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Thread: Squeezing the power out of a Supercap

  1. #1

    Default Squeezing the power out of a Supercap

    This would be my first post to CPF.



    I have a hub-dyno rear standlight of my own design, which without drawing the diagram, is a bridge rectifier, 5.5V 1F supercap, then an LM317 regulated to 2.1volts, fed across 1 red LED.



    Its not the only rear standlight, I have a BM tophat or whatever they call it. The circuit works just fine and the supercap from Maplin seems to cope with whatever comes its way. On test I used a Hornby controller and it didnt even get warm at 7v. Admittedly the load was minimal at that point. the Bridge Rectifier holds the dyno voltage down, as does the ohmic resistance of the cable from fwd to aft.



    Given the LED needs 2.1 volts, the supercap will drive it until it shows about 2.8v, at which point it can't squeeze through the LM317 and it fades. I had reckoned to use a PNP transistor, opening up when the output of the LM317 went low, thus connecting the supercap straight through for a bit more power, but that's not effective, nor can I make it work. Is there some voltage regulator IC or circuit design which can drag some more power out of the supercap for me?



    Jon, near Swindon

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    Default Re: Squeezing the power out of a Supercap

    How did that happen then? There were some nice replies to this before but I suppose things have been shaken up a bit by the rebuild.

    No matter, I have improved this design enormously and it stays on for ages, brighter than the Busch & Muller standlight above it and longer.

    A Low drop-out regulator, 3mm oval red LED with a bridge rectifier, smoothing cap and supercap (1F).

    http://www.maplin.co.uk/150ma-ultra-...rdercode=N68CA

    http://www.maplin.co.uk/superbright-oval-leds-45920

    I think I might be over-driving it but it's been good up to 25mph with the hub dynamo and hasn't given up yet. If it goes too quickly I might just put 2 of the LEDs in series to protect each other.

    Before the replies talked about boost/buck regulators etc. Is there is a reason no one likes these?

    Jon near Swindon

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    Default Re: Squeezing the power out of a Supercap

    I think I remember some discussion about this prior to the Great Crash. Wasn't I suggesting the Zetex ZXSC310 as a way to efficiently use the charge in the cap? It's a handy little gadget that runs down to 0.8v, IIRC.

    I like LDO's, but not as a led driver. There really needs to be some way to limit the current. Maybe if you put some resistance between the regulator's output and the led, just to limit the current?? I'd rather see some form of current regulator, preferably one that had a very low headroom requirement. I tried looking up a simple current regulator, and didn't see exactly what I wanted. National Semiconductor does make a simple current source that can deliver up to 10mA and only needs 1v across it to work.

    This would be an excellent excuse to learn about energy storage in caps! The fundamental equation is E = 0.5 x C x V^2. E is joules, and you'll want to calculate the energy when you start with a fully charged cap and the energy that's left when you can no longer make use of the remaining voltage.
    I'll throw out the other basic equations too: Q = C x V and I = C x dv/dt

    The basic theme is that you'll get more energy out of the cap if you can use the voltage down to a very low level. i.e. draining the cap down to 3v is using less of the available energy than if you can drain it down to 0.8v.

    I think the thread about standlights was recovered... it'll have a lot of the relevant discussions and ideas in it.

    regards,

    Steve K.

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    Default Re: Squeezing the power out of a Supercap

    I thought that was kind of the point of an LDO, it was a current regulator? I didn't get too much from its data sheet. The one I'm using is meant to be 150mA at 3.3v which to be fair is over-driving the LED but it's not burning out. I could add a resistor I suppose, that'd be reasonable.

    We got round to mentioning the functionalities before but I don't think you mentioned the ZXSC310 by name. 2 rather simplistic reasons I might not go with it. (i) This works nicely (ii) It'll be a bigger board. I'm a bit strapped for space in the old mudguard reflector/dynamo filament light housing, only just get the current setup inside it.

    I agree with the capacitor equations completely, it also shows me that I get a squared law increase in energy the higher voltage cap I can find. Not too many 10v 1F caps about though.

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    Default Re: Squeezing the power out of a Supercap

    LDO's are normally voltage regulators. The 150mA is probably the amount of current it can pass without damage. The datasheet should be detailed enough to tell you.

    did we discuss current regulators built from two transistors? It does eat up about a volt, which is a lot in a design like this. It sort of depends on what the forward voltage of the led is. Do you have any idea what it is? i.e. what the Vf of your led is?

