Two 5W Luxeon Stars in series?

[BentHeadTX]

Was pondering two 5W Luxeon Stars in series today.
That should handle the voltage of my 7.2V NiCad, now to handle the current. Figure mild undervolting of the 5w LS and some sort of current limiting should give off a rather blinding light for winter bicycle riding.

Any idea about a current limiter that could handle 2 amps?

 

[Nerd]

5 watt LS run at around 6.8-7.? volts? So if you connect them in series, you will be underdriving them. Maybe one will light up and the other won't. Why not in parallel?

 

[lux0]

R = (7.2 - 7)/.35A
R = 0.571 Ohms

Pr = I^2 * R
Pr = 0.07W

70mW is not a lot of heat.

 

[INRETECH]

If you connect them up in series, the voltage may not be even enough to make them glow

Run them in parallel

 

[BentHeadTX]

Thanks for the 7V tip on the 5w LS, now how to figure out how to do current limiting without resistors. I can imagine the heat a ceramic resistor would put off in that configuration!

 

[INRETECH]

The current for the 5w LS, are .750A not .350A

Mike
www.inretech.com

 

[BentHeadTX]

Thanks Lux0 and Inretech,
So using the formula with .75A and 8VDC (the NiCad fully charged) would equal 1.33333 ohm resistor and .75 watts of heat. For more protection, and a larger resistor to remove the heat... I will go with a 2W 1.3333 ohm resistor. (Maybe four 6 ohm 1/2 watt resistors in parallel)

Now, to get a LS 5W, Artic Silver conductive epoxy,a resistor and a block of aluminum! Thanks for your input on the correct output.

 

[PsycoBob[Q2]]

Rather than using a block of aluminum, an old Pentium-2 style heatsink would work well. Large, rectangular sink, old enough to be dirt-cheap.

 

[Jonathan]

Hey Bent,

I think that you should run the equation with a number of different battery voltages.

When your '7.2V' battery is fully charged, it might be as high as 8.4V, but only for a few seconds. By the time it discharges, it will be at about 6V. For most of the discharge period it will be at about 7.2V.

I think that you will find that the required resistance to get the desired current changes _drastically_ over the course of the battery discharge.

If you are going to go with a resistor for current limiting (and some current limiting is _essential_ when you use NiCd and NiMH cells to run LEDs), then you might want to rig a way to _change_ the resistance.

-Jon

 

[BentHeadTX]

Jon,
True, the voltage will sag on those NiCads so I calculated for 8VDC just for extra safety. I do have a bunch of heatsinks laying around (gotta overclock that computer after all!) Maybe use a cut-down Volcano7 copper/aluminum finned heatsink for fun. Should be a fun winter project when the white LS5's come out. Surely this board will "light up" with a lot of user mods and ramblings on the next latest-and-greatest LED. Thanks for the clarification on this one...can't wait until December!