How to find the slope of the tangent on the graph at X=0 and Y=0 for the given current-voltage graph

Hi all,

I want to find the slope of the tangent of the I-V graph (this is ploted from the raw data) at V=O and I=0. Can anyone help me to do this?. I don't have a good command on Matlab.

Thanks in advance.

Answer 1

[Star Strider]

I am not certain what you want.

See if the gradient funciton does what you want. It requires evenly-spaced data with known differences between the points, however you can get around that with:

dydx = gradient(y) ./ gradient(x);

That slope will probably be what you want.

If it is not, experiment!

[NITHIN XAVIER]

Thank you, Star Strider, for your response,

This graph shows the current-voltage curve of a solar cell. I want to estimate the Series and Shunt resistance associated with the device (which is the slope of the current-voltage curve at Y=0 and X=0, respectively ). Sometimes there may not be a value at Y=0, So curve fitting is also needed. To get a clear picture, I have attached the current-voltage raw data here. Please help me to do this.

Thanks for your attention.

[Star Strider]

See if this does what you want:

Tsc = readtable('solar cell data.txt','ReadVariableNames',0);
Tsc.Properties.VariableNames = {'Voltage_V','Current_mA'};
slope = gradient(Tsc.Voltage_V)./gradient(Tsc.Current_mA*1E-3);
ShuntR = interp1(Tsc.Current_mA, slope, 0);                         % Resistance
SeriesR = interp1(Tsc.Voltage_V, slope, 0);                         % Resistance
YatX0 = interp1(Tsc.Voltage_V, Tsc.Current_mA, 0);                  % Y-Coordinate For Shunt
XatY0 = interp1(Tsc.Current_mA, Tsc.Voltage_V, 0);                  % X-Coordinate For Series

figure
plot(Tsc.Voltage_V, Tsc.Current_mA)
grid
xlabel('Voltage (V)')
ylabel('Current (mA)')
text(0, YatX0, sprintf('Shunt Resistance = %6.3f \\Omega \n\\downarrow', ShuntR), 'HorizontalAlignment','center', 'VerticalAlignment','bottom')
text(XatY0, 0, sprintf('Series Resistance = %6.1f \\Omega \n\\downarrow', SeriesR), 'HorizontalAlignment','right', 'VerticalAlignment','bottom')

Check it to be sure I got the interpolated resistance values correct and didn’t reverse them.

Windows crashed (again) while I was doing this, destroyed my MATLAB preferences file, and took my MATLAB preferences backup with it. I’m so furious with Micro$oft right now I can’t see straight. Going Linux with my next desktop. I’m sick of this!

EDIT — (1 Nov 2020 at 18:36)

Corrected typographcal errors.

[NITHIN XAVIER]

Thank you so much.

[Star Strider]

As always, my pleasure!