LEDs, resistors & Li-Ion batteries

[sfer1]

I'm not 100% sure what resistor I need.

* Red Seoul P4 LED (forward voltage: 2.5V, mA: 800-1000)
* MR FX board
* AW 3.7V Li-Ion battery (2900mAh)

I understand the FX board needs ~0.5V, which would leave ~3.2V for the LED. A 1 Ohm 2 Watt resistor should be enough in this case.

Li-Ion batteries are 4.2V when freshly charged though.

If I used a 1 Ohm 2 Watt resistor, would the initial higher voltage of the Li-Ion battery fry my LED?

[KuroChou]

I wouldn't think half a volt would fry an LED.
Might shorten the lifespan, and wouldn't necessarily run it any brighter, but that small of a change shouldn't make much difference.

[jjshumpert]

no need for resistors with mr boards, think of it as a buckpuck with sound

[Rafalema]

Recently some have reported burnt red P4's driven by MR boards without resistor.

A small resistance resistor should do it, but I'm not an expert on this.

[jjshumpert]

i have a vader on the bench that has been running a p4 red for over a year with no issue

[sfer1]

no need for resistors with mr boards, think of it as a buckpuck with sound

That may be true for Luxeon III's, but Seoul P4's require lower voltage than them.

I tried driving a Red P4 by an MR FX board without resistor (I used a 4.8V battery pack then) and the LED got really hot within seconds.

[jjshumpert]

yeah they all do that buddy, thats what the heat sink is for

[sfer1]

The red P4 was secured to a copper heatsink, buddy.

[jjshumpert]

well like i said, i have an mr board running a p4 red, in fact im holding onto it as im typing this, and with it running it is warm, but the bh and heat sink takes care of that just fine. and i have had this led in this setup for a year with no issues what so ever. and with mr boards i have more experience than most members on this forum, so take my knowledgeable advice or dont, thats up to you

oh yeah forgot, this setup is running on 6 volts, which also is no problem for either the board or the led

[Skottsaber]

Shawn, the older batches of P4 reds ran fine off of higher voltages, but recently they don't survive and blow with the extra.
I have seen LDM WIP pics of resistored P4s off of Vader boards, and Wade confirmed he had blown a few in the shoutbox.

Sfer1, I don't know the exact output voltage of the MR board offhand, so I would suggest either testing the output with a multimeter if you have one, or waiting for somebody with more knowledge than me chime in here, and then we can calculate the resistor you require