Measuring current using USB 6009 with a Rheostat as shunt resistor

[kiatkiat]

Hi all, I'm a Electronic Engineering Student doing my final year project. I'm able to measure the current and voltage of a 9V battery, but unsure the method i use if it is correct.

For the voltage, I've no problem getting it. But for the current, I've connected a 1k resistor parallel with the battery. When I run the program, I've got a 9mA for the battery. I understand when using the DAQ assistant, for the setup for current measurement for USB 6009, I need to specify the shunt resistor as 'external' and the shunt resistor value (which is 1k for the above experiment).

Therefore, my question is when I'm using a rheostat as the shunt resistor, what value should i specify, as a Rheostat is a variable resistor, so it does not have a fix value. 

Or the rheostat cannot be the shunt resistor?

Really need help and thank you for the reply.

[GerdW]

Hi Kiat,

how do you want to measure current with a variable resistor? How will you determine R to be able to apply Ohm's law?

But for the current, I've connected a 1k resistor parallel with the battery

How do you want to measure any current, when you put a resistor in parallel to your voltage source?

Maybe you should not only look up Ohm's law in wikipedia, but also Kirchhoff's laws!

[sebster]

Maybe this will help....

http://www.ni.com/white-paper/7077/en/

[crossrulz]

This is really EE101 stuff.

Voltages are measured in parallel.  Currents are measured in series.  In order to measure current, you have to have a series shunt resistor.  We measure the voltage drop across that resistor and then do the calculation to get the current (I = V/R).  So by  having a variable resistor, you eliminated the possibilty of measuring the current (two unknowns for 1 equation will not let you solve).

[kiatkiat]

Hi GerD,

Thanks for the reply. I also find it strange if i put the resistor in parallel to measure the current. But I follows the DAQ Assistant 'curent connection diagram' which shows the shunt resistor is connected in parallel with the voltage source. So I'm abit confuse this part. Anyone can clarify this?

Thank you very much.

[GerdW]

Hi Kiat,

when I use the DAQ assistent I don't see any resistor in parallel with a source:

Source and resistor are in series as I have suggested…

[sebster]

The shunt resistor is normally a very low value which is put in series with the load. You then measure the voltage dropped across the shunt and use Ohm's law to calcukate the current.

So if you use a 1 ohm resistor as your shunt, (DON'T PUT THIS DIRECTLY ACROSS YOUR BATTERY!!!!) 1V drop across it represents a current of 1A., 200mV represents 200mA etc.

Maybe you should be using your rheostat as the load, which would make sense since this can be varied to give different current readings.

[apok]

High-Side Current Measurement: I dont recommend this if your voltage supply is beyond the Daq Ai specification once you disconnect the load

 Low-Side Current Measurement: once load is disconnected, the Daq is connected to GND(low side)....

caveat: your shunt is a load and can effect the "true" measurement of the DUT

[kiatkiat]

Hi GerD,

It is in Series. I see wrongly at first.

Thank you for ur help.

[kiatkiat]

As for why I need to measure the current using a rheostat is because I want to measure the voltage and current of a Solar cell using the 'variable load method'. Therefore a rheostat is necessary.

So back to the first question, can I use the rheostat to measure the current. If can, how do I specify the resistor value in the current set up measurement.