LM 317 in standlight circuit?

jack in the matrix

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I'm curious about the LM 317, a path not taken from the 2008 standlight thread, which I gleaned in 2011. I picked up a few while shopping at DigiKey, because I'm planning to go down that road soon, and was wondering if anyone out there knows anything about its use in dynamo standlight circuits? I've got the basics, but still have a few questions. Thanks.
 

Steve K

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the standlight thread was a while back, as well as being rather long. Can you point us to the circuit that you have questions about, or sketch out what you are thinking of doing?
 

Steve K

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http://www.candlepowerforums.com/vb...his-circuit-work-)/page2&highlight=Standlight

Post 35 on page 2 explains it all with an image of the circuit in post 39. I want to use that in my circuit, but across 2 of 3 LEDs. Having some problems.

The disadvantage of driving two LEDs with this circuit is that a cap charged to 5.5V will barely be able to put current through two LEDs.

Another disadvantage is that when the two LEDs are powered by the dynamo, they will be charging the supercap to a voltage close to 6V, which is not good for the supercap. It's not particularly good for the LM317 either.

The third disadvantage is that you won't be able to use more than a tiny fraction of the energy stored in the supercap. Once the supercap's voltage has dropped to 5V or so, the current through the LEDs will be very small and the LEDs will produce very little light.

Overall, you'll get much more light and for a much longer time if you use a single LED for the standlight than if you use two in series.
Of course, you didn't specify whether you would use two LEDs in series or in parallel. I'm assuming series, but if the LEDs are in parallel, the performance will be essentially the same as if a single LED was used.
 

jack in the matrix

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From that 2008 standlight thread, I came away with a different way to clamp voltage to the supercap, and a well charged 2 dollar supercap has been adequately powering 2 LEDs for 5 years. The standlights are acceptably bright for up to 5 minutes, the taillight (first in the series) more so, the headlight - enough so. For me, this exercise is about fully charging a super cap with the voltage regulator.


It's my understanding the LM317 takes various input voltages, and (with resistors) supplies a controlled output. But it needs some headroom-more input voltage than it outputs. To put out a controlled 5.5V, it probably needs about 6V, or more. I don't see an issue, unless there isn't enough (headroom), but if the LM 317 draws from all 3 LEDs, no problem. Hmm. Does it draw from all 3 LEDs in the circuit?
 

Steve K

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.....
It's my understanding the LM317 takes various input voltages, and (with resistors) supplies a controlled output. But it needs some headroom-more input voltage than it outputs. To put out a controlled 5.5V, it probably needs about 6V, or more. I don't see an issue, unless there isn't enough (headroom), but if the LM 317 draws from all 3 LEDs, no problem. Hmm. Does it draw from all 3 LEDs in the circuit?

With white LEDs, the voltage drop across the top LED will provide enough voltage to satisfy the headroom requirement for the LM317.
Paragraph 6.3 of the datasheet shows that the difference between the input and output voltage should be no less than 3v.
http://www.ti.com/product/LM317/datasheet/specifications#SCES1284837
 
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