    In a space constrained design, part of the math is to figure out how big of a cap you can fit, and then trade the cap's voltage against the capacity. Going to a higher voltage rating will mean the cap has to be larger. In some respects, the physical volume of the cap is directly related to how much energy it can store. The trick will be optimizing how much energy of the stored energy you can utilize. This sort of optimization is one of the essential skills of a good engineer.

    Steve K.

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    Default Re: Squeezing the power out of a Supercap

    Both the bits ofm information you asked for are at the links. I can't make head nor tail of the LDO datasheet and what Vout is meant to be. Vf for the red LED is meant to be 2.2V but if I'm at 3; it's not in excess of the stated maximum reverse; inside the other typical Vfs for the range. But there's no data for max Vf.

    Ultimately much better than a LM317 which droped out at about 3V no matter what it was regulating to. I could use the 8-pin version of this LDO to really set the voltage correctly but that'd be space I don't have to populate the rest of the circuit.

    As I've said, the circuit is in and working in the available space. This cap at 5.5V 1F. http://www.maplin.co.uk/double-layer-capacitors-98185

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    Default Re: Squeezing the power out of a Supercap

    Hmmm, perhaps I was overdoing it. Had to replace the LED. Found another spare 'normal' superbright red LED and threw a 18ohm resistor in there. It survived nicely after a couple of steep hills saw me at 34mph. I also got a spare oval LED and the right resistor for 3.3v should this one give up on me. I have to say, though, that I still don't really understand the problem of a voltage regulated power supply for an LED. Will Ohm's law not take over and whatever internal resistance the LED has limit the current in the appropriate way?

    Spearately, does anyone know what LEDs Busch and Muller use in their headlights? It's time I made a new front light to complement the halogen bulb with an equally bright LED standlight. And I'm thinking LDOs are the way ahead again.

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    Default Re: Squeezing the power out of a Supercap

    We got there, front light circuit made, tested and waiting an enclosure! Some bike shop ought to have something with a reflector I can bodge.


    Will this standlight work? Yes it will. Stayed on for 10 mins after 2 mins running with test source (Hornby railway controller) at 3 quarters power. After about 5 minutes it wasn't probably enough to even be seen by. But the one at the back of the bike with the same circuit lasts longer at full brightness than the Busch & Muller above it. (Admittedly the B&M turns itself off after 4 mins).
    Last edited by jdp298; 04-20-2011 at 01:45 AM. Reason: Got right links to pictures now

  9. #9

    Default Re: Squeezing the power out of a Supercap

    Hi There,
    I know this thread is a bit old, but I've just come across it.

    I've been experimenting over the last weeks with different configurations for bike lights using a shimano hub dynamo.

    It looks from your photograph that you're using an Osram LED, which is the one that I've found to be one of the better ones. However, I ended up using a 3W Dragon X plus. rather than a 1W, I just used a full wave bridge, with a !F 5.5v supercap (maplins) connected directly to the LED,

    As the dynamo only gives out a maximum current of 500mA, as soon as the LED draws this amount of current, the voltage is self limited to about 3.6V. There's a small amount of strobing at very low speed with such a low value capacitor, but over about 7mph it's not noticeable, and on coming to a halt, the standlight function works for several minutes, all be it as a light to be seen by rather than to see with!

    Also used the same configuration using two similar LEDs in parallel one with a 5 degree collimator, and one with a 30 degree. but this time used 2 x 10F 2.5v caps in series (to give 5F 5V) This set up works well, and there's no noticeable loss of brightness to one LED when the second one is connected.

    Both lights have been tested on the big hill where I live at up to 38mph, no problems.

    Just bought a couple of Samsung sunnix 6 180 lumen LEDs which are currently at the giveaway price of 1.72 euro post free from wwwLED1.de, these have a max current of 250mA, so 2 in parallel take the 500mA from the dynamo. the voltage required to drive these is 8.9V which the dynamo easily gives out, again self limiting when the max current is drawn. I've use 4 x 20F 2.5V caps in series ( to give me 5F 10V) and with the bike on a stand indoors the results look very encouraging. Certainly more suitable for high speed riding, so may have the finished unit switchable with the originally described unit.

    My only problem really, is the beam patterns, which I don't think are as good as using a reflector such as the B & M units. I did buy an excellent LED bike light from LIDL with a single LED mounted above the reflector shining downwards which gives a good beam pattern, but it's ultrasonically welded together, and I'm loathed to destroy it, however, when /if they come on offer again, (they were only about £8 I think ) I intend to get a couple and butcher them for the reflectors.

