On Wed, 23 Jan 2008 19:03:33 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

>On Jan 24, 12:05 pm, ehsjr <eh...@bellatlantic.net> wrote:
>> Dave.H wrote:
>> > I'm looking to convert a flashlight I have to LED, the flashlight uses
>> > a 6 volt "lantern battery but the LED's max voltage rating is 3.1
>> > volts DC. What value resistor would I need for this? I also want to
>> > run it off a 9 volt supply, again I need to know the value of the
>> > resistor.
>>
>> > The LED is part # Z4251 atwww.dse.com.au.
>>
>> I didn't see a constant current solution posted, probably
>> because it is costs more than using just a series resistor.
>> But constant current was mentioned, so here's a constant
>> current circuit that uses 1 watt resistors and an LM317
>> chip, and works either 6 or 9 volts input. It provides
>> about 347 mA to the LED.
>>
>> -----
>> + ----Vin|LM317|Vout---+-----+
>> ----- | |
>> Adj [3R9] [47R]
>> | | |
>> +----------+-----+ I = 347 mA
>> |
>> [LED]
>> |
>> Gnd --------------------------+
>>
>> The chip needs to be installed on a heat sink. I would omit the 47
>> ohm resistor to reduce the current to about 320 mA. That's kinder
>> to the LED.
>>
>> Dick Smith has the heatsink for $1.25, the LM317 for $2.10 and
>> the resistors for 10 cents each.
>>
>> Ed
>
>This is an easy enough circuit to build, I'll go with this one, thanks.

You might want to sub a low drop out regulator so it will work down to
the last dregs of the battery charge.

If you are running on batteries, a switch mode current supply is a
better choice (if battery life and cost are a concern). The idea is
to regulate the current without producing heat with the XS voltage. A
switch mode constant current supply will us a pulse width modulator to
regulate current and an inductor to smooth the current to the LED.
Unless you buy a ready made LED regulator, it is more complicated, but
with the numbers you're looking at you'd double the battery life with
a switching regulator.

But this discussion started with what resistor to use and resistors
aren't efficient current limiters . . . the regulator idea wastes as
much power as a resistor but protects the LED much better.

"More than one way to skin a cat" as the expression goes.
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