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RE: Re: Power optimization and some (stupid?) LED questions

From: Demetrius Anger   dangerwaste.org
Date: Wed, 18 Aug 2004 10:59:38 -0700

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> I should mention that I don't think there is such a thing as a stupid
> question.

Okay!  Somehow the LED - resistor question keeps confusing me and I know
that I've asked this question of people and gotten answers but somehow it
just doesn't stick or maybe just doesn't make sense in the first place so it
doesn't stick because of that.

My confusion about it is this:

Okay if I had a 6V battery and just one LED and I know that I've got a 2V
drop across the LED and I want 20mA running through the LED then I can say
there's a 4V drop across the resistor so I can say

V=IR

R = V/I = 4V / 20mA = 200 Ohms

But then say I put 2 LEDs in there.  Then there's only a 2V drop across the
resistor so

R = 2V / 20mA = 100 Ohms

But then say I put 3 LEDs in there.  Then there's a 0V drop across the
resistor so

R = 0 / 20mA = 0 Ohms.

So then I've got 3 LEDs in there put there's no way to calculate the
current.  What is the current?  Is it huge?  Like an ideal diode lets
current flow as much as it wants right as long as the voltage across it is
large enough so the current = infinite, but what is it in reality?

Like can you calculate based on specs from the manufacturer?

Somehow I thought if you did it so the voltage drop across the LEDs equaled
the source voltage and there was no resistor the current would be running at
the spec of the LED (20mA in this case) and that would be more optimal than
if you had a resistor in there just bleeding off energy as heat.  Is this a
misconception?

> I should also mention that I don't really have a proper answer
> for you.  I hope someone else can perhaps give you more
> information.  I will
> only say, that if it was me doing the project I would wire the
> batteries in
> parallel, (because 6 volts is sufficient to operate an LED, and
> batteries in
> parallel will last longer than a single one as you know).  Then with the
> LED's I would certainly wire each LED with it's own resistor, and wire all
> the  LED-resistor groups in parallel.  (In the event that only
> one LED will
> be on at a time, then all the LEDs can share a single resistor).  I'm not
> sure how long the LEDs will remain illuminated in this
> configuration, but my
> guess is longer than a week.

I will probably do this if no one else says anything.  I don't totally
understand why though.  Why would 2 6V batteries in parallel last longer
than one 12V in series?  Is it like if there's a bigger voltage drop those
little electron people try to get to the other side harder or something?
And why is it better to have them all in parallel?  I guess I'm still stuck
in the having resistors in there means there's power wasted idea.

When you guess they would run longer than a week would that be only running
at night or are you saying 24/7?  If I don't have to make a dark-activated
switch I be so stressed about getting that part to work.  Although I will
probably still try.  I just got a bunch of light-cell and light-resistors to
mess around with and have done some transistor / FET things so I think I can
figure out something.

Thanks alot for your help Sandor!

-aaron





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