Re: Hi Intensity LEDS

On Tue, 25 Oct 2005 10:03:11 +0900, H. Dziardziel
<hdzi@xxxxxxxxxxxxxxx> wrote:

>On 24 Oct 2005 16:01:14 -0700, "redbelly" <redbelly98@xxxxxxxxx>
>wrote:
>
>>
>><RJ> wrote:
>>> I must admit, I haven't been keeping up with
>>> advances in electronic components.
>>>
>>> I just bought a ( small ) LED flashlight.
>>> It uses 2 - AA cells, has 3 - LEDS,
>>> and the light output is amazing.
>>>
>>> As I remember, LEDs needed a current limiting device,
>>> usually a resistor.
>>>
>>> I've peeked in the flashlight case,
>>> and can't find any other components.
>>>
>>> Has the industry gone to a ( micro ) current limiting chip
>>> for use in these flashlights ?
>>>
>>> <rj>
>>
>>Using resistors to limit LED current is suited for when you don't care
>>about the power wasted in the resistors. For example in indicator
>>lights.
>>
>>In battery-powered flashlights you DO care about wasting power, since
>>that shortens battery life. Instead, some kind of active current
>>control which requires minimal power is used. Others in here know
>>better than I about what specific circuits are best for this.
>>
>>Don't believe for an instant that LED's are ever connected directly to
>>any voltage source, such as betteries. Small changes in voltage will
>>result in relatively large variation in current. Even if the LED is
>>lucky enough not to get fried when connected to a fresh battery, it
>>would become uselessly dim after the battery drops down one or two
>>tenths of a volt.
>>
>>Mark
>
>It depends on your needs of course, but two cheap dry cells
>directly connected to some white LEDs can put out usable light for
>ever. For example, LEDs nominally rated for 20mA can handle
>40-50mA . which is what some will draw from two fresh cells. That
>shortens their life but the resulting life loss is insignicant for
>practical purposes. Many cheap flashlites run the LEDs at these
>high rates anyway. The flash will be lost etc., well before they
>die.
>


I donno . . . the fact that the white/blue/UV LEDs are rated at 20 ma
and they have a higher dropping voltage says they are using more power
than red LEDs. More power means more heat; Heat kills
semiconductors.

I guess one really has to know how much luminosity is lost by
operating the LED over-current, and how much overcurrent ,for how long
.. . .

LEDs may last forever - given that they don't overcurrent. HOWEVER,
they do lose their light output over time. They lose their light
output faster as heat/current is increased.

I know - my LED "night light" has been great for the first year - and
essentially dead for the next 4 years. It still puts out light; but I
can't read by it, or even find the right doorway. It gets 20 ma at
`1.4 volts for the last 6 years.

My feeling/guess is that any "flashlight" that uses no limiting
resistor or current control and is connected to any low impendance
source, no matter how well matched, voltage-wise, is a poor design.

It may work for a time . . . but it ain't good design practice.

Unless white LED's have some special indestructibility clause?



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