    Hope this may have been of interest.....pete

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    Default Re: Squeezing the power out of a Supercap

    Quote Originally Posted by jdp298 View Post
    . . . . I still don't really understand the problem of a voltage regulated power supply for an LED. Will Ohm's law not take over and whatever internal resistance the LED has limit the current in the appropriate way? . . .
    At the same voltage, a change in temperature can cause the LED current to vary greatly.

    If you increase the LED voltage by 10%, the LED current increase TENFOLD.

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    Default Re: Squeezing the power out of a Supercap

    Why not use the one-transistor current regulator.

    LED connects to +ve and collector of NPN transistor. Current sensing resistor is selected to drop 0.6 volt at the the desired current - connects to Emitter and -ve rail.

    Two silicon diodes in series connect cathode(+) to -ve and anode to Base and to a resistor which also connects to +ve. The resistor should feed 5ma at (V+ve)-(1.5).

    You need to make sure the Supercap NEVER has more than 5.5 v across it.

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    Default Re: Squeezing the power out of a Supercap

    The 1F supercaps from Maplin are rated 5.5V normally, safe to 6.3v surge, so says their website. I've been wrestling with the front light, conceptually, for a little time now. Once I stop, the brightness immediately drops but then stays roughly constant for a couple of minutes before fading away again. I've never been too worried about thermal runaway as the safe range of the LED exceeds that which the LDOs regulate to. What I want is to stay at max brightness for as long as possible. I think the next step might be to take a similar risk to the rear light and do away with the zener, or at least push it a little closer to 6v. (Imagines heart attacks among the careful). And replace the 3.3v LDOs with 5V ones and a resistor in series with the same LED. It works out to only be 4 ohms.

    Separate question, my B&M DT toplight plus standlight is staying on for about 20 seconds max, even after a 13 mile commute. Is it getting old and has anyone ever successfully got inside one and fixed it?

  13. #13

    Default Re: Squeezing the power out of a Supercap

    Apologies if this post is out of sequence, I posted one yesterday, but it's not yet materialised. <br>
    <br><font color="#0000cd">?. See post 9.<br>
    <br>
    Bill<br>
    <br>
    </font><br>
    You say you're wanting the front light to stay bright for longer, I see from your circuit diagram, that you're using the two 1F caps in parallel giving you 2F in total, with the LED drawing a current of say 350mA, the voltage in the cap will fall pretty rapidly, so the led will take less current, and stay dimmer longer if that makes sense. There won't be enough energy in the cap to support full brightness for very long.<br>
    <br>
    I tried using two 20F caps ( in series for higher voltage as they were only 2.5V) that kept the led lit brightly when stationary for a lot longer, trouble was, that at the start of the ride, it took ages for the caps to charge up to the LED's operating voltage.<br>
    <br>
    Perhaps it would be better if we used (and a lot cheaper) some re-chargeable batteries?<br>
    <br>
    I've considered using protection circuitry, but really don't see the point, the LEDs I use safely take all the current that the dynamo will produce, so self limit the voltage. <br>
    <br>
    Think I could well have bought a commercial one for all I've spent on LEDs and caps! still it's not as much fun is it!<br>
    <br>
    I did buy an excellent LED bike light from LIDL with a single LED mounted outside the reflector similar to the B &amp; M unit, It runs from 4 AA cells and draws 420mA and 210mA on the two brightness settings, I did consider just connecting the dynamo directly to the (re-chargeable) cells through a bridge rectifier.<br>
    <br>
    Pete....Sheffield
    Last edited by Bullzeyebill; 10-12-2011 at 08:06 PM.

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    Flashaholic* Steve K's Avatar
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    Default Re: Squeezing the power out of a Supercap

    Quote Originally Posted by lumière de bicyclette View Post
    Perhaps it would be better if we used (and a lot cheaper) some re-chargeable batteries?<br>
    ......
    Pete....Sheffield
    that's the path I've chosen.

    The plus side is that I get long run times and fairly bright standlights. The negative is that the circuitry is more complex and I use a switch to disable the standlight when I park the bike (don't want to completely discharge the AA nicad). A CMOS timer could probably be added to get rid of the switch, I suppose.

