How to choose resistor for direct drive?

[dandism]

I want to direct drive an xp-g R5 with 3 AA cells (nimh or alkaline) but am unsure of picking a resistor. How do I do this?

-Dan

 

[ledpwr]

This website should help

http://led.linear1.org/1led.wiz

It will pick a resistor value for you but you need to know what the forward voltage is at the current you want to drive it at.

At 1500ma the average forward voltage of an XP-G is 3.55V but it could be a bit more or less.

I would advise using nimh as they have a steadier voltage and a much lower internal resistance so it will stay brighter for longer :)

 

[alpg88]

I want to direct drive an xp-g R5 with 3 AA cells (nimh or alkaline) but am unsure of picking a resistor. How do I do this?

-Dan

if you wanna drive it at 1a than you need
for nimh 0.3 ohm ,0.5w

for 1,4A drive
for nimh 0,2ohm 0.5w

i didn't even calculate for alkalines AA since they can't phisycly supply steady even 1A,
lithinum AA can, for them you need
1.2 ohm 1w for 1a drive, or 0.8ohm 1w for 1.4 amps draw.

 

[dandism]

Well, I use eneloops, but want to be able to use alkalines just in case.

 

[ledpwr]

You will be able to run it off alkalines and it will work without any modification but, as said it won't be as bright and it will rapidly dim to a lower current draw where it will continue to dim but at a slower rate. This means you will only get around 20 minutes before it gets below half power. (it could be a bit more or less)

 

[dandism]

You will be able to run it off alkalines and it will work without any modification but, as said it won't be as bright and it will rapidly dim to a lower current draw where it will continue to dim but at a slower rate. This means you will only get around 20 minutes before it gets below half power. (it could be a bit more or less)

I don't really care about performance with alkaline, I just don't want it to burn out the LED.

Thanks for the help,

Dan