Reverse current

[crossrulz]

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@kdm7 wrote:

And limit is + or - 20V.

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Be very careful with that.  Each input of the differential input still needs to be within the +/-10V relative to ground.  But if -VE is -10V and +VE is 10V, you are ok (there's your 20V differential limit).

[GerdW]

Hi crossrulz,

kdm has been told several times to read the manual. He should know it by now… 😄

[crossrulz]

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GerdW wrote:

kdm has been told several times to read the manual. He should know it by now… 😄

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Yeah, but that differential inputs is one of those tiny footnotes that is often missed.  I was almost burned by it once and I would prefer keeping somebody from making the same mistake.

[kdm7]

Hi Crossrulz,

I heard the current limit is around 0.243 A (for the NI 6008 card) for Analog Input. My solar panels wires contain around 1A. Is there a resistor inside the DAQ card which limits the current? Can I simply connect these two wires to the AI 0+ and AI 0-? (By doing this will it damage the DAQ card? 

[GerdW]

Hi kdm,

I heard the current limit is around 0.243A

Where do you "heard" this? The manual does not mention anything like that. Did you read the manual as suggested several times before?

Is there a resistor inside the DAQ card which limits the current

Yes, there is. It is shown in the manual and specified in the specs table. Did you read the manual?

Can I simply connect these two wires to the AI 0+ and AI 0-?

Yes, you could - with the risk of damaging your DAQ device. Did you read it's specs?

Come on: How often do you need to read the manual again and again to understand all of it's specs?

The USB6008 cannot measure current directly, you need to use a shunt resistor!

[kdm7]

Hi Gerd,

As long as the voltage is within the limits for the device,the input impedance of the AI device limits the current (as told in a different post). So according to this statement, I think I can directly connect. Why do you think it will be risky? as it has a resistor to limit the current.

Sorry I have problem understanding datasheets. I dont understand them.

[GerdW]

Hi kdm,

I have problem understanding datasheets. I dont understand them.

It seems you have problems understanding basic electrical principles/basic physics…

Where do you heard "0.243A"? Applying Ohm's law to spec values gives 10V/144kOhm = 69µA. How will you even input 0.2A (or upto 1A) into your little USB6008? Again applying Ohm's law will give you 144kOhm*1A=144kV: you would need a voltage of 144kV to have a current of 1A with the given input impedance!

That's all basic physics combined with values given in the specs. When you cannot understand all this you really should take some physics classes, followed by classes on (electrical) measurement techniques. That way you would not burn your DAQ devices or your PC, not mentioning hazards to your life or your coworkers life…

[kdm7]

Hi Gerd,

Sorry it was supposed to be 0.243 mA. I didnt notice that I hadnt typed m.

[crossrulz]

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@kdm7 wrote:

Hi Gerd,

Sorry it was supposed to be 0.243 mA. I didnt notice that I hadnt typed m.

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Still, that sets it at 144V.  That will destroy your DAQ in a heartbeat.

Again, you cannot directly measure current with a DAQ device.  You have to use a current shunt resistor (something like a 10k Ohm) and then measure the voltage drop across the resistor.  At 10k Ohms with 0.243mA, that will give you 2.43V, well within the range of your DAQ device.

[JB]

You started this thread with :

I am using a lem lts 6-np current sensor. I am able to get calbrated positive current. But, my problem is that I am not able to get negative current values on my front panel (after reversing the terminals).

The fact that you seem to have problems to use this module doesn't make the NI USB-6008 capable to measure currents directly.