    A lot of the issues with standlights come down to basic math. Any power used to charge the supercap or battery comes at the expense of the headlight (or taillight). The standlight run time is related to the size of the storage device as well as how effectively you can utilize the energy in the storage device. It also depends on how bright you want the standlight to be.

    There's also the question of the duty cycle of the standlight, i.e. what percentage of the time will the standlight be running. If you spend a lot of time stopped at intersections, then it's going to be very difficult to keep the standlight powered.

    While it's fun to tinker, sometimes it helps a lot to do the calculations and see what it even possible. This might avoid wasting time and money on ideas that don't work out.

    regards,
    Steve K.

  15. #15

    Default Re: Squeezing the power out of a Supercap

    I have a XM-L at the front and a XP-E (Red) at the rear running off a Shimano hub dynamo. I've found that a bunch (4) of 5.5v 4F supercaps that I picked up on eBay work brilliantly in parallel as a standlight for the XP-E, giving me bright rear light for 10 minutes or more.

    The XM-L drains the caps instantly so for the front light I'm going to need to discharge them through a resistor, probably using a circuit like this to get a some more voltage across the caps.
    http://www.candlepowerforums.com/vb/...t-work-)/page3

    The capacitors ovbviously have a reasonable internal resistance as they don't stop the rear light coming straight on when I start up, and they do take some time to fully charge.

    The rear standlight experience leads me to believe that supercaps are the way to go, I'll report back once my front light is done.

    Cheers
    Simon

  16. #16

    Default Re: Squeezing the power out of a Supercap

    I've been looking at various bike light projects for ideas to retrofit my old (ancient) halogen unit. I wasn't even thinking about supercaps until I spotted this thread!

    I'm likely going with a higher output main headlight, but a supercap circuit would be an excellent backup -- no worries of exhausted batteries!

    I'll be sure to post updates if I learn anything that isn't already covered here.

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    Default Re: Squeezing the power out of a Supercap

    Back to the drawing board. Have found some 50F 2.3v (yes, fifty!) caps on Farnell for sensible prices. So, incorporating the cct Steve K appears to have made work, here:

    http://www.candlepowerforums.com/vb/...t-work-)/page3

    I can cheerfully get on with things. But for a couple of questions:

    1) How do you calculate the Inductor and I(sense) resistor? I've been looking at the datasheets all evening and got nowhere. After all this time I'm not even sure how the ZXSC310 even works in the first place. I want to use a single 3.8v 700mA 3W LED or a 1W 3.5v 350ma one. Obviously the Standlight element could be lower current, but I want to know how to choose.

    2) In Steve's cct, here:

    http://www.flickr.com/photos/kurtsj0...7621965148305/

    I get holding the shutdown pin low when the wheels are moving. But the datasheet says 0.7v is needed to enable it. How do you get round this?

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    Default Re: Squeezing the power out of a Supercap

    Quote Originally Posted by jdp298 View Post
    ...
    I can cheerfully get on with things. But for a couple of questions:

    1) How do you calculate the Inductor and I(sense) resistor? I've been looking at the datasheets all evening and got nowhere. After all this time I'm not even sure how the ZXSC310 even works in the first place. I want to use a single 3.8v 700mA 3W LED or a 1W 3.5v 350ma one. Obviously the Standlight element could be lower current, but I want to know how to choose.
    page 9 of the datasheet (my 2002 copy, at least) has a table of suggested inductors. They are all 68uH and rated for about 0.5A. I ended up with a slightly larger inductance and higher current rating part that I got out of Digi-key. I picked the part about 8 or 9 years ago (I think..), so I'm not sure exactly how that happened.

    page 10 of the databook tells how to pick the current sense resistor. This sets the peak current in the inductor, which then sets how much energy is stored in the inductor. I think I experimented a little bit with different values, which I do recommend.

    I don't think there will be a problem with either LED, but the current out of the zxsc310 will in the 10's of milliamps, not the 100's.


    Quote Originally Posted by jdp298 View Post
    ...
    2) In Steve's cct, here:

    http://www.flickr.com/photos/kurtsj0...7621965148305/

    I get holding the shutdown pin low when the wheels are moving. But the datasheet says 0.7v is needed to enable it. How do you get round this?
    The datasheet says that the Enable pin can have a bias current of up to 1uA positive or negative. My experience has been that the pin sources current, and if you don't connect anything to it, it will float high. That's why I just added a switch to ground. You could add a high impedance resistor from the Enable pin to Vcc just to be sure that the pin is forced high. I think I avoided this to minimize the drain on the battery when the bike is sitting around.

    regards,
    Steve K.

    edit: as far as how the zxsc310 works, it's a simple boost converter. There's a decent wiki page on this type of converter.
    http://en.wikipedia.org/wiki/Boost_converter

    I'm avoiding calling the zxsc310 a switching regulator, because it's not really regulated. The output varies a fair amount as the input voltage changes.
    Last edited by Steve K; 10-22-2011 at 03:26 PM.

