Intensity vs Duration

A little challenge for all you electronic whiz kids. Everybody is into the light "intensity" business. I'm looking for duration as well. Would the following be possible: Power source: 3 AA or the typical Petzl 4.5V block battery. Light source: a single 1W LS. Requirements: Design a circuit that uses a minimum of 280mA (at that current the light intensity is more than sufficient) BUT the light is to last at least 10 hours! Is it possible or how close can you get? I've used a simple voltage regulated circuit powered by 3 Sanyo 1700mAH to obtain 5 1/2 hours. Do that and I can possibly generate a nice market for the seller!

Gonna venture a guess here:

You could just pulse it with a PWM. Don't they make those neat PWM circuits there in Africa? I remember reading about them in some other post...

Only thing is, the light may flicker if the pulse width (or is it the pulse gap) is too big. I'm quite sure you'd need a current regulator like Wayne's BadBoy somewhere in there to regulate the current, leaving the PWM circuit to switch.

Those PWM switches will be available in a CPF group buy fairly soon ... once samples arrive and some testing gets done. You read it here first.

I'm guessing, but wouldn't 10 continuous hours at a constant 280 mA be a something of a stretch for the capacity 3 AA cells, in series, offer?

If any regulator could do it, I suppose dat2zip's BADBOY circuit set to regulate at that output would come closest.

Do the math guys. 280 mAH x 10 hours= 2800mA battery capacity per battery. 3.6V for three rechargeables AA's in series, and 3.6V needed for the LS. That means this would work with a regulated circuit and alkaline or lithium AA's, but not with rechargeable.

Originally posted by Starlight:
Do the math guys. 280 mAH x 10 hours= 2800mA battery capacity per battery. 3.6V for three rechargeables AA's in series, and 3.6V needed for the LS. That means this would work with a regulated circuit and alkaline or lithium AA's, but not with rechargeable.

Click to expand...

<font size="2" face="Verdana, Arial">Ahh... but you're forgetting about the PWM. That should be able to stretch battery life quite a bit depending on the switching freq.

Here's how I see this.

280mA @ 3.3V is .924W.

Assuming 100% efficiency then at 4.5V (3cell) you would need .924W / 4.5V 205mA.

If you had NIMH typical batteries with 1800mA/HR rating and you could get 100% of that out of the batteries you'd get. 1.8 / .205 = 8.766 Hours.

That's absolutely ideal. None of the above things are ideal so factor 80% of that or 70% of that and you would get more realistally 6.1H and 7.0 Hrs constantly.

BTW: You would need what is called a sepic converter since Vin can be higher or lower than Vout. Assumption is dead batteries at 0.8V/Cell * 3 = 2.4V. LS ~= 3.3V.

Now, lets say we cheat and start with regulation for 50% of the time. This complicates things but it will generate a longer run time. I won't go into the specifics of this but you can take the area under a trapezoid to determine the average current. The trapezoid will allow you to stretch out the run time 2-5 times the constant regulation.

O, dang. tooo technical again....

Summary:

I don't think you can get 10 hours with 3 AA batteries and 280mA to a 1W luxeon load.

-WayneY

Sorry Wayne, I'm quite dense at times, but did your explanation factor in the use of a PWM circuit used together with a current regulator, or is it just the current regulator on constant load?

I think McGizmo raised a similar question in another thread about using a PWM switcher together with your BadBoy to get more runtime. Is there any short answer as to whether this is a good idea? TIA

The simple answer is no I did not consider or include PWM in the explanation.

I'm looking into several solutions, PWM, Pin controllable dimming, micro processor control smart functions etc...

-WayneY

Well, you what they say - shoot for the stars and you might hit the moon... OK according to the maths, only alkaline or lithiums might get close - alkalines would be OK since lithiums... $$$$!!! (over here in any case). What about the 3LR12 battery used in the Petzl? Normal burntime with a 3.6V 0.22A bulb is about 15-17 hours, with 3 AA alkalines it is 6-8 hours. With a 4V 0.5A Halogen bulb this drops to about 6 and 2.5 hours respectively. (You might note that Petzl is a bit more optimistic about the times, but above times were obtained through many sets of batteries in actual operating conditions.) Furthermore, there are loads of space inside the Petzl Zoom head, so designing a very small circuit is not required.

Hmmm. Was too lazy to do the math. Glad you did.

You could get 10 hours at a constant 280 ma if you could completely drain the batteries ... to 0 ma remaining ...

Isn't it battery in series = voltage adds while amperage remains the same?

Then, with a 1.7v cutoff by the BADBOY regulator (.8v for MADMAX) ... can you get all 2800 ma's out? Maybe dat2zip can help out on this.

Wow, I just posted an inquiry about putting a 1-watt LS in a Petzl Zoom in another thread. Any update on this? I was thinking of a Zoome version of Lambda's 3D Maglite Luxeon "Beamer" but I like the idea of balancing intensity with burntime. Any update on this?