Led Flashlight with Super capacitors V2

[sm0ky2]

the frequency, and the on-time ( duty cycle)
are the most important factors affecting efficiency
ideally, you want to pulse as quickly as possible,
and stay on as little time as possible.
this will consume the least amount of power, and greatest extended run-times

the only real requirement is that you stay above the response time of the human eye
otherwise you get a flashing, or "strobe effect" which makes the light look funny.

[MarkE]

the frequency, and the on-time ( duty cycle)
are the most important factors affecting efficiency
ideally, you want to pulse as quickly as possible,
and stay on as little time as possible.
this will consume the least amount of power, and greatest extended run-times

the only real requirement is that you stay above the response time of the human eye
otherwise you get a flashing, or "strobe effect" which makes the light look funny.

It doesn't quite work that way.  Switching at anything over about 100Hz will make flicker imperceptible. 

In most JT circuits and solar pathway lights the converter operates in discontinuous conduction mode.  Current builds up in the inductor and then the inductor energy is dumped into the LED load as a pulse.  For very clean, rectangular pulses, the amount of energy that goes into the inductor each cycle is:  0.5*V²*T_ON²/L.  The peak current increases as V*T_ON/L.    The motivation for switching at a high frequency like a MHz is to make the inductor small and limit peak current.  The devil is that the higher the switching frequency, the greater the switching losses.  Solar stick lights routinely switch at several hundred to six hundred kHz.  Discontinuous switching circuits are limited to low power because power only flows into the load during the transistor off time.  If the input voltage is 1V and the LED takes 3V then the output is powered for only about 1/3 the time and during that time the average current is one half the peak current.  That means that for 10mA average it takes 60mA peak.  That is why for higher powers, more expensive continuous conduction circuits are used, where the LED current is more or less constant.

[Hope]

Nice project. 

Assuming that your JT circuit works down to around 0.8V (many do better), then you can extract: 1-0.64/7.29 = 91% of the energy in your capacitors, or about 1990 J.   A AA alkaline battery by comparison holds about 2.4Wh or ~8800J.  So if you set-up with a pair of AA batteries, you would have roughly 9X as much energy available, but of course the unit would not be rechargeable.

Now, the only question is how much of your available energy that you are converting to light after going through your JT and LEDs.  Since you have a 40 hour run time, you are pulling about 50J/hour or 13.9mW from the capacitors.  State of the art power conversion and LEDs would yield about 2.6 lumens, (about the same as one candle ) yielding about 104 lumen hours.  You are using bulb style LEDs and JTs are notoriously inefficient, so you are probably getting more like about 0.35 - 0.50 lumens, which translates to around 14 - 20 lumen hours per charge.   If you change out the JT for a current controlled LED driver then you should be able to about double or triple your lumen hour product.  If you get some of the super efficient LEDs that are around then you can bring your lumen hour product up again by another factor of 3X or so.

Great input MarkE     Thank you for the comment,  it really helps to have someone active and interested in the projects end of the forum with the know how to improve a prototype.    What current controller would you recommend?

[MarkE]

Great input MarkE     Thank you for the comment,  it really helps to have someone active and interested in the projects end of the forum with the know how to improve a prototype.    What current controller would you recommend?

Depending on the power level you want and how you want to wire your LEDs, these ICs are all very efficient and low cost:
PAM2803 boost for one LED  Works down to 0.8V directed at 1W and 2W flashlight market.  Available from MOUSER
PAM2804 buck for one LED  Needs at least Vfd of the LED plus 0.2V  Very good for USB input.  Available from MOUSER
PAM2805 boost for one LED Works down to 0.8V Similar to PAM2803 but offers High/Low/Blink  directed at 1W and 2W flashlight market  Alibaba (beware counterfeits)
PAM2841 boost for series LED string up to 12 LEDs in series. Available from MOUSER

With careful design all of these are capable of ~90% efficiency and in some cases better.

[dinoth]

I bought on eBay LM2596 for US $2.47

http://www.ebay.com/itm/400802470941?_trksid=p2057872.m2749.l2649