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Automated IV measurement, changing sweep direction when threshold current is reached

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Hello, 

 

I am writing a labview program to automate the measurement of the IV characteristic of a diode-like device. The device starts passing current only above a critical voltage which is not known a priori, above the critical voltage the current depends ~exponentially on the voltage, and too high of a current can damage the device. With these facts in mind, I need a program that sweeps up the voltage applied to the device while measuring the current, and when the current reaches a specified threshold value, the voltage sweep should change direction and decrease until it reaches a user-set stopping voltage.

 

Hardware:

  • NI-DAQ USB6341 for analog input and output
  • Stanford Research Systems HV325 HV Power supply to supply the high voltage
  • Stanford Research Systems SR570 Current preamplifier to measure the currents

The program allows you to define the starting voltage, voltage step size, a threshold current, a maximum voltage (program stops if threshold current is not reached before maximum voltage is reached), a stopping voltage, and the time to spend on each step.

 

The program works decently well, but I'm having a problem where the voltage always increases by one extra step after the threshold is crossed and I'm struggling to understand why. I also noticed that an error that occurs if I set the step time to greater than 10 seconds, which I haven't figured out the cause of.

 

I'm still a labview novice so I'm sure there are some things in the program that are sub-optimal, so I'm open to any suggestions for improvement.

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Solution
Accepted by topic author ndrach

It is because you write data to DAQ despite of whether you want to stop or not.

santo_13_0-1679074883205.png

You should write to DAQ only if you want to continue. Enclose the write to DAQ with a case structure based on the decision.

 

Your code could be made a lot better, that is aside from getting the functionality right.

 

Santhosh
Soliton Technologies

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