  19. #19

    Default Re: Squeezing the power out of a Supercap

    Quick update for those that are interested, I've managed to come up with a simple circuit that gives > 5 minutes standlight for both front and rear LEDs seemingly without wasting any current from the dynamo when you're on the move.

    It uses a CREE XM-L for the front and a red CREE XP-E for the rear, connected in series so they can both get 0.5A when the dynamo is running. I then have 2 x 4F 5.5v super cap in parallel with the XM-L via a 47R resistor with a 1N4001 (D1) to limit the maximum voltage accross the cap as per the circuit linked to in my previous post. Where this circuit differs is that second LED, the rear, has 4x4F 5.5v supercaps in parallel with it and the 47R resistor has a 1N4001 (D2) in series to prevent the rear's supercaps from discharging back through this resistor and D1.

    The result is very bright lights as soon as you pull off (I'm guessing the supercaps can't suck in too much current) and once they're charged, e.g. after a minute or two of riding, you have standlight functionality for both front and rear.

    I can't upload a diagram at the moment but will when I get the chance, and if you're wondering about why I'm using 4F 5.5v supercaps it's because you can buy 10 for about £14 delivered on eBay which seemed like a good starter pack for experimentation...

    Now I'm going to stuff all this is some nice chrome vintage lights to finish the job...

    Cheers
    Simon

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    Default Re: Squeezing the power out of a Supercap

    The price for the supercaps sounds very good! Of course, I've only bought one supercap and that was a couple of years ago. Can you tell us the details of the supercap, such as manufacturer, model, p/n, etc?

    For comparison, when I search Digi-key for a 4F cap, it shows me a 3.3v part from Taiyo Yuden for $4.38. The diameter is 10mm and length is 22mm.

    regards,
    Steve K.

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    Default Re: Squeezing the power out of a Supercap

    Tell you what Simon, that's a decent enough and pleasingly simple circuit. Glad it works.

    After all that time examining the ZXSC310, I went with a copy of what I did before. I got another old style light off ebay 99p plus p+p; it's aerodynamic, but only from behind, and there's really not the space for a large circuit inside. The only variation is that I went with 5v LDOs and 3x 13ohm 0.5W resistors in parallel to give me 4.3ohms at over 1 Watt. Bit of an experiment to see if it either lasts longer or shines brighter on standlight. Initial results indicate neither, whaddya know? And there's a switch.

    The end result, tested in the park just this evening, is ideal. I think it's bright enough to ride any tarmac road regardless of ambient lighting. Photos to follow. The previous B&M halogen senso sucked the life out of the LED before and was a bit too warm a white. For anyone interested, it'll be on ebay itself some time over the weekend.

  22. #22

    Default Re: Squeezing the power out of a Supercap

    No idea on the manufacturer Steve, it's a coin cell form factor with just 4.0F 5.5v marked on it. Here's a link to the eBay listing (I am in no way connected to the seller) but this says nothing about who actually makes the cap.
    http://www.ebay.co.uk/itm/4-Farad-Su...item3cbdb1515c

    Googling seems to just bring up chinese wholesalers on alibaba.com rather than the actual manufacturer so I've no idea of the ESR etc.

    The diameter is a bit of a pain as I was originally hoping to squirrel them away inside a torch that takes a 18650 cell, however they are cheap for their capacity and the 5.5v rating helps keep the circuit simple.

    Having given this a bit more thought, I don't think there's any reason why I can't make the circuit more symetrical and charge the supercap for the rear light up to 5.5v as well, perhaps some more experimentation this weekend...

    Cheers
    Simon

  23. #23

    Default Re: Squeezing the power out of a Supercap

    wow. Ive got some motivation to seek supercaps in the order of a dynamo circuit.

    I have two 2.7v 150farad caps... but am really concerned about being able to charge them reasonable on the cycle from dynamos.
    Any ideas that trek in stride with the other posts thus far?

