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From: ehsjr on 9 Dec 2009 00:01
John Doe wrote:
> 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?

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.


>
> http://led.linear1.org/1led.wiz
>
> Should that current limiting resistor calculator be used?

No, it does not give the proper wattage resistor for worst case
for your design. Also, you need to make some assumptions
when using a current limiting resistor, and the site does
not mention that. Finally, the forward voltage for your
LEDs is not constant - it varies, depending on how much
current is drawn.

Ed

>
> Thanks.
>
>
>
>
>
>
>
>
>
From: ehsjr on 14 Dec 2009 23:32
John Doe wrote:
> 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.
>
>
>
>

The key is that it uses 8 NiMh. At full charge, they will provide
~11.44 volts which is far lower than you would get with your
14.4V tool battery, which can provid over 17 volts immediately
after being taken off the charger. If you go with the 14.4V
battery, use the LM317 constant current circuit. If you don't,
and use the circuit in the Coleman, you'll fry either the circuit,
or the LEDs or both. My guess is that the Coleman current
limiting circuit is nothing more than a 1 ohm resistor, which
is fine for a 9.6V battery & those LEDs. Heck, they might not
even use the resistor.

Ed
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