Fatman regulator question

Hi everyone, I'm working on a small project and have a quick question about the fatman's capabilities. Is it possible to wire 6 luxeon I stars in 2 sets of 3 (3 connected in series, and then each group of 2 connected in parallel) with the fatman set to 1000mA to power all 6? Can it handle it? Thanks!

no can do.. i want to do the same thing..i'm using (2) fatmen.. george has some special drivers that can handle that much but they are much bigger in size..

basically the easiest way to calculate the capability of the FM is to calculate the input current.. it has to be less than 2A.. since your are talking about 21W.. and FM is a boost converter.. the current would be a lot higher than 2A.

you can always check the specs on taskled.com and if you can't find the answers.. email george.. he is 'the bomb'.

Thanks for the info - I'll be sure to check out the site to double check. I re-read my post though and now I think I made a mistake: what I want to do is use 6 Luxeon I stars each driven at 300mA. I know the Fatman has a 12V limit, and I would likely be using 2 18650 2400mAh LiIon cells to power it. If each Lux has a Vf of roughly 3.2 volts, that is 9.6 volts at 300mA that the fatman has to be able to provide per set - if wired parallel with the other set then - if I'm understanding everything right - the Fatman would need to provide 9.6 volts still but at 600mA? For the W=V*I formula that would only be 5.76 watts of power - so to rephrase (though I'm sure you are still probably right anyway), I want to know if the Fatman can power 9.6 volts @ 600mA from a 7.2-8.4 2400mAh source (assuming I added everything right). Thanks again,

Dan

Oh.. much different question.. 6W is a much different animal... i've never tried to use it for 1W devices..

600mA is only about 1/3 of the input max on fatman.. and it's a boost driver so 7.2V -> 9.6V looks good to me.. i've used fatmen to output to 3 lux 3's..

I just looked it up and it's good for input up to 12V and output up to 16V.. so.. you could probably do 5... but not six.. other than very under driven.

I would look at the myriad possibilities.. you can put the stars in parallel/series combination.. in other words.. if you put 2 strings of 3.. you are looking at 700mA to drive them at 350.. and now you 1/2 the voltage and are at .. 9.6V.. which is still above 7.2..

now.. 9.6V x 700mA = 6.72W.. divide by .85 and get 7.9.. and that's the power from the battery.. now divide by Vbat (assuming two LiONs).. and the current is: 2.13.. whoops.. just a bit high.. ok.. so multiply by 300/350 since you only wanted to run them 300mA.. i was going for max.. and now you are down to 1.83A from the batteries which is more in-line with what they'd like to supply anyhow.. so.. that's the answer i would try..

2 strings of 3 series lux1s.. you can probably run them close to 325mA in this configuration.. the fatman will know it's limit.. just turn it up 'til it stops going up and back it off a bit for an operating margin.

-awr

2 strings of 3 ought to be fine... if the Vf sums of each string are almost exactly the same and stay that way. You might even want to "burn-in" your Luxeons so their Vf's stablize.

The only real limits I'm aware of on the fatman are that it's a boost driver so Vin (2.7V-12V) needs to be less than Vout (up to 16V), and the supply current cannot exceed 2500mA.

Thanks for the info! I just noticed a few mods being done with the new Cree parts and I think I might try this with those instead. One last question though (not necessarily fatman related) - right now I'm driving 3 lux iii's at 900mA and 10.5 volts for a total power output of about 9.45 watts. So if I was to wire up 3 cree parts (assume 3.5 volts Vf again) for 300mA each would the overall power draw be one third (3.15 watts) since it is only a 3rd of the power? If so, is it possible (again assuming the exact same Vfs) to wire up 3 sets of this in parallel for the same 900mA at 10.5 volts? With these new 40-60lm parts from Cree (and so small too - though I would still get the heatsink and just file it down to separate the + and - connections) it is very tempting to have a 9-led light putting out 360 to 450 lumens for the same amount of power as 3 luxeon iiis! Thanks again,

Dan

sounds like you did the math correct.. it's a bit tricky trying to balance the Vfs so that each branch runs the same level... most LED based lighting designs that have strings like this have a series resistance at the beginning of each string.. with lux's could be like 0.1 or 0.3 ohm... that way if that string trys to draw more current it will have more voltage drop on that series resistance.. and level out the current.. 9 LEDs lots to fit in a small place.. but go for it.. i have a 19 emitter design.. do a serach for 'gigasonic' on cpf /ubbthreads/images/graemlins/wink.gif

-awr