# Low-Cost Three-Quadrant Single Solar Cell _I-V_ Tracer
## 3 Results
### I-V Curves Obtained  
By means of the integrated serial RS232 port, the microcontroller can receive commands to configure the sampling range desired for the measurements. The first traces presented were configured to sample the first quadrant and a small section of the second one close to the zero-voltage point, obtaining the usual _I-V_ curve in the generating region of the cell at a maximum resolution. Figure 18 shows the graphical representation of the dataset acquired for a measure of a single PV cell, using our device for _I-V_ tracing and the platform with the LED board for illuminating. The first quadrant (positive voltages) and some of the second quadrant (negative voltages) can be seen. The curve is the typical curve of a good cell, with the first zone at low voltages, almost horizontal; that is, without changes in the current (almost _ISC_) and then a \"_neck_\" zone ending in a straight zone up to the open circuit (\(V_{\text{OC}}\)). From these values (at the beginning and the end), it could be possible to extract the values of \(R_{p}\) and \(R_{s}\), respectively. The second quadrant of the curve is not traced because the firmware of the main IC is configured, in this case, to obtain the curve with the maximum number of points at the first quadrant zone. As mentioned before, it is possible to also obtain an _I-V_ curve with a small number of PV cells in series by simply changing the feedback loop of the voltage measure Op-amps. For this purpose, we fabricated a mini-PV module with six cells that were a sixth of a cell of the size used. Therefore, the mini-module could be mounted in a size just slightly larger than the single cell, so it fits in the modular platform and could also be illuminated with the same LED system. The corresponding _I-V_ curve can be seen in Figure 19a. As expected, the noise in the current measure is more notorious in this case, because the values of the current are lower (the current depends mostly on the cell surface for the same cell technology). For this reason, a software Savitzky-Golay filter was used over the raw data. The filtered data are shown in Figure 19b. For the full three quadrants, tracing the appropriate sampling range selection command was sent to the microcontroller. In this case, the two voltage sweeps described before were performed and the sampling covered the three-quadrant curve. Figure 20 shows the full _I-V_ curve of the cell measured; we should note how the microcontroller had adjusted the sampling rate to perform a better resolution for the usually measured first quadrant, where more details were desired to extract the basic main parameters of the cell. More specifically, in the second quadrant and the first part of the first quadrant (from left to  
Figure 18: The _I-V_ curve of a real cell with the device and the platform proposed for irradiance of 750 W/m\({}^{2}\). right), the current was very high (so the variation in voltage was very fast) but the current variation was low (so the variation in the current was slow). Here, we could sample slower without loss of quality. After the change of the slope in the first quadrant, the situation was the contrary. The current was, at each step, lower, so the variation of the voltage was slower, but the variation in the current was faster, so we needed a faster sample rate. As explained in Figure 4, the curve was traversed naturally by the current flowing from one capacitor to the other, so the factor which determined the speed of the voltage variations in the capacitors was the current. Figure 21 shows a detail of this curve zoomed in to the first and fourth quadrants. In the figures, it can be appreciated that there is a _gap_ between the positive and negative current sections. As mentioned before, it was impossible to reach the 0 V point (because the current tending to zero would result in time tending to infinite), but this was not a problem, because the _gap_ data were easily interpolated from the tails of the two sections.