[sdiy] voltage and LEDs

[Stewart Pye]

The specs you have given said 1000mA max for the LED. I'd try it at 
about 500mA and you can go up from there.
Here's an app note on using the LM317 as a current source:

http://www.onsemi.com/pub_link/Collateral/AND8109-D.PDF

Using the circuit in Fig1, Fig 2:  if you use a 2.2 ohm resistor for R 
sense it will output about 560mA.
For the power rating of the resistor: P = V*V / R   =   1.25*1.25 / 
2.2   =  0.71W
So use a 1W or preferably 2W resistor, and you'll need a heatsink on the 
regulator. Larger the input voltage the bigger the heatsink.


Stew.


 



db wrote:
>
>
> So do these LEDs actually need 1000ma to drive them, and a single 7805 
> is pushed to it's limit but powering one?
> So maybe a 2ohm 5watt resistor in series?
>
> I'm still having a hard time with this concept of "constant current".
> What makes current constant?
> Not just a regular power supply?
>
>
> At 2/11/2010 07:41 AM, George Mattson wrote:
>
>> The forward voltage is what the diode is going to use up across the PN
>> junction. It just "goes away"
>> Actually, that's the voltage required to get the current to cross the
>> barrier region. Consider it the PN road bump.
>>
>> Using 5 V, subtract the 3.9V Vf from 5V to get the difference of 1.1V.
>> 1000mA max current is 1 Amp. Using ohms law divide 1.1V by 1 Amp to 
>> get 1.1
>> ohms to limit the current.
>>
>> That's the minimum resistance necessary to prevent burning up the LED in
>> regards to the specs. Anything less will just dim it down. Usually, 
>> you can
>> get by with half the current. Anything above a certain limit doesn't 
>> let the
>> LED get any brighter, just dissipates as heat at the junction and 
>> shortens
>> the life of the semiconductor.
>>
>> But, 1 Amp is also the high limit of the 7805
>>
>> You'd either need a huge heat sink and a fan to keep it cool. Or, use 
>> two in
>> parallel and let them split the load.
>>
>> The 1.1 ohm resistor at 1 Amp is going to dissipate 1.1 Watts of power
>> (P=IE)
>>
>> I'd suggest using a 2 to 5 W resistor and mounting them above the 
>> board to
>> let them get some air to cool down.
>>
>>
>> My $0.02
>>
>> George Mattson
>>
>>
>> I want to drive a 3watt led
>> # 1000mA - Max Drive Current
>> # 3.90Vf - Forward Voltage
>>
>> Can just use a 7805 to get 5v, and then drop it with two diode in 
>> series to
>> get 3.6v?
>> Or do I still need resistors in series with these LEDS?
>>
>>
>> dennis.barton:skylab2000[socal]
>>
>>
>>
>> : 9.0.733 / Virus Database: 271.1.1/2680 - Release Date: 02/10/10 
>> 11:38:00
>
>
> dennis.barton:skylab2000[socal]
>
>
>
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