July 5, 2004
The high temperature hit 95 degrees F (35 degrees C) today. Thanks to Wilmington's lovely location next to one of the biggest
bodies of water in the world (the Atlantic Ocean) we had lots of humidity to go along with it. Combine
these two together and you end up with a day best spent indoors.So what's to do after you've played with the cats, watched TV, mindlessly surfed the internet, cleaned house, called some friends, and tried to read a book or two? I decided to perform science experiments!
This experiment was to calculate how much resistance was needed to safely light 8 LEDs on an old electronics experimentation kit when powered with an 18.8 volt, 3 watt solar panel sitting in near full sun. I was curious if the solar panel produced too much voltage even when run in series through the 8 LEDs. I would find out if a resistor needed to be wired in the circuit to keep from burning out the LEDs.
If you're beginning to think it must REALLY be hot outside to make this sound like the best use of my time, you're probably right. :)
The things I know:
- Solar Panel produces 3.2W @ 18.6V (170mA)
- LEDs are rated for about 40mA (0.04A) each at 3 volts.
- I have 8 LEDs wired in series
- I need to find what size, if any, resistor I need to use
We know that the voltage drop across each of the 8 LEDs is the same. 18.6V / 8 LEDs = 2.3V/LED which is well within our acceptable limits!
Since I now know I don't need to use a resistor, what is the internal resistance of each LED? Let's use Ohm's law to find out.
Ohm's Law is Resistance(R) = Voltage(E) / Current(I)
Resistance(R) = 2.3V / 0.04A
Resistance(R) = 65 Ohms
I found that by inserting a resistor in the circuit (pictured below) I was able to dim the LEDs by varying amounts. Even an additional 100 ohm resistor still allowed enough current to pass to brightly light the LEDs.
Eureka! It works. Okay, I'm hoping for cooler weather tomorrow...gotta get out of this house...