Bad Boy question

[StoneDog]

Hi all, I'm hoping someone can help clear up something that for some reason is confusing the heck out of me.

I've got a 7LED (5mm) assembly pulled from a Streamlight ProPolymer 4AA. Each LED appears to have its own 22ohm resistor soldered to it. If I wanted to do something crazy like install the assembly as-is into a 2D light I would need a BadBoy, MadMax or some other boost converter to get decent light out of it. This much I understand. /ubbthreads/images/graemlins/smile.gif But, if I want to run the little LEDs at spec or a little over (say 30ma each) could I get away with a custom-ordered BB210 or does the added resistance mean the BB needs to generate more current than this?

I'm really confused... If I'm reading the resistors right (red, red, black, gold) they're 22 ohms each and with 6v on fresh alkalines the poor little LEDs are getting 6v/22ohm = approx 273ma each? That just doesn't seem right?!

Jon

 

[stefx]

Re: Bad Boy / Confusing situation...

I=(6V-Vf)/(22ohm+Ri)

If the LED's Vf (at this current) is 4.2V and the internal resistance of each AA cell is 2 Ohm, then the LEDs are driven at 1.8V/30ohm = 60mA.

Stef(an).

 

[StoneDog]

Re: Bad Boy / Confusing situation...

Stefan,

OK, I knew I was missing something there, thanks!

Any thoughts on whether the BB needs to regulate output at 210ma to get 30ma to each LED?

Jon

 

[StoneDog]

Can anyone offer a suggestion? I've got a 7 LED assembly, each LED has a 22ohm resistor attached. I'd really like to put this in 2D light to use for emergencies but need a boost circuit to get useful light out of it. I would guess I need a BB210 or so to get 30ma to each LED. Is this right or is the calculation more complicated with all that extra resistance?

Anyone?

Jon

 

[Flatscan]

Have you used the light stock at all? It's so well known for overdriving (and thus burning out) the LEDs that I'd just pitch it, or at least put it in a non-critical application.

The BB is a constant-current driver, so the resistors don't affect the total amount of current at all. They do, however, affect the distribution of current among the 7 LEDs, probably making it more even. Using a BB210 should work, but it's suboptimal.

Running LEDs in parallel off a constant-current source seems like a contradiction of sorts, but I can't think of a better way to set it up. I was about to suggest rewiring them in series (assuming that's possible), but none of the readily available drivers boost enough.

 

[StoneDog]

Flatscan,

Thanks for the response. No, I haven't used the stock light for more than 60 seconds total. The ProPoly 4AA now houses a custom heatsink and an R3J LD being driven at about 250ma by a DB converter (SL should do this with all of their ProPoly LED lights!!!).

Why is a BB210 suboptimal in this case? It will have to boost a bit more to overcome the extra resistance - is that why?

Jon

 

[StoneDog]

Well, I went ahead and ordered a BB210 anyway. I will be putting it and the LED assembly into a $5 Ray-o-Vac "industrial" 2D light. /ubbthreads/images/graemlins/icon15.gif It's actually a nice light for what it is, even has a little 3-led voltage indicator that goes from green to yellow to red. Assuming this all works I will play with it a bit and then stow it away in a closet for emergencies.

I'm expecting long rutimes as the converter should draw less than 350ma on fresh "D" cells. The light won't be impressive by any stretch (especially compared to the typical modded flashlight here on CPF) but it should be quite useable.

Jon