Help on resistor value - is that right?

[gad]

I will put a 3w led in a mag 4c cells. The value of the resistor calculated is 2ohm, 2 watt. I have found the 2.2 ohm, 2watt. Is that right? I will not overdrive the led?
Thanks.

 

[red_robby]

1 amp is still in spec for a LS3

 

[Crux]

If the Lux3 is a K suffix (i.e. SVOK, SWAK,...) then its voltage drop at 700mA is 3.51V to 3.75V. This drop will increase slightly at higher current.

With 6 Volts (4 alkaline C cells) applied and 3.75V for the Lux3, a 2.2ohm resistor will drop 2.25 volts and limit current to 1.02 amp.
However, the power dissipated by the resistor is 2.25V x 1.02A = 2.3watts. The resistor will get quite hot and may fail. A 3,4,or 5 watt would be a better choice but it will still get hot, 2.3watts has to go somewhere.

If the Lux3 is a lower voltage J type (3.27V to 3.51V) then things get worse for the resistor and efficiency.
The 2.2ohm resistor would limit current to 1.14 amp and dissipate 2.8 watts. If the Lux3 volage is higher then things get better. Its easy to see where a convertor is useful.

Running a Lux3 from 4 NiMh cells is almost perfect, the resistor can drop to 1.2 ohm and dissipation is only 1.2 watt at 1 amp of drive current. I'm not sure how runtime will be affected, NiMh have less capacity than alkalines but they hold their voltage very steady over the discharge curve.

I've rambled enough... if you're still with me.

Sleep bekons me, Crux /ubbthreads/images/graemlins/sleepy.gif

 

[springnr]

Three cells would be better.

 

[DrJ]

.

 

[gad]

Thank you people. You have help me a lot. I think I will try to found a 2.2 ohm, 3watt resistor, even for use with alkalines, cause I don't want to live on the edge with the led.

Really thanks.

 

[gad]

Just another question. I have another mag, but 3D cells. I am thinking about a 0,68 ohm for put in it a 3w. Good choice?

 

[Crux]

[ QUOTE ]
gad said:
Just another question. I have another mag, but 3D cells. I am thinking about a 0,68 ohm for put in it a 3w. Good choice?

[/ QUOTE ]

Yes, 0.68ohm at 3watt or even a 2watt would work here.

 

[G Pilot]

Can I but in here....... what is a K-ohm? I have some laying around marked 4.7k-ohm 1/2 watt?

Thanks

 

[The_LED_Museum]

A K-ohm is 1,000 ohms. So that 4.7K resistor is 4,700 ohms.

 

[INRETECH]

If you use the 4.7k resistors, your LED might glow - but I don't think it will put out the amount of light you are looking for

But, your batteries will last a LONG time

 

[G Pilot]

Thanks......maybe Radio "Shaq" will let me exchange them?