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Periodic voltage drops


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#1 helmik

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Posted 25 June 2012 - 09:08 AM

I am Using Labjack U3 LV. I want to measure drop of voltage on one resistor. Because the voltage was too big (around 10V) i parelly added two big resistors and i measured the voltage on resistor R3. The values of resistance and circuit is shown in the attached picture. Now for maximum voltage V_s=12V i should have drop of voltage on R3 around 2,4V (max). I measured it on labjack in a way shown in the picture (connected to FIO2 and FIO3), in fact i could see the value of around 2,4V, however it was periodically dropping to 0V. Period is around 10s and time when labjack shows U=0V is around 2-3s. I measured the voltage also with multimeter and there were no drops of voltage. What can be wrong? Can you help me solve this problem?

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#2 LabJack Support

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Posted 25 June 2012 - 09:23 AM

See Section 2.6.3.6 of the U3 User's Guide for information about voltage dividers:

http://labjack.com/s...s-guide/2.6.3.6

You want the biggest resistors you can use, so set your R2 to 10k. Then choose R3 so that at your max voltage the divided voltage is <2.44 volts. For example, if you set your R3 to 1.8k, then 16V in will result in about 2.44 volts out.

Also, in your drawing you don't show a ground connection. FIO2 is fine, but the other side should be connected to ground:

http://labjack.com/s...l-analog-inputs

#3 helmik

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Posted 25 June 2012 - 09:52 AM

Ok thank You for help. I am sorry for probably another stupid question, but my experience in the field is extremely small ;-) So: my source is a box that is constant current source. To get correct ground connection i should connect GND from labjack to source box or just connect second lead (from picture) to GND instead of FIO3?

#4 LabJack Support

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Posted 25 June 2012 - 09:59 AM

Everything at the bottom of your drawing is the same thing, so you can connect anywhere there, but simply moving the FIO3 connection over to GND sounds the easiest.

#5 helmik

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Posted 25 June 2012 - 10:00 AM

Thank you very much for your help. I will try it tomorrow and hope it works.


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