Torch battery configurations..

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It just hit me... why are there torches with 4AA cells? The Turtlelite II, Energizer Lantern and the Princeton LED torches are prime examples.

White LEDs require a forward voltage of 3.6v. 3 AA cells give you 4.5v nominal (dropping fast as you use it) and a dropping resistor should be the perfect configuration.

So why use 4 AA cell? It doesn't seem to make any sense unless a DC-DC converter is used. The extra battery will only be used to heat the resistor.

And (I feel like ranting today), the Inova 5x. If it only has a dropping resistor and assuming a 6v nominal voltage, the 2.4v across the resistor will suck 40% of the battery power. Not very efficient is it?
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Sean

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Someguy:
...the Inova 5x. If it only has a dropping resistor and assuming a 6v nominal voltage, the 2.4v across the resistor will suck 40% of the battery power. Not very efficient is it?
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<HR></BLOCKQUOTE>

No it isn't!
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It's my only complaint with the X5 is that the battery life could/should be longer.
 

bikeNomad

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Someguy:
It just hit me... why are there torches with 4AA cells? The Turtlelite II, Energizer Lantern and the Princeton LED torches are prime examples.

White LEDs require a forward voltage of 3.6v. 3 AA cells give you 4.5v nominal (dropping fast as you use it) and a dropping resistor should be the perfect configuration.
<HR></BLOCKQUOTE>

Three cells only let you run the voltage down to about 1.2V per cell, which doesn't use much of the battery's energy.

With four cells you can run down to the nominal 0.9V per cell.

Also, you have to consider the range of LED voltages that you might get in a production run. With more cells you will get a more consistent brightness regardless of actual LED forward voltage. No flashlight manufacturer is going to hand-select resistors.
 

Chris M.

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...and you must remember- the body of the Turtlelite/Dorcy Cool Blue/whatever- was originally designed as an incandescent light. All they did was stick a LED bulb in there instead of an incandescent one. Same goes for PT`s lights- they can also be found with bulb/reflector modules in there too, under different names.


Of course mr Nomad up there has the "proper" answer! Basically, longer burn time. Efficiency is largely irrelivent since white LEDs are no more efficient than a decent filament bulb anyway.


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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>With four cells you can run down to the nominal 0.9V per cell.<HR></BLOCKQUOTE>

But thats irrelevent if you're only using a dropping resistor. With 4 cells, you'll have a nominal voltage 6 volts and you have to design the circuit so the dropping resistor will have voltage of 2.4 volts.

Irrespective of the voltage of the cells, the resistor will always drop the voltage proportional to the resistance of the LEDs. If the voltage of each cell dropped to 0.9v, the LED will definately not get the whole 3.6v

Also unlike incandescents, most LEDs use very little current, thus the voltage drop will be slow and gradual.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>the body of the Turtlelite/Dorcy Cool Blue/whatever- was originally designed as an incandescent light. All they did was stick a LED bulb in there instead of an incandescent one<HR></BLOCKQUOTE>

Thats true and is most probably the reason why they're using 4 cells. Its a patchy solution IMO.

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Basically, longer burn time<HR></BLOCKQUOTE>Thats my arguement. It shouldn't run longer!
 

StuU

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That's my approach to led conversions. I try to go from 4 cell to 3 cells. I can't tell a difference in burn time when doing this- in fact, I'd say the 3 cells seem to last a bit longer.

One nice touch- Use the 4th cell space to put in a variable resistance feature. I have used a DPDT switch that is hooked up to a varying set of resistors. I start out with 47 ohm/led and when the light starts to drop, I flip the switch and am down 10 ohm/led. Primitive but effective.

Anybody tried a small potentiometer here?
 
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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>I start out with 47 ohm/led and when the light starts to drop, I flip the switch and am down 10 ohm/led. Primitive but effective.<HR></BLOCKQUOTE>

Smart!
 

Jonathan

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<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Someguy:



With four cells you can run down to the nominal 0.9V per cell.


But thats irrelevent if you're only using a dropping resistor. With 4 cells, you'll have a nominal voltage 6 volts and you have to design the circuit so the dropping resistor will have voltage of 2.4 volts.

Irrespective of the voltage of the cells, the resistor will always drop the voltage proportional to the resistance of the LEDs. If the voltage of each cell dropped to 0.9v, the LED will definately not get the whole 3.6v

<HR></BLOCKQUOTE>

Actually, the current to voltage relation of the LEDs is not nice and linear, so you can't think of it as a resistance.

A better approximation of an LED is to think of it as a constant voltage drop in series with a low value resistor. The LED is then placed in series with the external dropping resistor. As the battery voltage drops, the constant voltage drop of the LED remains constant, and the voltage drop in the various resistances must fall. This happens naturally as current drops.

So when the battery voltage falls to 3.6V, the LED will see all of the 3.6V, but the current through the resistor will be zero (no light).

With a 3 cell pack, you go from full current with fresh cells at 4.5V to zero current at 1.2V per cell or 3.6V for the battery pack. With a 4 cell pack, you go from full current with fresh cells at 6V, to about half current at 1.2V per cell with a pack voltage of 4.8V to zero current 0.9V per cell.

At a cell voltage of 1.2V, with the 3 cell pack you are finished, but with the 4 cell pack you are still running at 50% current, which is probably >50% light output.

If you can switch to a lower resistance, as some folk do here, then you can get full brightness as the battery discharges.
 
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