DIODE CHARACTERISTICS USING USB 6008

[Neal M]

Hello NUCLEAR LAB,

Yes, you can definitely measure diode IV characteristics with your HW/SW. 
I searched the web and found <a href="http://cnx.org/content/m13711/latest/" target="_blank"> this link</a> to a lab that uses LabVIEW and a
6008/6009 to do what I believe you are trying to accomplish.

What it essentially says is that you use an analog output to provide a known
voltage and you then measure the voltage drop across a known resistor to figure
out the current.&nbsp; You then plot the known voltage against the measured
current.&nbsp; The circuit would look something like this (taken from the link
above):

<img src="Loading Image...">

The programming is also shown in the lab and should be pretty self explanatory.&nbsp;
It uses the DAQ Assistant to do the AO and the AI and then plots the response.
I hope this is what you are looking for, please post back if you have any
issues. Message Edited by Neal M on 12-13-2007 10:19 AM


DiodeTest.JPG:
http://forums.ni.com/attachments/ni/250/35734/1/DiodeTest.JPG

[NUCLEAR LAB]

Hi,
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; Thanks for your mail. just now i tried the same and got the result.
now i have one more doubt: can we measure resistance directly using USB 6008. I have read some discussions already regarding this from the discussion forms but i didn't find what exactly i was searching for. so can you please help me in this.
thanks once again.

[mallorim]

Hi Geetha,

You ask a good question. The USB-6008 is a voltage controlled device, and therefore unable to measure resistance directly. However, there are methods to measure resistance indirectly, which require you to have a resistor of known value. The Developer Zone article <a href="http://zone.ni.com/devzone/cda/ph/p/id/209" target="_blank">Resistance Measurement - Indirect Method</a> gives a great explanation of how this is done. Basically, you use the resistor of known value to perform what is commonly referred to as the Voltage Divider Method.

The <a href="http://digital.ni.com/public.nsf/allkb/408C3D76B0B5690C86256E4500056DDF" target="_blank">Knowledge Base Document ID 36NHULSF:</a> <a href="http://digital.ni.com/public.nsf/allkb/408C3D76B0B5690C86256E4500056DDF" target="_blank"> </a> <a href="http://digital.ni.com/public.nsf/allkb/408C3D76B0B5690C86256E4500056DDF" target="_blank">Using a Multifunction DAQ Device to Determine the Value of an Unknown Resistor </a>explains how to connect the two resistors to an E-series DAQ card. You can refer to the User Guide of your USB-6008 to get the proper pin numbers and how to perform differential measurements.

National Instruments does make hardware that will take resistance measurements directly, mainly Digital Multimeters which can be found on our website ni.com. If you are interested in how the multimeters perform the resistance measurements, the Developer Zone article <a href="http://zone.ni.com/devzone/cda/tut/p/id/3981" target="_blank">Resistance Measurements</a> is helpful.

I hope this explanation helps,

Mallori M.

[Neal M]

Hello again GEETHA,

I believe that third DAQ Assistant is to essentially reset the voltage
output.&nbsp; Those DAQ Assistants (I believe) are set up as On Demand, so the
output will stay at the last value it is given.&nbsp; After the loop ends, the
last DAQ Assistant puts the voltage back to 0 so that there is no current in
the circuit when the program has stopped.

The 0.0098 is a combination of 2 values: The actual resistance of the resistor
you are measuring the voltage across (labeled R1 in the diagram, 9.8 Ohms in
this case) and a scaling value (1/1000 converting A to mA).&nbsp; Thus
9.8/1000=0.0098. Since you are measuring voltage across a known resistance to
calculate the current, the entire calculation (Using I=V/R) looks something
like this:

I = V / R = ( (V_in - V_out) / (9.8) ) * (1000mA / 1A) = (V_in - V_out) /
0.0098

I hope this clarifies, please post back if you have any more issues.

[adrian moochy]

guys, please help me,,, what are the step by step procedure in making the vi file, i am very new and i need to report on it this saturday,,please please reply asap guys,, i really need your help, and also how to configure the xy graph so there wont be a broken line? and can u give me a very very simple program that i could use the usb 6008? i would really appreciate it guys,,,thanks in advance...

[Dennis Knutson]

If you follow the link in Neal's first post, you will find an example VI. It is an image so you will have to do some of the work yourself. Try to reproduce what is there and if you get stuck or have a problem, attach what you have written. Also, please be specific in describing what does not work correctly.