Which converter?

[Kill-O-Zap]

I would like to drive a 5W LS, preferably even overdrive, with some converter or combination of converters (parallel I suppose) fed by just two NiMH C cells. I don't care if the converter(s) are operating in "regulation" mode or just huffin and puffin trying to meet demand, since the NiMH's themselves will provide as much regulation as I require. However, efficiency is a concern; I'd like to keep it above 75% if possible. I don't mind using multiple converters, and I suspect I'll have to, but...

which converters can do what I want? and how many would I need?

What would a single BB do when faced with a 5W LS (let's say K Vf rank) on <2.4 Vin? What would a MM+ do? Is it safe to try, or would I fry the converter?

If there's no solution, I could use 4@1/2C cells, but they are only 1300mAh vs. 4000+ for the C's, so I'd lose quite a lot of runtime, that's why the contorsions to use just two cells.

Thanks for any suggestions, especially from Wayne /ubbthreads/images/graemlins/grin.gif.

 

[dat2zip]

Madmax is out. Can't use it to drive a 5W.

As for Badboy. Let me do a quick calculation. Per my technical web site (which isn't very technical) the formula for drop out is:

Vin * 1.5 = (Vf * Iout)/0.8

Lets compute Iout for a drop out Vin of say 2V. That's a good number which translates to 1V/Cell which is a common reference to a dead cell.

2V * 1.5A = (6.8V * Iout) / 0.8

Solving for Iout = ~.35A

So, using one, two or more 0.35A Badboys, they would hold regulation to 2V.

Two 0.35A Badboys would give 700mA of regulation.

The closest to a BB350 is the BB400. Two BB400s would be 800mA of current regulation to the 5W.

Your battery demand will be up to 3A+ at or near end of regulation.

Wayne

 

[Kill-O-Zap]

Thanks Wayne!

I'm surprised at what these little buggers are capable of, to be able to drive the 5Watters even at 350ma from 2V. So then a BB400 would drive a 5W with full regulation at 2.4 V? Somehow I had come away with the impression that the cutoff was a lot higher than that. Impressive.

I hadn't realized how high current draw at the batteries would be; of course that's not the converters' fault, even with a 100% efficient converter that's still >5W out on 2V in, that would still be over 2.5A on the batteries. I am a little worried about such high current, particularly through the switch.

The 4 cell configuration may be safer, plus it gives the same performance with just one converter, though it gives up ~35% on the runtime due to the low capacity of the cells. Not sure which way I'll go, but at least the 2 cell route seems viable.

Thanks again for your help.

 

[Entropy]

[ QUOTE ]
Kill-O-Zap said:
Thanks Wayne!

I'm surprised at what these little buggers are capable of, to be able to drive the 5Watters even at 350ma from 2V. So then a BB400 would drive a 5W with full regulation at 2.4 V? Somehow I had come away with the impression that the cutoff was a lot higher than that. Impressive.

I hadn't realized how high current draw at the batteries would be; of course that's not the converters' fault, even with a 100% efficient converter that's still >5W out on 2V in, that would still be over 2.5A on the batteries. I am a little worried about such high current, particularly through the switch.

The 4 cell configuration may be safer, plus it gives the same performance with just one converter, though it gives up ~35% on the runtime due to the low capacity of the cells. Not sure which way I'll go, but at least the 2 cell route seems viable.

Thanks again for your help.


[/ QUOTE ]
Last time I ran the calculations, it was about 2.26v for a BB400 driving a 5W. Depends on Vf though. I was assuming 7v or 7.2v (Can't remember - Basically depends on the Luxeon binning.)

So many mods so little money... /ubbthreads/images/graemlins/frown.gif