Just how inefficient is the 2 stage tail cap for L4/E2L?

Delta447

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I've seen many references to the fact that using a resistor to create a two stage tail cap from your L4 or E2L clickie is inefficient, but that the added utility more than makes up for this fact. I believe you still get extended run time when using the low mode, but are you wasting a lot of battery? Does the ohm of the resistor have any impact? Thanks.
 

wasBlinded

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Much of the battery's power is indeed being wasted bythe resistor, but at low drive levels, the lumens/watt of the LED increases so you get some of that back. Even with the overall inefficiency of a resistored low, you still get markedly prolonged runtimes, such that I don't think its a practical issue at all.
 

Wolfgang_Ludwig

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A Mce2s makes out of my L4 a light I can use for 99% of my requirements. L4 without 2-stage is almost always too bright, has too little runtime and doesn't stand on tail. With a 20 Ohm-resistor I get > 8 hours of light. That is enough for 3 nights when used in my house in Papua Newguinea. With rechargeable Pila the L4 is even more versatile.
 

Long John

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I often heard this theme discussed. But what's efficiency in your opinions.
The post, Wolfgang L. descriped, is a very good example for a usefull application. So it's very efficiency to use this kind of brightness level changing, to have the choice for less or more light and to encrease the runtime from about 1 hour to about 8 hours.

In the resistor will be energy changed into heat, but that's the price we have to pay for. Absolutely no problem for me.
Specially in a case of the L4. It has only one brightness level = full output.

Do you want longer runtime? So you can carry spare cells = 100 % efficiency in some people's opinion.

I prefer to have the choice between 2 levels (or more), even when heat will occur at the resistor.

When there is the possibilty to change the levels by the driver, the setup will be more efficient. But perhaps not, because the driver itself isn't 100% efficient.
So to use a Led as a lightsource isn't 100% efficient, because it will produce heat too.


Best regards

_____
Tom
 

McGizmo

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I won't try to do the math but the resistor is as stated, a device that converts electricity into heat so it is a drain on efficiency if efficiency is based on lumens/watt.

Let me use a lame analogy here. Lets say you have a big commercial vehicle with a turbo-charged diesel engine that is state of the art in terms of fuel efficiency and it gets 8 mpg. You also have an old beat up moped with a shot engine that only gets 30 mpg. If you just need to get yourself to the store and back then the moped will suffice and it will get you there and back better than 3 times more on the same gas expense as the truck would. In this example, the moped is a more efficient solution based on mpg but it is much less efficient in terms of energy converted to power of propulsion.

Although the power efficiency of a resistored two stage light may be relatively poor due to the resistor, the run time increase can be so significant that the gain in light hours makes it a much more efficient device having the two stage option. If you can increase your run time by say 20X with a useable level of light, how significant is the efficiency of power conversion from watts of electricity to lumens of light? A two stage constant current device might give you a 50X increase so it is certainly more efficient and even desireable if available as an option.

With most flashlights and in most applications, I suspect that the real loss in efficiency is at the very end of the trail. Simply put, you generate light at some level of efficacy only to have it wasted as it is not needed or even on target. We spill more light than we need or use, in most cases. If you are using a light that is outputting 80 lumens but you only need or are only getting 4 lumens on target, then the waste there makes a resistor insignificant in comparison.

Don't get me wrong here, I prefer a device that excels in efficiency at all levels of energy conversion and delivery but I also realize that the photons needed and delivered on site is the most important consideration in overall efficiency in task accomplishment.

Efficiency can be defined and measured using different aspects of a system and the level of efficiency will vary as defined.
 

Delta447

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Thanks to everyone. The knowledge of this forum is intimidating sometimes. That pretty much answers my question. Bottom line is that extended run time = extended run time, whether 100% efficient or not. I can live with that type of "inefficiency". Now if I wasn't such a wuss to throw my tail cap into a pot of boiling water...
 

lightrod

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The resistor in my L4 two stage switch is about 26 ohms, and cuts high level output by 1/10th to 10 lumens. Runtime not shabby at all, and IMO only goes to further emphasize the points already made....



Output starts after the quick initial drop at about 52 lux (beam center at 1m) and holds 90% of that out to 7 hours. Hits 50% at 10.5 hours, then has low/emergency light for several more hours.

Interesting that FLR (Doug) got am almost identical runtime to 50% on the L2 low level which is rated at 15 lumens. I'd guess if the L2 low level were cut to 10 lumens it would run about 15 hours or abour 50% longer than my L4.

Is that a big deal? It is not. I REALLY like my L4 and Don's point is well taken here. Having the two stage mod makes this light's usefulness go WAY up. The not-so-great low level efficiency is insignificant in comparison. Why would I complain about 10 hours of runtime instead of 15, when on high level I only get a little over 1 hour, and in practical use only infrequently want/need the high level anyway?
 

Illumination

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Are the two stage switches still available pre-made? I know Sandwich Shoppe sells a switch kit, but I'm pretty confident that I will end up messing up my switch in trying to add the two-stage component. I'd rather buy a pre-made switch. I looked on Lighthound and couldn't find it either, though they do have beamshots on the 'Information' page.



Also, does anyone know how these two stage switches compare to the SF L1, L2 and U2? Does Surefire use a resistor, or do those switches work in some other way. (I know there are electronics in the flashlight body of the L1, L2 and L4). From my basic understanding of electricity, I would imagine those lights work the same way.
 

McGizmo

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Illumination,

As to availablilty it is very limited at present. The "FT" solutions should still be available. I don't know what the future has in store for these but there is some discussion going on, including the "standard" components.

The SF two stage switch that is used on the L1, L2 and A2 is the same switch in all three lights. It has a 10 ohm resistor in series with the ground path on the low level. The electronics are black boxes to me but there is a voltage drop across the 10 ohm resistor when it is in the ground path and this voltage drop is energy converted to heat in the same manner as the other 2 stage resistor switches. In the cases of the L1 (latest revision) and L2, to my understanding, in low, the circuit will provide a constant current drive level to the LED. So you have the inefficient ground path imposed by the 10 ohm resistor but your circuit is operating at some level of efficiency and in current regulation.

Of primary import me thinks is consideration of light level and runtime and not electron to photon conversion efficiency, in any of these 2 stage set ups.

There are much more eficient means of variable output control if it is handled at the converter level and not a "waste" or "bypass" valve on the voltage supply from the battery itself as with the resistor tail caps switches. The SF U2 for instance handles its output selection up front and at the converter level. There are a number of PIC and PWM drivers out now that are at the converter level. The "X2" converter's of dat2zip manage the constant current settings at the converter level.

The resistor in ground path is a very pragmatic and viable add on solution for electronic circuits that don't have other means of output control. It is most viable in cases where it chokes the power to the LED way down so that the net is an inefficient chunck taken out of a small magnitude of power in flow. I.E. if one were to use strictly a resistor on a L4 to get its low level of output close to that of the L2 on low, the loss through the resistor would be significant and the runtime on low would suffer noticibly.

Some of our resident EE's could do a much better job on this subject! :green: (in addition to repremanding me for my buttcher job )
 
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