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From: John Doe on 8 Dec 2009 12:19
ehsjr <ehsjr(a)nospamverizon.net> wrote:

> LEDs are best driven by a constant current supply,

That looks like a clue... Thanks.

> like the one drawn below.
>
> -----
> +14.4 ---in|LM317|out---+
> ----- |
> adj [R]
> | |
> +---------+---[LED]-[LED]-[LED]---+
> |
> Gnd --------------------------------------------+
>
> Choose R for the current you want by R = 1.25/I
> According to the LED data sheet, the maximum current is 1 amp,
> so R must be no less that 1.25 ohms. The resistor would dissipate
> 1.25 watts at that value, so 2 watts or higher should be used.
>
> The LM317 will dissipate ~3.3 watts nominal at that current, but
> over 6 watts if the battery is at full charge voltage (over 17
> volts) and taken immediately off the charger and connected to
> the circuit. So, size the heat sink accordingly.
>
> You could use a 7812 in the same configuration with R = 12/I,
> but the resistor would need to be about 10 times the wattage
> of the resistor used with the LM317.
>
> I would recommend running the thing at well under maximum current
> for longer run time, less heat, and longer LED life. If you use
> 3.5 ohms for R, I will be ~357 mA, R will need to dissipate about
> 1/2 watt, and the LM317 dissipation will be under 3 watts worst
> case, and about 1.6 watts nominal. You'd get more than triple
> the run time that way, too.

What happens when only a current limiting resistor is used, provided
that current is kept under the maximum safe current for the LED?

http://led.linear1.org/1led.wiz

Should that current limiting resistor calculator be used?

Thanks.









From: John Doe on 14 Dec 2009 22:19
ehsjr <ehsjr(a)nospamverizon.net> wrote:

> John Doe wrote:

....

>> What happens when only a current limiting resistor is used,
>> provided that current is kept under the maximum safe current
>> for the LED?
>
> The current through the LEDs changes as the battery voltage
> decreases. If that produces acceptable results, it is the
> simplest way to go. Note that the LED brightness will decrease
> as the voltage drops, which may make it unacceptable.
>
> Using a constant current source keeps the brightness of the LEDs
> at the same level - until the battery discharges too low to
> maintain the current.

At the moment, I am using the circuit that came with the Coleman
LED spotlight, minus the sealed lead acid battery charger and
momentary switch circuits. Using eight AA NiMH batteries... The
OFF current is about 3 microamps, so I guess the wiring is
correct. After charging, the ON current starts out at about 910
milliamps and quickly drops to about 700 milliamps. The current
drops slowly to about 600 milliamps and then slows to a crawl.
Apparently it is not a current limiting circuit, but maybe
efficient enough.




--
thanks to the replies
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