Resistor value help

MYREDCHEVY

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Still working on the solar light project. This is what I got. After a full day charge the batt. had a voltage of 3.6v. I figure they should be 2.4 (rechargeable AA type- 1.2v?) The question is how to find resistor value, so I don't cook my LEDs. I'm going with a single blue one. Its rated at 2.1 foward v. and 2.8. max. Now correct me if I'm wrong. Subtract foward voltage from supply and divide by current to get resistor value. 3.6-2.1(dev.).02=75. I then would use a 75ohm resistor. Did I do my home work right?
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Lux Luthor

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Personally, I don't use these kinds of calculations. I take my battery configuration, run it through an ampmeter and a potentiometer to the LED(s), and adjust the pot to give me whatever brightness/runtime configuration I'm looking for.

I always check for heat if I suspect I'm overdriving the LEDs, and I do a few runtime tests before deciding concretely on a resistor value.
 

Slick

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My approach is even more low-tech than Lux's technique. I grab my *selection of resistors and start BIG to be safe and measure current draw with my DVOM. When I get it as bright as I want, within acceptable current flow I stick with that resistor. I try, and I do mean TRY to really avoid using a resistor, but rather matching power as close as I can get (without losing the magic smoke)
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* visit an electronics surplus house and stock-up on a wide variety of resistors.
 

Lux Luthor

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You could call what Slick and I are doing the low-tech approach, but I would call it the experimental approach. When you try to calculate for "correct" resistance, you have to consider the variation in the LEDs, and the fact that different battery configurations will have different voltage drops due to internal resistance.

These effects can quite be large, and are difficult to model mathematically. If you're totally lost, and can't even make a ballpark estimate, then these kinds of calculations could certainly narrow things down for you. But ultimately, you still need to test it.

So if you simply have enough sense not to try to drive a white LS with 2AAA NiMHs, or a red Nichia with a 9V battery, you can dispense with these calculations, and go straight "into the lab" and find the resistance you're looking for that way.
 

remuen

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As Lux Luthor already mentioned the LEDs vary. To really make it correct you need to measure each single LED.

For this you need a 0.1 ohms resistor, a DMM and an adjustable power supply. Connect the resistor in series to the LED to measure the voltage drop accross this resistor. You then have to adjust the output voltage of the the power supply until the measured voltage drop does correspond with the current you want to drive the LED. You then can calculate the resistor you need.

Details and more explanations about all this can be found on my Electronic Basics page.
 

WarpWave

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That is the way I do it.
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Myred please do not feel left out, I too have had this road-block. nor do I have $100-$300 for a big time power supply. but if you are like me and do not have all this "tech" stuff you are going to have to learn the "feel" of it all much like slick does. I would say go to radio-shack and get a breadboard, 4,6,8 battery holder, and some very cheap LEDs and a hand full of resisters. then do the math like you are doing it now. then try a resister with twice the value or more, see what it looks like, see if it gets hot over time, think about it's MCD value, and then push it a bit till it gets hot and even blow-up a few in the name of learning. but keep in mind we all want a bright cool or at least not hot LED, there will be some resistor heat.

There are some web sites too that may help.

There are lots of guys here that know there stuff and will help so keep this post up-dated
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As I am new to all this too I can't give you any real answers
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WarpWave
 

MYREDCHEVY

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Thanks for all the replies!! I like your "methods" better than the math anyways. Yesterday I stuck a green LED in there, to see what it would do. Good thing I didn't use a resistor: cause theirs not much light output. Guess I'll keep trying.
 
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