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    Default Re: Squeezing the power out of a Supercap

    They'll take a few minutes to charge by dynamo, for me though the size would stop me from using them on a light build.

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    Default Re: Squeezing the power out of a Supercap

    Yeah, they're pretty massive. Some simple numbers: Energy on capacitor at full charge = CV^2 = 150*2.7^2 = 1093Joules per cap = 2186J
    Energy from a dynamo providing 7v at 0.5A = VAt rearrange for t = 1093/7*0.5 = 312s, or just over 5 minutes, assuming 100% charging efficiency. In reality it's about 60 to 70% so more like 6 or 7 mins. With 2 caps, suddenly you're at 12 or 13 mins. Have a long commute do we?

    I run two 1 Farad caps at 5.5v for each of 2 front lights and another single cap at the back. = ~90J. Lights come up in about 2 seconds and full brightness once stopped after 20s or so. The nice thing about the LDOs I use is that they fade the 1W front LEDs quite quickly from full on but keep going as pretty reasonable lights to be seen by for a good 5 mins. The rear LED, just being a superbright red, stays full brightness for 3 or 4 mins and then fades gradually, but it takes a lot less current and so only needs 1 cap.

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    Default Re: Squeezing the power out of a Supercap

    Quote Originally Posted by krienert View Post
    wow. Ive got some motivation to seek supercaps in the order of a dynamo circuit.

    I have two 2.7v 150farad caps... but am really concerned about being able to charge them reasonable on the cycle from dynamos.
    Any ideas that trek in stride with the other posts thus far?
    How about a circuit which feeds most of the Generator current to the LEDs so you get some light instantly, and any surplus current charges the supercaps.

    Keep in mind that when you put two supercaps in series, you MUST have a balancing circuit to ensure that neither capacitor goes over its voltage limit.

  27. #27

    Default Re: Squeezing the power out of a Supercap

    I am starting to see that the 150 farad caps are a little over kill for only a 10 mile trip (20 round trip). I average about 20mph so its really only a 30 min trek.
    If anyone wants one of these I might be inclined to share if someone can be of help in working out a working standlite circuit.

    Maybe I should start a new thread though... as this might be diverging from this post.

    Heres a hook thou...
    I have some of the mentioned 5.5 volt 1 farad caps-sounds like these will work much better. Although I have tryed a couple circuits, im having a rough time getting
    them to charge and power my test 'lamp'. Actually, I never got the leds to lite even after a moderate pace on the workbench (floor w/bike upside down) for about 1
    and a half minuits. At which point, I stop - so to not over load the cap. Yet when I short the tabs, there is no visable spark - showing no charge at all is getting through.

    The setup is a 0-10vdc 500mah output (magnetically rotated stepper motor) through a shokty bridge, then to the leads on the cap and to a small array (x9) 5mm cool white leds.
    Mearly a test setup right now, and I still cant get the diodes to illuminate with the supercap. It works fine with 3700uf 50v but the leds dont hold enough to be 'stand' lites.

    Should I go ahead and bump this to another post?

  28. #28
    Flashaholic*
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    Default Re: Squeezing the power out of a Supercap

    Quote Originally Posted by krienert View Post
    . . . . and to a small array (x9) 5mm cool white leds. . . . .
    This sounds like a 12 volt LED array ???

  29. #29

    Default Re: Squeezing the power out of a Supercap

    not quite.
    just a small test rig. It could sure run 12 v, but would be pushing it.
    Its actually the small board from one o thos cheapy $2 aluminum
    lights from the junk box. It has 9 leds (1 center-8 outer) ran in parrallel not series... if that helps?
    (originaly powered by a x3-AAA block)
    Last edited by krienert; 11-01-2011 at 05:24 PM.

  30. #30
    Flashaholic
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    Mar 2011
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    Default Re: Squeezing the power out of a Supercap

    There are quite a few standlight circuits cutting around these parts. Mine's pretty simple but lacks absolute efficiency and brightness, while Steve K's is the platinum plated solution but quite a bigger build. Diagram of mine here:
    http://www.flickr.com/photos/jdp298/...7626884852454/


    What's the normal current draw and voltage requirement of the white LEDs? If 3 AAAs do it, then that's about 4.5v. If you can find a 5v Low Drop-out Regulator, then put a series resistor, knowing the current, before the LEDs, then that ought to work. I protect the front lights' supercaps with a zener diode, but that's really for paranoia's sake, I don't use one at all in the rear light.

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