[sdiy] voltage and LEDs

[Oren Leavitt]

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?
>

Dennis,
A constant current source maintains a preset current through a load (in 
this case the LED) regardless of the input voltage and load resistance - 
like a voltage regulator maintains a preset voltage regardless of input 
voltage and the amount of current the load draws.

You can use an LM317T adj voltage regulator as a 1A (Assuming your LED 
needs 1A) constant current source with just one external resistor - I 
think there's an example in the 317 data sheet.

The 317 circuit goes in series between the power LED and V+ most any 
unregulated DC voltage. Be sure to heat sink the LM317. Keep in mind 
that the LM317 will dissipate more heat the higher the input voltage used.

Before you use your expensive LED, to verify that it works, feed it into 
a 1 ohm power resistor. You should measure a steady 1 volt across the 
resistor (1 amp through 1 ohm) regardless of input voltage.

An unregulated 6VDC 1+A wall wart would work great and not run the 317 
too hot.

- Oren

> 
> 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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