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Need Advice


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#1 keithtan

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Posted 07 May 2010 - 08:47 AM

Hi there, I need to do a project with Labjack U12 to demonstrate a simple experiment on ohmís law. With the GUI using Labview, interface using Labjack U12 and the circuit. The circuit is to be made up of a potential meter so that the resistance can be varies via the GUI and the current and voltage is to be read back by the Labjack U12. Please advice if this setup is possible for this experiment. And if possible is there'a a potential meter that can be recommend. Many thanks in advance.

#2 LabJack Support

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Posted 07 May 2010 - 11:06 AM

It's possible to set something up that would demonstrate Ohm's Law, although to measure current I would recommend using a fuse-protected ammeter. In the examples folder of the Labjack U12 installation files is LJscope. This Labview program operates like a simple oscilloscope, and could be set to show one channel as the input voltage, and a second channel as the modified voltage. Using a small potentiometer (200-1000 ohm) I was able to get nice readout of the voltage difference when the potentiometer was adjusted (Constant Current). My source was 5V DC 1.2A To show the relationship with current you can simply remove the ground on the potentiometer, and use it as a variable resistor; putting the ammeter between the output and ground. (Constant Voltage)

#3 LabJack Support

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Posted 07 May 2010 - 11:22 AM

The circuit is to be made up of a potential meter so that the resistance can be varies via the GUI and the current and voltage is to be read back by the Labjack U12. Please advice if this setup is possible for this experiment. And if possible is there'a a potential meter that can be recommend.


LabVIEW GUI: It is easy to make your own, but you might start with the example "simple eanalogin loop with chart.vi":

https://forums.labja...hp?showtopic=84

If you want to use a ready-to-run program, I would use LJlogger, LJsimplelog, or LJstream, rather than LJscope. LJscope would be when you want to maximize speed and act like an oscilloscope.


Excitation: You need an excitation voltage to put across your pot. I suggest just using +5V, although you could use an AO channel.


Current Measurement: Typically you measure current by measuring the voltage across a current shunt, which is just a know resistance. That is what a DMM does. In this case, you can just use the DMM beforehand to measure the total resistance across the pot, then by measuring your excitation voltage during the experiment you can calculate current through the pot.


Voltage Measurement: Measure the excitation voltage and pot output voltage using analog input channels. Keep your pot resistance lower to minimize errors due to the analog input bias currents on the U12 (our newer U3/U6/UE9 would virtually eliminate this issue). I suggest 1k at most, although you could certainly do down to 100 ohms if using +5V as your excitation.

http://labjack.com/s...users-guide/2.1


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