LM317T Voltage Regulator + LEDs

[samnissan2003]

Is it safe to have the regulator for 2 volts to power up an LED without a resistor?

 

[JohnBrown]

Depends what you mean by "safe". I wouldn't do it.
If you search these forums you'll find a wealth of differing opinions amongst recent threads.

 

[samcheetah]

i agree with JohnBrown. an LED is basically a diode and if you have seen the Voltage versus Current graph of a diode you will see that after the voltage across the diode exceeds the knee voltage (0.7 for silicon diodes) the current rises exponentially. ideally the current will be infinite for any voltage greater than the knee voltage. now as you have said that you are using a regulator, you will definitely have some ripple on the output. so the voltage might go more than 2V and thus alot of current might pass through the LED. thats why an LED is operated with a current limiting resistor. the resistor limits the current to a value that ensures the LED is not damaged. and the diode action is the same. when the voltage across the diode exceeds the barrier voltage (knee) the LED emits light but the current doesnt go to infinity because of the current limiting resistor

i hope that helps

 

[checkmate]

I too agree that it all depends on your definition of "safe". In practice, I've run LEDs straight across 5v rails before and it din't blow straight away. Does that mean it's safe? The resistor is there for protection, but it's there more for a peace of mind for me. There's lots of other design issues to worry about other than deciding whether to save that cheapo resistor.

 

[TheOne]

now as you have said that you are using a regulator, you will definitely have some ripple on the output. so the voltage might go more than 2V

Ripple? Correctly configured the 317 will have between 60-80dB ripple rejection depending if a cap is used or not on the ADJ pin.

 

[Nigel Goodwin]

Is it safe to have the regulator for 2 volts to power up an LED without a resistor?

It's certainly not advisable, and would be a really poor design choice!.

 

[TheOne]

Also different colored led's have different voltage drops. You can configure the 317 as constant current source but it is cheaper and simpler to use a resistor

 

[samnissan2003]

LED
HLMP-EH08-Y20DD

i need to light up a large number of LEDs, the regulator circuit i have does have a cap for filtering purpose.

 

[samnissan2003]

Im pretty familiar with electronics, i even have a degree, but i already forgot most of the stuff, and the funny thing im still paying school loans, anyways im working on a car tail light made from LEDs, ive made one in the past, its a very risky setup but not a single LED has gone out...

http://www.nissanclub.com/forums/showthread.php?t=124857

now this is the new light im working on

http://www.imagecave.com/albums.asp?album=Altezza_Project_LED

the LEDs im using are

HLMP-EH08-Y20DD

im very happy with these lights, very bright, i tested one some time ago, it can handle 145mA and still be on with out burning, ofcourse at these conditions the life of the led goes down the drain