1. Intro
With this curve tracer you can measure the characteristic curves of a single photovoltaic cell.
2. Graph
3. Libraries needed
Unzip the Dac library and place it in the standard Arduino library subfolder \libraries\. The libraries Streaming.h and Flash.h from Mikal Hart should also be installed. Download these libraries here: http://arduiniana.org.
4. Hardware curve tracer
The curve tracer is based on an Arduino Duemilanove microcontroller board. See the photos. You can ask me for the schematic. The solar cell is loaded with a power MOSFET BUZ10 which is used as a variable resistance. The gate is driven slowly by an increasing voltage so that the drain current increases from zero to the maximum current that the solar cell delivers. During this process the voltage and current are measured and print to the screen. We use the Arduino simple 10 bit DAC to drive the BUZ10. Connect the solar cell with very tick wires because the resistance should be low, don’t use connectors. We need a 12V adaptor for the supply.
5. Software
The hardware must be calibrated first, see below.
- Unzip the Solarcell curve tracer project file. You can ask me for the recent software version.
- With the Arduino Software run the project.
- Set the 4 preferences in void loop():
|
bool printCurve = 1; // 0 = without curve, 1 = with curve |
See here for Troubleshooting. For the latest software and hardware updates you can contact me.
6. Create the graph in Excel
- Start the serial monitor and wait until the values are printed to the screen.
- When ready, copy the results from the screen with ctrl c. Select the numbers and the row "mV mA mW".
- Paste the results in Excel with ctrl v.
- Immediately look below the Excel table and select “use wizard”.
- Use “finish” to exit the wizard.
- Insert a XY graph from the first two columns.
7. Calibration
The hardware must be calibrated first. Required instruments:
- Laboratory power supply which can deliver a constant current of about 5 A.
- Accurate multimeter, minimum 5 A.
The calibration procedure is included in Curvetracer.pde. Use this Excel file for automatic calculation of the values.
8. Illumination sources
8.1. The sun
Doing reproducible measurements with the sun is difficult and we have to wait for the summer to do tests. The measurements have to be corrected for the current sun irradiation [W/m2]. This can be done with a solar irradiance meter.
8.2. Solar cell testing light source
8.3. Slide projector
This is the poor man solution. The light intensity is about 1/3 sun.
9. Solar irradiance meters
The TSL230R light sensor IC with Arduino interface. Instead of photo diodes, the TSL230R is special designed to measure irradiation.
10. Slide projector illumination
With a slide projector we can do reproducible measurements independent of the varying sunlight. The light intensity is about 1/3 sun. The advantage is that the solar cell is hardly heated. I have find out that the accuracy can be quite high when some preconditions are met:
- Mains variation correction
- Solar cell cooling with a ventilator
- Testroom without daylight
- Fixed mounting frame
- The slide projector must be turned on three hours in advance
Note that the reflection is also measured because the tested solar cell is placed at an angle of 30 degree; this is the average sun angle in the summer in the Netherlands.
11. Corrections
11.1. Reproducibility
Measuring wear and different encapsulation materials require a reproducibility of less than 1%. Therefore it is necessary to correct for the mains variation and temperature.
11.2. Mains variation correction
Measurements at for instance solar cell encapsulation materials require high accuracy. Due to mains variation the light of the slide projector varies more then allowed. See here an example of mains jump of 6% in Maastricht, the Netherlands.
Note that the output light power is proportional with the square of the mains voltage. I have made correction software that eliminates completely the influence of the mains variation.
11.3. Temperature correction
I measure a temperature coefficient of about 0.4%/°C. By measuring the solar cell temperature directly with a LM335 temperature sensor, the temperature influence can be corrected.
12. Forum
You can reply on this article on the forum here.
