For some time I have been intending to create some sort of solar powered project but never quite got around to it. However, a few weeks ago I decided to do something about this lack and purchased some solar cells, which were rated at a nominal 6V and 90 mA maximum, with a size of 120mm x 38 mm. These voltage and current ratings are nominal ideal values in very bright sunshine and optimum loading and I was not expecting to get 90 mA at 6 V. But I wanted to find out the actual characteristics so I setup the following circuit and went outside.
I used a low cost switched resistance box to provide the variable load. It only provided four decades of resistance and I do not think it is particularly linear or accurate, but it was cheap and is all I have, so the measurements might not be that accurate. However, as solar cells are so dependent on the light level the non-deal resistor box does not seem too much of an issue.
Outside in the Sunshine
I only have one digital multimeter (so it must be time to get another one now) so I was forced to dig out my old analogue multimeter which I have had since I was 16!. It isn't that reliable due to movements of the switching PCB inside the casing as well as some corrosion, but it was all I had so I had to use it. It has been some time since I used an analogue multimeter so I forgot to zero the scale before taking measurements and I kept getting confused with which scale I was looking at and what the divisions were, so I ended up taking the measurements again. Also, as it was quite sunny and bright and my eyes are not what they were it was more of challenge than expected. Then it took me so long that by the end of the second attempt the sun had moved and the solar cell was partly in shade, so I had to take the measurements for a third time. Definitely time to get another digital multimeter.
Taking the Measurements
Although it was a sunny day there was a high level hazy cloud cover which did reduce the intensity of the sun somewhat. However, as I couldn't do anything about that cloud and it seemed to be fairly consistent I decided to make a set of measurements. It is predicted to be bright sunshine all day tomorrow so I might repeat these measurements to see what changes there might be.
I entered the value in Google Sheets, calculated the power (P = V I) and plotted the following graph.
From the graph, as well as the tabulated measurements and calculated power output it can be seen that the maximum power output (maximum power transfer) is at approximately 100 Ohms. The interesting part of the graph is in the 0 to 300 Ohm region so I have replotted using just those values.
It can be seen from the graph in the region 0 to 100 Ohms that there is some variation and unexpected mini-peaks and this is probably due to errors in taking the measurements (eyesight problems) and/or short term variations in the intensity of the sunshine. The graph from 100 Ohms onwards is not as expected either. A better shaped peak should appear and this might be because no measurements were taken in the region 100 to 200 Ohms. When I repeat these measurements in brighter sunshine I will take measurements every 10 Ohms between 100 and 200 Ohms. It may well provide a much higher peak.
It is important to find the value of resistance at which this peak occurs as this is the maximum power transfer point of the solar cell.
It is interesting to note that this maximum power transfer point does vary considerably dependent on the intensity of the sun and is the region why solar panels used for power generation use an adaptable power convertor which tracks the internal resistance of the solar cell as light intensity varies, in order to always achieve maximum power transfer. I hope to be able to illustrate the change in internal resistance of the solar cell in a later Blog when I can find a sunnier day.