The cell we will use for our solar tracking experiment is made from crystalline silicon and has a maximum voltage rating of. 55V and a maximum current rating of 300 ma. The cell can be picked up in your local Radio Shack for about $5. Although the cell doesn`t produce much power, this experiment can certainly be scaled to handle higher wattages.

Power is the measure of energy per unit time. It can be calculated by multiplying voltage by current at the output of the solar cell. Because our GP-3 Board can only measure voltage, we`ll need to create what is called a current-to-voltage converter circuit. This circuit takes current from the photovoltaic cell and runs it through a couple of operational amplifiers to produce a corresponding voltage (1V corresponds to about 10 milliamps). Figure 2 shows the circuit constructed for measuring current from the solar cell. The operational amplifier (also known as a differential amplifier) is a great circuit for sensing or measuring just about anything because it keeps the voltage at one input at a reference point determined by the other input. The first opamp is the current-to-voltage converter. It takes the current from the photovoltaic cell and converts it to a voltage determined by the 100 ohm feedback resistor. Unfortunately the voltage produced at the output of the first opamp is negative (which the A/D channel on our GP-3 board can`t measure). We need to flip the polarity of this voltage using a second op amp set up to invert the voltage sign. This positive voltage is then fed into pin 10 of our GP-3 board, which is actually channel #0 of our A/D (Analog to Digital) converter. The charting program consists of a User Control that plots data points on an XY Plotter. (See my previous article, An XY Plotter User Control in...

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