Powering LED's

I'm working on making a very bright set of "headlights" for my
remote-controlled car. I decided to go with white LED's because my RC
car is power'd by a nitro engine that produces some good vibration
which would probably make incandesent lights not last very long.

I bought 20 white LED's which are spec'd for a forward voltage of 3.0V
min, and 3.8V max. I read that I should wire them in series, and not
wire them in parallel, but I'm planning to power them with 3 NiMH AA's
(3x1.2v = 3.8V). Since that would be the same as the max forward
current, can I get away with parallel wiring, and not using resistors?

I am new to electronics, and from what I have read online, it seems
that resistors are used when your power source produces more voltage
than your electronics can handle. So if I am correct, 3 NiMH AA cells
should produce a tolerable voltage at all times (perhaps too little
when they get drained). So am I correct in assuming that I can wire all
the LED's in parallel, without resistors, to the 3 NiMH AA's?

For more information about the LED's I bought, the eBay page for them
has tables of info on them:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66952&item=7514693502&rd=1

Lastly, I'm curious about how long my 3 NiMH AA's will provide good
power to the LED's. It looks like my LED's will be drawing 20mA each,
and I plan to use all 20 LED's, so that will be 400mA total. My NiHM
AA's are 2400mAh, so my first assumption is that they would last about
6 hours. But then I realized that would be about 6 hours to fully drain
my batteries. So this leads me to the question of when my batteries
will start to get to the minimum forward volage for my LED's: 3.0V. I
have no idea how to calculate that... any tips?

Thank you,
--Farrell F.

 

Thanks guys, so now I have a few more questions:

To clarify, is this what Don is reccomending:

[+]LED,resistor[-]
[+]LED,resistor[-]
[+]LED,resistor[-]
....

and I'm still a bit confused about what resistor(s) to choose. I was
reading this page on how to power LEDs:

http://wolfstone.halloweenhost.com/TechBase/litlpo_PoweringLEDs.html

and it leads me to believe that resistors are specified for both
resistance (ohms) and wattage (watts). Since now I will be using
resistors, I was thinking of going with 4 AA NiMH batteries to increase
runtime. So this leads me to the question of which resistors to choose.
Is the 39ohms, or the 22ohms resistor better suited to this task? and
what wattage? and just so I can better understand this, is it better to
pick a resistor rated to handle more ohms and more wattage? or less?

I noticed a section on that website I linked to above about combining
both the series and parallel methods,
http://wolfstone.halloweenhost.com/TechBase/litlpo_PoweringLEDs.html#WiringLargeNumbersOfLEDsInSeriesParallel
and was thinking of maybe going that route. Is that a good idea, and if
so, how would that affect my resitor selections.

If I should go with the hybrid serial-parallel route, won't I need to
have a battery that supplies more volage? 4 NiMH AA's would be 4.8V...
not even enough for 2 LED's. (I plan to use all 20 LED's) I was
thinking of perhaps buying a pair of rechargable 9V batteries, but 18V
is only enough for about 5 LED's. I do not want to use too many
batteries as it would weight-down my RC car... and since I have a large
supply of rechargable AA's I would prefer to use them. Would I need to
get another device to increase the voltage... what is it called, a
transformer?

Sorry, I'll bet I'm really confusing stuff up right about now. So back
to basics... Is there a nice webpage or book that describes the basic
electronics vocabulary, and basic devices (diodes, resistors,
transformers, capacitors, transistors, etc...) I think this is my main
problem, because the webpage I linked to above did a great job of
defining how the equations worked, it's just that I don't quite know
what voltage, wattage, current, power, etc. are, and I'm going off of
guesses and past expierences.

Thanks again,
--Farrell F.

 

Thanks Tom!

So since you reccomend that I should not go with a transformer, I guess
my assumption of how to wire my LED's in my previous post is correct?
e.g.

[+]LED,resistor[-]
[+]LED,resistor[-]
[+]LED,resistor[-]
....

No? and does it matter which "side" of the LED the resistor will be
attached to?

And now I think I will go with a 4 pack of 1.2V NiMH AA's as my power
source, so will you please check my math and tell me if I am messing
things up:

* 3.0V is the min my LED's are spec'd for, so would it be bad if I aim
to provide them with 3.6V, just a little below their 3.8V max?
* 3.6V per LED, 4.8V power source leaves me with 1.2V to resist.

1.2V/.02A=60 Ohms
1.2V*.02A=.024 Watts

And since I am not familiar with standard resistor specifications, I'm
guess I should still go with a 1/4th Watt resistor, for approx. 60
Ohms... and should I round up or down?

I looked online at Radioshack.com and found 100 Ohm 1/4 Watt
resistors... do they sound about right?
http://www.radioshack.com/category.asp?catalog%5Fname=CTLG&category%5Fname=CTLG%5F011%5F002%5F014%5F001&Page=1

Thanks,
--Farrell F.

 

Thanks yet again

A few more questions, hope you don't mind...

By 5% and 10% I assume you mean the carbon film percentage? What does
this mean, and how does it effect the results?

And what exactly is "drop" voltage? The amount used by the LED?

Thanks,
--Farrell F.