Capacitor/Battery Solution. Hold temporary charges

Discussion in 'General Electronics Chat' started by John5788, Aug 5, 2009.

  1. John5788

    Thread Starter Active Member

    Apr 2, 2009
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    I'm trying to just make a system that can hold a temporary charge for about 5 or more seconds after being plugged/unplugged from a 12V power source. system will consist of a 3V PIC24H, and a few LEDs.

    anyone have sample projects I can look at to see what to do? I have a feeling an RC circuit would be in place somewhere, but not sure what other kinds of elements I would need.
     
  2. steveb

    Senior Member

    Jul 3, 2008
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    Supercapacitors will work. The following module can be charged to 15 V and can be fed into a 3V regulator to power your system. If you use a low dropout regulator, the system will be powered while the capacitor discharges from 12V down to nearly 3 V. You can do some quick calculations based on your current draw, but Im quite sure this will last more than 5 seconds.

    http://www.maxwell.com/ultracapacitors/products/modules/bpak0020-15v.asp
     
  3. John5788

    Thread Starter Active Member

    Apr 2, 2009
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    thanks, never looked into the supercapacitors. these work pretty much the same as normal capacitors, but with huge capacitance?
     
  4. steveb

    Senior Member

    Jul 3, 2008
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    They are pretty much the same, but the voltage ratings are usually very low 2.5V to 2.7V. The modules attain higher voltage ratings and are designed with series connected capacitors and some method of voltage balancing. You could make your own module using standard supercaps.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    I have had this for many years but not tested it. I think perhaps most of the circuitry can be replaced with a LM2578/LM3578
    take a look at it
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    @steveb: any ideas on the pricing of these modules?
     
  7. t06afre

    AAC Fanatic!

    May 11, 2009
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    I will say that they are not so expensive. If you evaluate price against Farad. I see that you live in US, so do a search on Digi-key (supercap). I live in Scandinavia so it is hard for me to say if things are cheap or not in US
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    Actually, I never priced out the modules. Supercaps are fairly cheap now, but they may get a premium price for the modules.
     
  9. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    No problem. That is the first time I have seen the ready made modules. I'll give the manufacturer a call.
     
  10. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    The Maxwell ultracaps and modules are available from www.mouser.com. The modules are in the $150-200 range, so yes, a little pricey.
     
  11. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    if a cap does not do it look into small nicad battery. probably will last much longer than 5 sec. does that matter?
     
  12. t06afre

    AAC Fanatic!

    May 11, 2009
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    I think you mean a NiMH type or ;). Anyway I think a supercap solution should last for at least 5 seconds depending on the supercap size. And of course current consumption
     
  13. John5788

    Thread Starter Active Member

    Apr 2, 2009
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    thanks for the replies.

    I was wondering if the supercaps would work at the lower voltage levels.

    I salvaged a bunch of radial caps from an old computer power supply to test this out. I wired them all in parallel and got somewhere around 8000uF. I watched with the meter and noticed that there is a large voltage drop that is exponential once my main power to the board cuts out.

    I'm wondering now if I used the supercaps, ~3.3V, that if I charged it up, then cut power, wouldnt the voltage level drop below 3.3V and stop powering my PIC24H? unless it holds a good charge for a long time.
     
  14. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    a cap has a voltage proportional to the charge in it, so as it dischanges the voltage falls. Rechargeable bats. have a flatter discharge curve and are cheap which is why I suggested a nicad. ( i did mean nicad, but a nickle metal bat would be fine too ).
     
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