|MadSci Network: Engineering|
Beam me up, Scotty! As I'm sure you found out, LED's come in a large variety of colors, sizes and brightnesses - but, they all share some common characteristics such as maximum voltage and current limits that must not be exceeded or they will be destroyed, maximumm operating temperatures, and optimum operating voltage/current ranges; outside of which performance either decreases or does not significantly increase. By performance, I'm referring to light output (luminence) vs. power consumption and increased heat production. To get more specific, an average LED operates in the range of about 1.9- 2.02 volts and 10-40 milliamperes, with a MAXIMUM of about 2.4 volts and 50-70 milliamps. Therefore, using a 9-volt battery (or any power source above about 2.35 volts), you must supply a series resistor to keep the voltage and current within maximum operating parameters. POWER SOURCE: While it makes ecological sense to use solar power, you will find that the Cadmium Sulfide photocell (CdS) is unsuitable for a variety of reasons. Firstly, they typically don't produce enough voltage or current to power anything like an LED, and secondly, the output varies considerably with the amount of incident light, making it virtually impossible to maintain a stable light output. A silicon photocell would be better because it has much higher current output with the same amount of light but again, control would be difficult. The 9-volt battery is a better choice with its' more or less constant output, but it would not be able to power multiple LED's for a very long period due to its' limited ampere hour capacity. A much better choice would be 2-"D" cells in series, thus producing 3 volts at a fairly high current capacity. You would still need to use resistors to reduce the voltage to under the maximum rating of the LED(s). RESISTOR SELECTION: Using Ohm's Law (I=E/R), you can calculate the size of the resistor required. Assuming you want to operate an LED at 30 milliamps and you're using 3 volts as your power source, you can plug them into the equation to come up with the resistor size. In this case, E(3 volts)/I(.03 amperes)= 100 ohms. Experiment with different values of resistors until you get the desired brightness, but remember - some LEDs produce a lot of light and some produce only a little with same voltage and current. Use VOM in series with the battery and LED to make sure you don't exceed about 50 milliamps! I would suggest you use a 1k potentiometer, adjust it for the desired brightness and current draw, then disconnect it from the circuit and measure what resistance it is set at (using your VOM). Use that for your fixed resistor for that specific LED. Repeat the procedure for all of them. The human eye is more sensitive to green light, about 532 nanometers (nm) wavelength, then red light, about 628 nm, so different colors will require more or less "drive" to get the same apparent light output; which bring us to the next topic- SERIES or PARALLEL? If you were using the same LED for all 20 or so (like the Hewlett Packard HLMP-EG15-PS000), you could theoretically use either a series or parallel hook-up because every LED within that batch is matched (electrically) to every other LED within that batch. For a series hook-up, the supply voltage would have to be increased to about 48 volts because the voltage drop accross each LED is about 2.4 volts(MAX.), and no resistor would be required. Because you are using many different LEDs, the series hook-up is not practical nor would it work properly due to the differing electrical and luminance characteristics. The parallel hook-up is much more complicated because the resistance of an LED(or any diode, for that matter) varies with the voltage applied across it. Figuring the value of the required resistor requires a special formula which takes into account a parallel resistor formula, LED threshold voltage, temperature coefficients, etc., and is beyond the scope of this reply. CONCLUSION: Use the above mentioned potentiometer/VOM method and use a seperate resistor for each LED. Remeber also, that LEDs are POLARITY SENSITIVE. If you reverse the voltage they will not light at all or, at worst, they will be destroyed if the "maximum reverse voltage" is exceeded. Go to this address for more info on LEDs. Agilent is the semiconductor division of Hewlett Packard. http://www.semiconductor.agile nt.com/ If you have need any further clarification, contact me directly at: firstname.lastname@example.org May you future be "bright"! Your not-so-mad scientist, Karl
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