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Running a LED on 6v?
I know this isn't exactly related to FIRST, but I don't know of any other place to get the opinion of hundreds of electrical engineers.
I'm going to mod my calculator, and light up the screen with a blue LED and some fiber optic cables. I can get the fibers from a cheapo Ikea lamp of something, but I'm having trouble finding a suitable led; the calc runs off 4 AAAs (6v, in case you're anti-multiplication). I really don't want to pay $3.29 for Radioshaft's 6v-max-rated led, but I can't seem to find any other 6v ones (and i'm beginning to suspect that they are liars). What would you suggest? Possibly only wiring it to 3 of the batteries somehow, or putting a voltage regulator on there? Maybe wiring two leds in parallel (I'm en electronics newb, if you can't tell). Thanks for your help. ~Chris Hammond |
Re: Running a LED on 6v?
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Re: Running a LED on 6v?
LEDs don't exactly run off voltage; they run off current. Being a diode, they have a characteristic voltage drop across them, so you do need to have at least that much voltage supplied. But the important part is a current-limiting resistor.
jgannon's analysis is fine. A blue LED at 2.5 volts and 10 milliamps sounds about right. Red or green LEDs usually have a much lower voltage drop, and often like much less current, though some are perfectly happy at up to 25 milliamps or more. If you can, you should look at the specific ratings for the LED you're using. |
Re: Running a LED on 6v?
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Pertaining to the delicacy of internal electronics: I have already cracked it open and examined it. I found a place to mount the led and run the fibers with a minimal of dremeling and wiring. Also, I plan on simply running the fibers up the back of the board and wrapping them around the front of the screen to get a nice front-lighting effect, since backlighting would require a lot more involved and breakage-likely operation. |
Re: Running a LED on 6v?
More information than you ever wanted on LEDS and their usage:
Dan's Data LED Caselight article |
Re: Running a LED on 6v?
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i can solve the voltage difference between my calculator batteries and the led rating by putting two in series, but then if i have too much current flowing over them i'll get thermal runaway (because their resistance goes down with temperature increase) and they'll burn up, since i can't fit a resistor in there without lowering the voltage too much. alternately, i can use 1 led and a resistor (which i calculated to be 96 ohms, or the closest standard size) The single-led solution seems best, but i don't know if one will be enough to light up the screen very well. Would i have a significant decrease in light if i ran them at lower voltage? |
Re: Running a LED on 6v?
I'd go with the single LED, but I think you should be able to fit a resistor in there for the double LED.....
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Re: Running a LED on 6v?
will it affect the brightness if its run at, say, 2.5 volts instead of 3.6?
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Re: Running a LED on 6v?
it's always running at 3.6V. The voltage drop across an LED is a fairly fixed thing. The resistor affects how much current will be passing through the LED. Basically, if it's a 3.6V LED, that means the resistor will HAVE to have 6V-3.6V of voltage across it. So the resistor has 2.4V across it. Then you just use Ohm's law to set the resistance of the resistor to get the desire current through the circuit.
Now, the current is what actually affects the brightness of the LED. there's probably a graph of the LED's brightness vs. current somewhere if you look hard enough. |
Re: Running a LED on 6v?
i already did the ohm's law thing. But if i were to use two leds in series to double the voltage drop to 6 volts, then i wouldnt need a resistor for voltage, but the current would still be too high.
You know what, i just realized that if i wired them in parallel I would just need a resistor to drop to 3.6v and 50mA. duh. I guess that's what the stress of winning a regional does to you. |
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Re: Running a LED on 6v?
to give each its own resistor, would i split the power supply wire, then put a 100-ohm resistor before each led, or would that not have the desired result?
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Re: Running a LED on 6v?
that'd be exactly what you'd do. then tie the other end of the LEDs together.
EDIT: And I just noticed you're from team 68. Too bad you couldn't have asked me this while my team was at GLR, it'd have gone a lot quicker. Congrats on the win. |
Re: Running a LED on 6v?
Yeah, I kinda came up with the idea on the car ride back. It was dark and I wanted to write a calculator program.
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Re: Running a LED on 6v?
Chris,
I would not recommend two LEDs in series for this project. As the batteries discharge it is possible to fall below the voltage at which the LEDs will stay lit (i.e. the forward voltage drop of a two standard LEDs is 2v + 2v=4v.) When the battery falls below the "on" voltage, the LEDs will go out. The calculator will continue running down to maybe 3.5 volts or well below the "on" voltage of a white LED with series resistor. As a matter of fact, I am not sure you will have satisfactory results with the light pipe idea. Before truly modifying the calculator, try breadboarding your idea to see if it works. The angle of lighting on liquid crystal displays can have a huge effect on viewing. Try a flashlight and fiber optic to test your idea before jumping into the LED wiring. You will also find a huge difference in operating time as the LED will consume much more current than the calculator. |
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