LED 101 - Question To The LED Gurus

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Not sure if this belongs here or on the Homemade mods side, but it does have to do with a straightforward generic question about Light Emitting Diodes in general:

Why is a given LED, say a Luxeon Star for example, not as bright when running on 3 AA cells, as opposed to the same LED running on 3 D cells? I can understand the 3AAs running out of juice sooner, but as for power and brightness, aren't the voltages of the batteries identical -- 4.5V?

 

[vcal]

Becos the "D" batts. voltage under the higher current load needed by the Luxeon will be higher than the smaller AAs under the same load.

I'm not sure about being a "guru" though....

 

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by mr.glow:
Becos the "D" batts. voltage under the higher current load needed by the Luxeon will be higher than the smaller AAs under the same load.

I'm not sure about being a "guru" though....

<HR></BLOCKQUOTE>

Thank You, Mr. -- "Glo-ru". Close enough to guru for me, especially when compared to me! So those three Ds will be flowing greater than 4.5V and those 3 AAs will be flowing less? In other words the lit device itself (LS in this example) acts as a voltage "pulling" device?

 

[vcal]

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by MR Bulk™:
Thank You, Mr. -- "Glo-ru". Close enough to guru for me, especially when compared to me! So those three Ds will be flowing greater than 4.5V and those 3 AAs will be flowing less? In other words the lit device itself (LS in this example) acts as a voltage "pulling" device?<HR></BLOCKQUOTE>

Since the Luxeon (in your example) operates at a recommended/nominal 350ma, it's much easier for the larger "D" batt. to deliver that current than the much smaller "AA"-even if it's an alkaline. Because of that, the voltage will "compress" less than with the smaller batt.,-end result.., the voltage under Load will be greater with the "D" batt.

Fyi-LEDs are really primarily current driven devices, and secondarily voltage driven.

 

[lambda]

Charlie,

What really is going on is called internal resistance. Batteries also act like resistors; the smaller the battery, the larger the internal resistance (generally speaking).

If shorted by a piece of wire (low external resistance), the D cell will provide about 8 times more current, than the AA cell can. The internal resistance limits the maximum available current under heavy load.

Also, when drawing a heavy load, say 1 amp from an AA cell, the voltage may drop to 1.2V, where a D cell may maintain 1.45V at that current. Again, because of internal battery resistance.

Hope I've made some sense...

 

[papasan]

yeah, what he said.

it's similar to why a garden hose puts out more water than a drinking straw.

 

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A-HAH! I got it! And that's why the Hydra XP4 with D cells is slightly brighter than the C cell version. Got it!

Thanks Dr. Lambda, and everyone else who responded to this poor, uninformed, and at times thoroughly confused, newbie...

 

[Kill-O-Zap]

First, a quick introduction. I'm fairly new to CPF, I've been mostly lurking for a number of weeks. I am getting really caught up in the custom LS light craze (was hacking up an RS 3C earlier today). Love it. Love CPF. Where has this been all my life?

And what on earth was I doing with those cheesy supermarket bought flashlights?

Ok, onto the main subject [sorry, long winded], which I realize now, after having written it, belongs not in LED's, nor in Mods, but in Batteries. Oh, well here goes anyway:

In addition to the other explanations, Mr. Bulk may also find it interesting (or maybe I just like to hear myself talk

) that because NiMH cells have *much* lower internal resistance, three *AA* NiMH cells, rated 3.6V, when driving a real heavy load, say three LS's, direct drive, may actually outperform 3 alkaline C's rated at 4.5V because the alkalines may drop to 3.6V or less under that kind of load, whereas the NiMH's will barely budge.

What's more, as they begin to run down, the alkalines progressively drop in voltage such that it stays down even without any load, whereas NiMH's will hold a fairly constant voltage as they are partially depleted. That means more constant light output, without a regulator from the NiMH's. So if the NiMH AA's didn't outperform the alkaline C's from the get go, they may catch up soon.

But wait, there's more: under heavy load, the alkalines are less efficient than NiMH's (it's all goes back to the internal resistance) so even though their rated maH's may be much higher than the same-size NiMH's, alklines will probably conk out completely sooner, a phenomenon well known to digicam users. Again, all of this applies to heavy loads only, a single and maybe even a dual LS will probably do better on alkalines.

And since I've gone this far, I might as well mention that NiMH's are also reusable (duh

) and environmentally friendly. Not that any of that means anything to me, I just want BRIGHTER, LONGER

So what does any of that mean to you and your 3D quad regulated LS? Maybe nothing. When you start getting into D size cells, the alkalines themselves pack a whallop, whereas the NiMH's in that size are sometimes C's or even just AA's in a bigger case; pretty lame. The regulators should take care of the voltage depression. So the alkalines may be your best bet after all. (did I just waste your time?

)

Regards,
Sal

 

[treek13]

Hi All,

Just want to say thanks to Mr. Bulk for asking a good question and to everyone who answered for providing information.

You learn something new every day and now that I've met my quota I can get back to killing my brain cells with mindless entertainment.

Seriously though, this newbie is appreciative of a chance to pick up a little knowledge.....thanks!

 

[Jonathan]

Since this is a topic that I've been interested in for a while, I'll jump in with my $0.02...

Battery 'voltage ratings' are nominal values; they are a name that we give to the battery to roughly describe the things, but the values are in no way 'set in stone'. As has been mentioned, batteries can also be described by there internal resistance, their state of charge, their internal temperature, etc. All of these factors will influence the output voltage of the battery when a load is applied, and additionally, the amount of current flowing through the load will strongly influence the battery voltage.

I want to reinforce the suggestions above: under 'significant' current loads, low resistance batteries often deliver more energy to a load than higher resistance batteries with more total energy storage. Much of the energy gets wasted in that internal resistance. Many lighting applications are just on the edge of what cells can deliver, so that for many lights NiMH cells will provide better output than alkaline cells.

SureFire has made a science of carefully matching the lamps to the internal resistance of lithium cells...if they tried to get much more power out of the cells, then cheaper alkaline batteries would actually perform better, and if the drew less out of the cells then the voltage would be too high for the lamps.

I want to take issue with one of lambda's points above, about the greater current handling capability of D cells. While it is true that D cells have more total capacity than AA cells, as well as a lower internal resistance, the ratio of internal resistance to capacity actually favors AA cells for 'high drain' applications.

(Note: this is specific to Alkaline batteries...) As I recall, D cells have about 6-7 times the capacity, but only about 1/2 the resistance of AA cells. This means that at low drains, the D cell will supply about 6-7 times the total energy as an AA cell, and at moderate drains the D cell will maintain somewhat higher voltage than the AA cell.

_But_ if you put some AA cells in parallel, you can get even lower internal resistance. For moderately high drain applications (on the order of 1 amp), 3AA cells in parallel will have lower internal resistance, and will deliver more total energy than a single D cell. For even higher drain applications (in the 2-3 amp range), _2_ AA cells in parallel can out perform a single D cell (though at these power levels, both will be doing pretty poorly, and you should be using low resistance NiMH cells).

Given that one can pack _4_ AA cells into the space of a single D cell (with a bit of squeezing...they will fit in a maglight), for many high power applications, you are better off if you can replace the D cells with a larger number of AA cells.

I'd bet the 'hydra' packed with 12AA cells rather than 3D cells, would be brighter still...or at least would maintain its brightness for longer (depending upon voltage regulation issues.) Of course, it would be a bit more difficult to load

-Jon

-Jon