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Running a LED off a capacitor

Discussion in 'LEDs and Optoelectronics' started by Liquid design, Dec 11, 2011.

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  1. Liquid design

    Liquid design

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    Dec 11, 2011
    This is my first post as electronics are not my main focus. I know from reading many posts that LEDs are a main question for multiple users. Hopefully, I did not overlook the answer to my question on a previous post.

    Here are a few things I am looking at or considering:

    1. I am looking to run a 470nm LED (blue/blue-green wavelength) off a capacitor for an extended period of time. I want to use a capacitor since there will be an extended period of time in between uses (could be months) and batteries can get a memory if not stored correctly.

    2. I am also looking at the aspect of speed of charging which, if I did my research correctly, can be achieved with capacitors.

    3. Also, this will be in a sealed system so I will not be able to change batteries.

    4. I was thinking a 1-1.5F capacitor for supply to a single LED but unlike most uses I want to dim the output of the LED (not sure how to calculate lumens) and am hoping this will extend the burn time. Could a thrysistor/ joule thief help the run time?

    5. Size is a big factor but can be adjusted for off the shelf components to keep the cost down.

    6. This is a project I am wanting to market if possible depending on cost as this is only part of the entire project.

    I have approached several firms on designing this for me but the responses I get are less than I was hoping for (either too simple to waste their time or WAY too expensive for me to afford). If this is not the appropriate forum then I apologize but let me know either way as I am trying to figure this out a.s.a.p.

    Or, if possible, direct me in the right direction to get this accomplished in a less expensive manner

    Thanks in advance!
     
  2. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    You are going to need a lot more than 1F to do what you want. Given a current draw of 5ma, your 1F capacitor will lose 1V every 200 seconds. And the highest voltage I could find on a 1F capacitor was 5.5 volts so 1100 seconds if you could use all the energy in it. If you can find and LED that is bright enough at 1ma you are still talking about 1 hour or so of runtime off a 1F 5.5V capacitor, and this if you can use an efficent switch mode current controlled power supply. This is why people are still using batteries.

    Bob
     
  3. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Of course I could be grossly misinterpreting what you mean be an extended period of time. Why don't you tell us:

    How much current the LED willl draw and
    How long it needs to run.

    Bob
     
  4. Liquid design

    Liquid design

    3
    0
    Dec 11, 2011
    Bob,

    I think you are right and not misinterpreting it. I am looking for quick charge (not hours on end), and the most rum time I can get while keeping size down. Ideally, I would like to keep some light going for a total of 8 hours. I know this is out of the question and that is where quick charging could come into play.

    Again,this is not for reading just illuminating a tube about five inches long and about 1" I.D.- want a glowing effect.

    If a battery would be feasible I would be open to suggestions, but keep in mind that this thing could sit on a shelf for 9-10 months without any attention or maintenance. I think we all understand how this could affect rechargables. Unless there is a new battery out there I am not aware of (and this could easily be the case) I have been looking for other options that could work.

    I thank you for your input and I am looking forward to hearing more suggestions.

    P.S. there are no parts set in stone so current draw is up to whichever manufacturer's LED I use. If I need to I will look up different LEDs and post the current draw if this helps.

    Mike
     
  5. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Well, I was thinking of a lot more then 8 hours when you said and "extended period of time". So I think it is feasible to use supercapacitors for this purpose.

    You want to run the LED off a current-controlled DC-DC converter. The converter should be of the buck-boost topology so that it will operate off the widest possible range of voltages, allowing you to draw as much energy from the capacitors as possible.

    Since supercapacitors tend to be low voltage, you could use several in series to get to the max input voltage of your DC-DC converter. Same size capacitors in series have only half the capacity, but they can store twice the voltage, and the energy in a capacitor is proporitonal to the square of the voltage, so you can put more energy in 2 1F capacitors in series even though the you have only a 1/2F capacitor.

    And the higher voltage also lets you extract more energy. If your DC-DC converter operates on input from 2 to 20 V, then using a 5V capacitor would allow you to use only 84% of the energy stored, whereas a 20V capacitor would allow you to use 99%.

    Hope this helps.

    Bob
     
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