For Stingmon

vcal

Flashlight Enthusiast
Joined
Dec 16, 2000
Messages
3,074
Location
San Gabriel Valley
What's the principal reason an LED draws less power as the batteries run down? I know the efficiency increases w/less current.
-My Pelican L1 accidently got switched on, and stayed on for about 7 hrs. whilst I slept. (With the little 40mAh Nimh batts. it was still usable[output comparable to my white Infinity] after being left on All that time).
shocked.gif
-I measured the mA draw before recharging the batts. and it measured about 4mA.
confused.gif
 
To try and put it simply, the closer your voltage drops to the LED's threshold voltage (going downward from its rated operating voltage), the lower the current becomes, until almost no current at all flows and no more light is produced.

So in your little Pelican, as the battery voltage decreased, so did the current demands of the LED.
Eventually, the voltage on the LED will sag very close to the LED's threshold (somewhere around 2.4 to 2.8 volts for white), and current will be measureable in microamps, not milliamps. And at current that low, even dead batteries can chug along for quite a long time. Even little half-dead button cells will run for a long time with only 1-4mA on them, and most modern LEDs can produce usable light even at that low power. And it will take progressibly longer to dim out as the current demand of the LED continues to decrease.
That's why your Pelican was still usable.
But since most of the battery chemistry was used during the brighest several hours, there won't be very much left to run the LED even at a few mA, so another night will probably do it in.

Now, if there had been an incandescent bulb on there, the current will actually increase as the voltage drops and the filament cools.
In some cases, you can actually watch the flashlight go from dull yellow to dull orange to cherry red and finally to black extinction in a matter of minutes or even seconds because the bulb continues to draw more and more current, eventually exceeding the remaining capacity of the batteries and sucking what's left of them away quite rapidly.
Even at 0.1 volts or some other rediculously low value, the bulb will continue to draw current - as much as the nearly dead battery can generate. That's why you can sometimes find "stone cold dead" batteries in an incandescent flashlight that was left on, but in an LED flashlight that was left on until it went out, some voltage can still be measured - at least until the remaining good chemicals in the battery become neutralised or react with one another in storage (the self-discharge thing we all know and love).
 
Thanks for that info.-mr.Sting
-interesting that the incand. wants to draw proportionately more juice as the batt. dies. -All the more reason to respect the solid state bulb
smile.gif
 
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Stingmon:
Now, if there had been an incandescent bulb on there, the current will actually increase as the voltage drops and the filament cools.<HR></BLOCKQUOTE>

No, Stingmon, you have a flaw in there: When the voltage drops, the current goes down in any way. But the current goes down less than the voltage. For example, when the voltage drops by 10%, the current only drops some 3% or even less. That's why an incadescent bulb makes a simple and easy constant current regulator.
For everbody who has an adjustable power supply (and I think every electronics tinker has one) it is easy to check. I will do it this afternoon for a Surefire P60 (if there is any interest for it) or any other popular bulb.

The second main difference in between incandescent bulbs and LEDs is that LEDs gain a little in efficiency when current drops and incandescents go down fast.

And BTW, this all is not true for all incandescent bulbs, there are also ones where the current goes down even faster than the voltage.
 
Top