel to channel isolated DAQ-Device, eg. NI 9225 (600V ch-ch isolation).

I would suggest to call your local NI office and talk to an engineer with all the details of your application.

 

 

Thanks,

Christian

 

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Thanks Christian,

 

But the ground of the computer is connected to the ground of high voltage machine. So if there is not any isolation between DAQ ground and computer ground, then there should not be any difference what DAQ I use.

 

Also, why should I use channel to channel isolation DAQ? The pins of the DAQ are all from the same category and their ground can be the same (please correct me if you think I am wrong).

 

As for transducer, I may use either an OpAmp circuit, or just a simple 2 resistor voltage regulator. For the first case, I will need an extra voltage supply and it may get noisy near zero (I have been told), but for the second one, it is simpler and since my DAQ input impedance is higher than my resistors, I should be fine except the grounding. So that's why I want to figure out how the DAQ grounding works, so I know which method I should use.

 

I am new to this field, please correct me if I am wrong.

 

Thanks,

Baran.

 

 

 

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@Baran wrote:

Thanks Christian,

 

But the ground of the computer is connected to the ground of high voltage machine. So if there is not any isolation between DAQ ground and computer ground, then there should not be any difference what DAQ I use.

 


 

 


Well, first off you really should re-think this for personal safety reasons.  It may work But, I would not want to be in the room.


"Should be" isn't "Is" -Jay
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🙂 High voltage as maximum 200 peak. But probably even less than that. 

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Hi Baran,

The USB 6009 ground is referenced to your PC chassis ground and therefore, you can only input a +/- 10 V signal with respect to your chassis ground.  If your system has a ground reference that is 200 V above earth ground, you will not be able to feed your signal into the 6009 directly.  Let me resummarize the few options below:

 

1. If using the 9225, this has 300 V RMS channel-to-earth ground isolation and would properly handle your +/- 10V signal that is 200 V above earth ground.

2. Use a resistor divider network.

3. Isolate your +/- 10 V from the 200 V reference.

 

Note that placing your chassis ground above earth ground is not recommended.

 

Thanks Baran!

 

-Brandon G

National Instruments
Precision DC Hardware Engineer
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@Baran wrote:

🙂 High voltage as maximum 200 peak. But probably even less than that. 


200 Volts is plenty if you want to get dead!   yes, dead as in ceasing to remain alive- (I type poorly and spell badly, I WANT to be clear here. )  Sure it may work, I would appriciate the sign at the door saying " enter at your own risk! - you may contact LETHAL voltages."


"Should be" isn't "Is" -Jay
Message 7 of 14
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200mA through your heart will kill you. 200 volt in you fingertips does not do any arm. it only tickles a bit. 10 kilovolt ++ will maybe burn your finger off, but you will not die. Even ESD is over 1000 volt, but you don't die, only the poor electronic components.

 

If you get 200mA through your heart, the blood will start to get thicker and you will eventually die. So in other words, it's the current that kills, not the voltage (If it's not waaaaaay to high though hehe)

Regards,
Even
_________________________________
Certified LabVIEW Associate Developer

Automated Test Developer
Topro AS
Norway
Message 8 of 14
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Thanks everybody. My background is control theory and I have worked on paper my whole life, and this is the first time I am dealing with power systems. So I am not that familiar with the technical issues as well as safety. So thanks for the comments and concerns Smiley Happy

 

 

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Thank you Brandon for your informative response. I have some questions and I appreciate if you could answer. 

 

If I use 6009 or probably upgrade to 6210, then my DAQ voltage should be +/-10V. So I need the voltage transducer anyway.   (just 2 resistors, connected to each other; one port to 200V, one to ground and the common port to DAQ). But the question is should I connect the resistor's ground to DAQ ground? In that case my motor ground is floating. Right?

 

What if I use differential inputs. Then again, which ground should I connect my resistor's ground?

 

What if I use laptop with battery. Maybe it looks silly, but I was thinking if I am using battery then my computer ground is not earth. Should I be fine then?

 

How can isolate my +/- 10 V from the 200 V reference?

 

Many Thanks for your time and help,

Baran.

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