Voltage, Current and Led's.

S

sketchy

New Member
Hi all

This is a bit of a general electronics post so i dont know if i am allowed to post it here but if not sorry.

I know what ohms law is and i know how to work out the resistance for an led. But I do not understant what is more important voltage or current.

I was told once if the voltage is right then the load will only draw the current it needs. When i connected my luxeon led to 3.6v by way of 3 nimh cells insteadof drawing is rated 350ma it drew 1.2a and died DOH!!! I though because the voltage was right i did not need a resistor.

Now i the final project i will be using a power supply that flutuates from 10.5v to 8.5v so to run one luxeon i need to regulate it the question is if i use a lm317 and a series resistor i can limit the current to 350ma but i think the voltage will still be 10.5 to 8.5v will this kill the led. does voltage matter if the current is limited ro should i limit the voltage and forget the current. Or have i got compleatly the wrond end of the stick.

Well im confused and now my head hurts :)

Any help much appreciated

Thanks

Joe.
 
B

BeanieBots

Moderator
When driving an LED, it the CURRENT which is important.
The voltage across the LED will vary with current, temperture, age and a few other things. You need to control the current.
If you have a LM317, with just one reistor, you can make a constant current source. Check the app notes in the datasheet.

To drive a luxeon LED at 700mA from a 8.5v to 10.5v source you really should be considering a constant current buck regulator to save wasteing all that volt-drop as heat! (but that's another subject!).
 
D

Dippy

Moderator
Heat makes things die.

LEDs (and many other things) do NOT follow the 'voltage-is-right' law.

Light bulbs do.

You must limit the current going through an LED.

Yes, a regulator and suitable resistor by all means.
Usually, R = (Vsupply-Vdropled) / Current.

But BEWARE. Some specs for those power LEDs refer to:-
A) Pulsed performance
and VERY IMPORTANTLY B) adequate heatsinking.

Power LEDs, unless premounted on a big thing, need HEATSINKING.

ALL the Data Sheets I have seen give land info for soldering and heat dissipation.

LEDs are NOT MAGIC COOL BULBS. They get hot, very hot when pushed to the max.

Pukka power LED drivers use Constant Current drivers, usually PWMd (switched buck or similar).
If you are vaguely interested have a look at Zetex website for inspiration.
 
E

eclectic

Moderator
@Joe.
Firstly, look again at posts 2+3. Then print them.
As well as being expensive, LED's smell 'orrible when they pop.

Second, have a good read of these pages

http://www.kpsec.freeuk.com/components/led.htm

Third. Power supply varies 8 – 10 volts?
How's the wallet?
Wouldn't you be safer investing in a current-limiting
bench power supply?

Lastly, you'll need something like these
http://www.rapidonline.com/Electronic-Components/Resistors-Potentiometer/Power-Resistors/3W-Metal-oxide-resistors/66471

“ordinary” resistors go pop with high power LED's.
(how do I know?)

e
 
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