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Runjhun

Current Measurement with NI-9227

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Hi,

 

I want to use NI-9227 on NI-cDAQ 9188 chassis for current measurement.

 

When the load is engaged an instantaneous current of 25 Amps for microseconds is observed using Oscilloscope.

 

The specifications of this card says,

Input Impedance = 12mOhms (across the channels Ai+ and Ai-)

and Instantaneous Measuring Range is 14 Amps.

 

My concern is whether this card will still support this in-rush current or the channel will go bad or the card will not work ?

 

Any suggestion is welcomed.

 

Regards,

Runjhun.

 

 

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Most voltage DAQ cards state they have a sampling range.  Let say they are -10V to 10V.  Then they have a rated maximum input voltage say 20V, then sometimes you see a overvoltage protection at say 30V.

 

This current card appears to be different, where it has a normal sampling range of 5ARMS, but a maximum sampling current of 14A if it is a single sample reading.  All other documentation I found doesn't mention current over 14.9A.

 

http://www.ni.com/datasheet/pdf/en/ds-194

 

Will this mean your card will blow up if 25A is put into it for a microsecond?  I can't say.  Best case it is fine, worst case it blows up.  My guess is it is somewhere in between and it will work fine for a while, but will shorten the life of the card and some day that input will stop working.  A better design would be to use some kind of current measuring device, which changes the range of the reading, which you then can scale.  Something that maybe has a 0-30A range, and converts that to 0-10V.  I know Omega, and Analog Devices make hardware like this, but many other companies do too.

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Do you have any extra channels? Just throw two in parallel and sum the currents*

 

*Warning, following electronics advice from random internet patrons may blow up your equipment

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You may have some luck with getting your local sales guy to put you in touch with R&D to give you an "unofficial" max limit on the hardware that will at least give you an idea of how terrible that current will be for that short duration and possibly an idea of what components could fail.  Worst case they'll tell you that you absolutely cannot exceed the specs listed.  I agree with Hooovahh that these specs seem a bit different than the typical spec sheet.

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The reason the spec sheet reads strangely is partly due to this being primarily aimed at AC applications.

 

The overcurrent protection is for 10Arms which gives you the peak value for 14.051A peak. This is rated for 1 second but remember that is at AC so the rating for the 14A instantenous is probably much shorter.

 

Will it survive? possibly, but I think it is certainly out of specification for the card and will reduce the life.

 

These cards basically work by having a small shunt resistor feeding a very low range voltage measurement. That 12mOhm resistor is the part that has to take the brunt of the current and will likely fail. If you need the fast response then you can use an external shunt resistor which you know can take the load and the NI 9238 which has a 500mV input range.

 

The easier option is an external transducer as mentioned above but be aware of dynamic characteristics and precision with these as they are often worse than a shunt based measurement although have the advantage of affecting the system less.

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If it were me, I'd go with the solution that can work today, using two channels to read and adding the currents as suggested.  I like these quick and dirty solutions some times.  But yeah the right way would be to buy the right hardware for the task.

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