What kind of power resistor do I need?
I'm an 8th grader doing a science project using photovoltaic solar panels. I need 5 resistors, but I have no clue what kind I need! The solar panels are small thin flexible ones that say 3.6V (I think that's the maximum amount of volts it can get...) But I need to be able to track the amount the solar panel gains each day, and I read that I need a power resistor and a digital multimeter. Could someone please help me find these 2 items?
Any help is greatly appreciated.
Thanks! =]
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A photovoltaic solar cell puts out a voltage, but to get power out of it, it has to deliver a current. Power = Voltage x Current. To get current you need a load, which is something that draws power when a voltage is across it. A resistor will do this. A light bulb will also do this. As you take more and more current from the solar cell, the voltage produced by the cell will be reduced. If you had a light bulb across the solar cell and added another bulb, the brightness of each bulb will then be lower than if you had only one bulb.
Also, as something shades the solar cell, it will produce less voltage and less current. If you connect one wire (we call the wires “leads”) of the solar cell to one lead of a multimeter and the other lead of the multimeter to one lead of a resistor and the other lead of the power resistor should be connected to the last lead of the solar cell. The multimeter should be set to read current (mA, for milliamperes). I would suggest the value of the resistor should be between 200 Ohms and 1,000 Ohms. It does NOT need to be a power resistor, just a regular resistor.
Alternatively, use a light bulb in place of the resistor. The bulb should be rated for anywhere from 3.5 Volts to 4.5 Volts and no more than about 50 mA.
I’m familiar with those little 2 inch wide solar dells that you can buy at radio shack. They’ll put out about 20mA in bright sunlight. The power is only about 70 milliwatts so any small 1/4W resistor will do. The value should be about 200 ohms. The voltage on this resistor will always be less than the maximum of 3.6V so you can track it.
If you are tracking the output each day for a science projuct at school, then I would do this: You can attach the POS and NEG terminals of the solar cell to the “current” input of your digital multimeter. This will “short out” the solar cell and give you maximum current that the cell can put out.