NI DAQ 6008 for solar cell characterization

[gparticle]

Hi, I've been doing a solar cell characterization setup by using USB DAQ 6008.

Basically, I would use the DAQ 6008 to input and sweep voltage through the solar cell from V=0V to V=5V (forward bias) and measure the output current of the solar cell.

I am expecting a current near micro amps. I've tried using a resistor and input the voltage across it back to the DAQ 6008. So I would have a data where the x-axis is the bias voltage and y-axis is the voltage output of the solar cell which i then convert to current using I=V/R. The problem is I am getting only a linear graph and not forming a knee as what solar cells should behave. Is there something wrong with my setup? I am using a variable resistor and tried about 500 Kohms to 900Kohms. Is it also possible to just use a transimpedance amplifier to have voltage reading per bias voltage? 

Thanks!

[GerdW]

Hi gparticle,

I am using a variable resistor and tried about 500 Kohms to 900Kohms.

The AO of your NI6008 is able to drive up to 5mA with a range of 0…5V.

Applying 5V at a resistor of 500kOhm will drive just 10µA.

What is the (expected) resistance of your solar cell?

Which voltage drop do you expect at your resistor and solar cell? (I guess you are using a voltage divider here.)

Did you compare that expected voltage drop with the accuracy and resolution of your USB6008 AI channel specification?

[gparticle]

Hi GerdW, 

Thanks for the quick reply. 

I will attach the schematic diagram of my setup. 

Basically, I use the DAQ 6008's AO0 to input 0 to 5V to the positive terminal of the solar cell. The negative terminal of the solar cell should give me a current which I need to graph with the input voltage. I used a variable resistor to have a voltage reading from the current and input it to the DAQ 6008's AI0. 

Is my circuit correct? or will it work if I use a transimpedance amplifier (current to voltage converter) instead of a variable resistor? Thanks

[GerdW]

Hi gparticle,

I used a variable resistor to have a voltage reading from the current and input it to the DAQ 6008's AI0. 

Please read the specs of the USB6008 and note the rather low input impedance of the AI pins.

Keep this in mind when calculating the expected voltage signal!

(The USB6008 is not good to measure low current/voltage signals because of their resolution, accuracy and input impedance!)

[gparticle]

Thanks GerdW,

Would changing the variable resistor with a current to voltage converter ( transimpedence amplifier) work for the purpose of my circuit? Or a current sensitive preamplifier maybe?

[GerdW · Accepted solution]

Hi gparticle,

"any" kind of amplifier would make sense when you want to monitor small currents…

[gparticle]

Thank you so much GerdW.

Will work on the circuit for now. Thanks

[JÞB]

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@gparticle wrote:

Hi, I've been doing a solar cell characterization setup by using USB DAQ 6008.

Basically, I would use the DAQ 6008 to input and sweep voltage through the solar cell from V=0V to V=5V (forward bias) and measure the output current of the solar cell.

I am expecting a current near micro amps. I've tried using a resistor and input the voltage across it back to the DAQ 6008. So I would have a data where the x-axis is the bias voltage and y-axis is the voltage output of the solar cell which i then convert to current using I=V/R. The problem is I am getting only a linear graph and not forming a knee as what solar cells should behave. Is there something wrong with my setup? I am using a variable resistor and tried about 500 Kohms to 900Kohms. Is it also possible to just use a transimpedance amplifier to have voltage reading per bias voltage? 

Thanks!

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With all respect to GerdW I'm going to jump in.

WTFAYT?  

a solar cell outputs a current proportional to the luminosity incident on its surface! Driving it with a "Bias" and expecting its output to be proportional to that applied voltage is just silly or insane.

[Blokk]

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@JÞB wrote:

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@gparticle wrote:

Hi, I've been doing a solar cell characterization setup by using USB DAQ 6008.

Basically, I would use the DAQ 6008 to input and sweep voltage through the solar cell from V=0V to V=5V (forward bias) and measure the output current of the solar cell.

I am expecting a current near micro amps. I've tried using a resistor and input the voltage across it back to the DAQ 6008. So I would have a data where the x-axis is the bias voltage and y-axis is the voltage output of the solar cell which i then convert to current using I=V/R. The problem is I am getting only a linear graph and not forming a knee as what solar cells should behave. Is there something wrong with my setup? I am using a variable resistor and tried about 500 Kohms to 900Kohms. Is it also possible to just use a transimpedance amplifier to have voltage reading per bias voltage? 

Thanks!

---

With all respect to GerdW I'm going to jump in.

WTFAYT?  

a solar cell outputs a current proportional to the luminosity incident on its surface! Driving it with a "Bias" and expecting its output to be proportional to that applied voltage is just silly or insane.

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No, it might make sense. After all, a solar cell is just a semiconductor. You can still measure IV characteristics over it, which might give you important info about the performance/quality...

http://plasticphotovoltaics.org/lc/characterization/lc-measure.html

edit: NI white paper: http://www.ni.com/white-paper/7230/en/

[JÞB]

Ahhhh... The "Output Current" is negative. D'Oh